Page 1

*3

Speaker-Box Construction AND@NsrRucrloN Hou) DES¡GN ENcLosuF.E Trs pRlNcFlEsoF c'ooDSPB^KER prinfor any speakerproiecl regardlessof box tyPe By heeding üose procioles, váu can üuild with confidence and preveDtlong debugging furniture' ""trr"J. ryo., "t" io"tined toward novelty in your choice of vou can exprcss it iD your sPeakor eoclosures by such superncrsl ásoectsofdá"ign as extemal appearance.For l¡ose desiSnleaturesüat aff;ct üe sound, stick to basics

CHAMCTERISTICS ENCI.OSURE DESIRABLE The sinele most deshablecbaBct€ stic ofa good enclosureis that it does not ieúouslv alter ths sou¡d of the sPeakersinstallsd in it A sDe6kersvst€mis different hortr a musical instrument in on€ important thl speakershould produceno sound of its o$'n An instrument a^soect: ,.lau."" toio¿, " "p"akei reproducesit lf üe speal'eraddsan¡'rtring walls il is producingdistortion lf üe eDclosure io the sienalit receives. bass tlue üe weaken "it""t" i"aitfv, they will cotor the sound and shápe enclosüe A bad *"0"""" ¡v aúsorbinglovl-f."quencysnergy can spoil ieproduction by lumping the inevilable air r€so¡ances a¡ounáa nat¡owbandof ftequsncies,Producinga boomysystem SHAPE ENCLOSURE Elclo$res with nonpa¡allel wslls, such as triangular boxes' have wiü internalreflectionsMost spealerbuilders as well fewe¡oroblears üey are because ," orftét"u¿lo f"o", "hooseorünary box-lüe" enclosures _look most of üese the lüe spealers To male """u ro ¡oil¿ and üev no intemal male "nciosr¡es,uuoide*tremsshapesAs a rule of thumb sn enclosüre Such dimension more than thee times that of any other'


24

Speaker-EoxConlruction

can act like a resonantpipe. You can violate üat rule if you follow üe suspestions sivenlat€rin Lhfuchapter'Át the otheraxbeme.if all dimenofall ure eorlal, siñ. vou havea cube.which is oneofüe worstshaPes used Thk ratio' 0 6:1:1.62. prefened is r€Üo of dimensions One by golden ratio used properties, is üe ssme acoustical bere for lts mason to you have a sPecial p¡'ramids. Unless Egyptian artists since th€ the cor¡ect 3_1 lists Table a choice. ratio fu ratio, this useanothe¡ Sood intemal dimensions for spea.ke¡boxes wiü net intemal volumes ftom 400 cubic inches (0.23€ubic foot) to 10,000cubic inches {5.8 cubic feet). Table 3-2 lists a range of cubic volümes measued tn liters with inteúal dim€nsions in cántimete¡s. If you use üe data ftom th€se tabl€6,you¡ enclosu¡ewill have about 10 percent overvolum€to allow soace for clrive¡s, cleats, ajtd oüe¡ internal Psrts. The cohrmns are úbebd D1,D2,andD3. In timespsst,üe smsllestdime¡sionwasussd for enclosuredepth. Now, to obtaio better iúagin8, a Ía¡¡ower box with qeater depti ii often desi¡ed,so D1 becomesthe enclosurehontal üdth. Foi li.t"n"r" wbo preferbassresPonseto imsglnS or bsve a vt¡oofertoo l6rse for the nanower box, m€l(€D2 the \ ¡idth Intem¡l D¡mensions Table3-1. Colden-Raüo for SpeakerEnclosur€r,Entlish units. Des¡red NetVol¡¡me (Cu.In.) 10000 8000 6000 5000 4000 3000 2500 2000 1800 1600 1400 r200 1000 900 800 700 600 500 400

Approx. Tot¡l (Cu.In.)

r r000 8800 6600 5500 4400 t300 2750 2200 1980 1760 1540 1320 1100 990 880 770 660 550 440

lnternal Dim€ris¡ons (ln) D3 D2 D1 13'/." 121f 11" 10,/;' 9l{ 8'/i 8"

6'h' 6fi 6'

22'/: 20'A" 1at 17.[ 16'/r" 14't{ 14" 13" 12'h" 1211'h" 11" 10f," 10" 93t¿"

36' 331'' 3O'/"" 2a'h' 26'4" 24" 22'/i 21" 20v," 19'4" 185/i 17r: 16sK 16li 15\/,'

8'/," ayi 7'{

14\li 133/r" 12'/í


sháPe 25 Enclosurc

'"?"r;il".u1',of l;i,iJi'^.'Jfi#P¿Ti,T"* Desired NetVolume (Liters)

Iotal

lnternalDimensions

(Liters)

D1

D2

D3

34.0 32.0 29.7 28.7 27.O

54-8 51.7 4t.9 46.2 43.6 40.4 38.0 15.3 32.1 30.2 28.0 27.O 25.4 23.6

88.8 83.7 77.6 74.9 70.6 65.4 61.6

150 100 90 75 60 50 40 30 25 20 18 15 '12 10 8 6 5

I10 99 83 66 55 33 27.5 22 19.8 13.2 l1 8.8 6.6 5.5

(cm)

23.6 21.9 19.9 18.7 16.8 15-8 13.8 12.E 10.9

20.6 18.8

51.9 48.9 45.4 41.8 41.2 38.3 36.0 33.4 30.4 2a.b

*fi*ru-e'*$,r,

il;:ñ] s#*i,so;ffiiu$$

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in EnclosureDesign SomeRecenlTrends

27

The thúd rule isr if the vertical line of drivers is placed off cente¡, make the secondspeakerboffd of your stercopair a mirro¡ image ofthe ñrst (Fig. 3-2). This allows you to plac€ the spealels so that in both enclosuies the drivers will be ofl centered inward, or outward, rather than having one enclosure with the drive¡s inwsid a¡d the oüer with the divels outward from the spacebetween you¡ speakers.

DESICN IN ENCLOSURE SOMERECENTTRENDS A gene¡ationago therc were as many horizontal enclosuresas vertical d¿signs,but that is no longe¡ so. In fact, some curent speakercab' inets are io skinny and tall that they oÍce would have been considered extreme. Figures 3_3and 3_4show some wavs that you can use a tall' na¡row enclosure while miniñizin8 the dsk of piPe resonance lnse¡ting a partition conve¡ts üe pipe to an ordinary box Placing the d¡iver at a point that is one-fifth the distance from the end of the pipe supprcssesthe fi{lh harmonic, a major causeof resonancei¡ closed piP€s. Ifyou add stuffing to the enclosure,results can be quite satisláctory. Columnar speakers. such as those shown in Figs. 3-3 and 3'4, have seveml practical advantages.The nanow front usually produces better Ete¡eo imaging than the wider f¡ont of conventional boxes AIso, the height ofthe cabinetsputs the sound at ear level without the need to place ihe speakersatop other Pieces of fumiture or on separat€ sta¡ds.

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Fígüe 3-2. when dt¡ve' arc nounted off cente, the t||o speakersshould be ñiíol ir¡,agesof each other The namal anngenent ¡s ta ¡nstall the drivers so that they arc offset towad the niddle arca of the s¡ereospace.


?a

Spe¿leÉ8o\Conetruction

,:y:;:,i:;, ,?;i.-*

i;3í:,;í;,::,::

" tuitd a to||e, speake,fheto||e, pa,tof ke cotunn serves

instailedat a f¡fthof thetotatpipetenlthion thetop, iif#1 ,¡s,

:fffi'+il:;il"',T:';"',1".1i.,ff :$;"*:l::,:ll";l*nltn** have,alr-eadynoticed the proliferationof speaker -.- lou nrobablV

asme oes,gnrn | ¡gs.3-5 and 3_6.Suchslandssre designed to r¿iseancl


Som€RecenrTrends in Enclosur€ Desien 29 tip back spealels of intermediate size. Most medium-size speslers sound much more open and natuml when placed on such a stand. The secondkind of Btandyou might have seenis the type shown in Fig. 3-7. Thesestsnds are useful for ulhacompact to compact-sizespe€lers. As speaker designers have become more álert to problems of difhaction causedby some kinds ofcabinet const¡uction, they have made chsn8osin the extemal shape of speakerenclosures.Figure 3-8 show6 some colrlmon ways speakerswere once installed in poorly designed boxes. In th€ cabinet shown in Fig. 3-9, the worst features of Fig. 3-8 have been corected. Figues 3-10 and 3-11 show somo ways to solve the problem ol placing a grille fiame over üe front of the enclosu¡e. When üe grille is installed on standoffs, as in Fig. 3-11, sound can move horizontally wiüout hitting any interfering projections. If yoüi drive¡s are installed ñom the hont ofüe enclosurcand you plan to use an ordinary halne to hold the g¡ille cloth, you can cover th€ speaker board area betwe€n üe d ve¡s and the fra¡re wiü thick felt or

FiSue 3-5, A useful stand fór speake\ oÍ intermediate size. The doping platforn l¡tu and til$ rhe spe.aker,nak¡nq the sound mote open and natural.


30

SP€aker-BoxConsructioñ

rigt¡e 3-6, *

Íigtte 3'7

iñ FiS t'5 * *^O t 'hePt¿nsshown

bv plac¡ngúen on inproveddranatically be can speakers Many srnall


SomeRecentTrc¡d,in E¡closureDesign

-11

Fig"rc 3-8. A bad speakerenclosurc. Ihe drivet installation behind the boad and the shary pro¡ecting edqes inteíerc with sound wave propagat¡on

F - - - - - - - i

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Figu¡e3-9. A good speaketenclosure.lherc are ro obsttuctionsto inteíere with

foam (Fig. 3-12). You can buy sheets of foam at carpet deale$ Such foam is designed fo¡ carpet u¡derlaymenti it comes in va¡ious colors and thicknesses.The %-inchfoa¡r shouldbe about riSht for most speakers. Cut tho fosrn to fit the space and glue it to the speaker boa¡d, the smooth surface against the board. Such material will absorb sound that might be rcradiated hom üe spealer board, causinSinte¡ference wiü sound hom the d ver. You can hear the impmvement wiü some spealels easilf but it is Dot as obvious wiü oüels. To test the usefuInessof thiÁ beatment, tempo¡arily tack or stapl€ the foam to the speakerboard on one of your stereo sp€alers and compare the sound wiü the untlsat€d speater. If you cs¡ hea¡ a difference, it is wo¡th doin8.


32

Sp€aker-Boxconstrucr|on

lin¡n8

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dE a g¡tte taÍE' slue leh ot foan atons

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Materials 3J

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Íoan, canbe glued to thespeakel ÍEür¿3-t2. A soundabsoóent,suchas fettor the dr/'f]rsto rcducediffnctton'

MATERIALS

matedal is %-inch (19 cmj Tbe most commooly used speal(er-box

{medium-densitv p*i"leuo-áorMD.F pry"i.¿. iffi*lilirul ihick¡ess ihould be choser to oatch encl(

:li}:xt#,ltr'#"i*:':" rrt;iy"tii"tl",'gi,i'fi -same to.pan6lsize A Sood üickness so you shouldadiust lhickness tor r" tó use'¡-inch ir'g cml mate¡ial in. soc.losures ñ" "f ü"-l

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p€nelsintite;l,S,",h, larse ""-tl"ahonsBrace ""." g¡a¡ good a pl)'wood choose

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-lf you usefir'ask for tattlás cause " p-"i. *¿ f";se layersca-n

otiot"¡-qtvY:.91 üuTnÁ:''c,"¿" lr.vr'i.fr ill :::f.:I"il,[tJ; enclosur€ by 1 ¡ncn {z : tle enla¡ge wood.

iii"-t¡"',"i"".i-*"ii" *i t h iocb(r 27cmlPaflic',i"11$,**o*,*

1i,ilil*,r"u"-'r"'p*'ra" r""i;:"":'1""1'"J':,F;H"T:ffi 'l;¿;t""iff;'"'l'Jrffi:lJH

j"ff :rS":i."J# wellpaper.


Ext.aWallTrealment 35 Particle board is a good choice becauseof its hiSh density' lack of voi¿lso¡ loose layers, ;d low cost some builders pÉfe¡ high-density the oarticle bo6rd -iile oüets choose medjum-density fiberboa-rdin pafl¡cre Lelief lhat it is betler dsñped You can identi¡' hiSh-density Uo"rJ¡v "". in¡C l¡"particlesize althe sdgesoftheboard-ThehiShha"-smali particles and smoot¡ edges which mates il ¿"*irv'¡o"t¿ *"i". io ¿,r". co""tt""tion adhesiveis a good, low-cost choice of material to us; for bonding parti.le bodrd or €ny kind ol fiberboard lt ioinls ror dont matcb wel[, epoxy is better'CPoxyc€n be qurte expenslve place doioqan enlirecabi;et bul itworks well for Eluingbrscesinlo b"iw?un oppo.;t" punels Cul lhe brac" Io fil ss snugly as possible then the atr"ctrit *iifr epoxy,u"ing nails to hold the bracein place until contact close it requires glue but is useful wood eooxv\els.Carp;nter's oithé parts for ma-ximumstrenSü. ó; disadvanlageol hjgh'densityps-rücleboad is that il dulls ordinary saw bladesi After o-nly one pass tlrough üe- board, ^üe ¡lade needst; beresharpefled,Fo¡ fastercutting ard longerblade life' choose a tlade with carbiáe teeth to cut paficle board Such bladescan be danger;us if.,sed lncorr""tly lfyou get a blade with carbide-tipPedte€th' follow these precautions: . Use a blade guard on your saw. . Don't use tñe blade in a saw that exceeds the úaximum recommended rpm for üe blade. The maximum rate probably is printed somewhereon the blade Package. . Savethc orieinalpack€gefor blade'toragc\ahennot in use . Use careat áU tirn"s;n handlingthe blade lo ensu|pthát you nevec qurtace bump lhe Ldrbjdeleelb a8ainslsny

WALLTREAIMENT EXTRA If vou are a perfeLlionist. thete are some e\ba touches üal can improíe almosrary speakerenclosure.parlicularlythosemadeof ply' -;d. one kind oibr;.e úral occupiésvcry litlle spaceis on€ üat ties anv opposing walls togeüer' Such a brace need not be thick becaúse That brace the compress sbetchor to movemust tJriaut "| to. ttt"rtváu" movement is in üe düection ofgreatest stiffness' Almost any enclosure wall ia¡ be made less resonant by making üe wail into a composite of more üan one kind of material lt's a parricularlygood ideaü add ¿n] mdl€rialthal can damp üe walls suchas ""on"f,'.É"n"g material You can often 8"1 scraps of asphalt roofing hJm constructlon silos. If ¡ ou glue pieces of il lo the cent¡al arcaof üe wall Danels,the walls will be relatively dead to sound You can use asphjlt cementfor elue but it takcsso long lo setthal ) ou m¡ghtprefer adhesive.one all-purposeadhesjvemade by Macco a a iaste¡-actinp


SPeakerBoxConsruction

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ConstructionRules

JZ

boad and back'suchasthesg Fiqrrc 3-14. Close-fittingcleatsbehindthe speaker hélpprcventa¡ teaks. üe speaker hol€s to ioitrt. then install the back. You can r€ach throügh taulk the ioints atound the back

iHi:i:Üli:Jl"iHH;"$ -.tll:'*;ttlm:iT"1"f hsr:oi;¡" r,fa'1ir;?il*i'i,*::.":",1i;; o",nq"ls rorq"1 3,o"o.t "#:*,iil::::"f *á i:ffi ;,"- "i,¡"*'"..*" i,j:1.."Tll

'**:'J;"-*-i"';Hi' .:T:,"J,i,,ffi $it"#T:":'iPf

* t",:T::fi""i"'J,,"J".:il. 1$i!1r"11;1,, 5,'""*::.,i",:lül ,"''ü11 inte' onrhe mareriar ;iliií"::tli"':iiliilil;'"l''",iio"-'ins l;l[ilJffi"::; "*""iy;l"'",'ffi ;1"'#;ff ffi:*fi',::Tfl


38

Speaker'BoxCo¡struction

tiEue 3-15. Add cornetglue blocks,scrcwedot nailedin, for strength.

dimension will absorb rea¡ ¡eflections. h üe real world, you should aim for a ma¡gin of safety srld put damping mate¡ial on every interior wall except üe speaker board (Fig. 3-16). For ñnal adjustment, use your ear. Add more Eat€dal to any system üat sounds loud at mod€st volume levels. Ma.kesu¡e the wslls n€ar Oreütoofe¡ a¡e heÁvily cove¡ed snd, if necessary, ha¡rg a cuftain of mate¡ial b€hind the woofer o¡ frll the spac€ b€hind it. when ñtüng the area nea¡ üe woofer, don't üse ñberSlass. It can get into üe voice coil region and cause problems. For such areas,use polyester batting or Acousta-stuf Il there is a single_note boom, ty shetching a layer of úate¡ial over ü€ back of the woofer, stapling it to the speaker boa¡d. Make sule this dsmping pad is shetched üghdy. In additioü to absorbing reflections, you can use dsmpinS matedal to incroase the effective cubic volune of the box. It does this by chsnging the operation of üe box air ftom adiabaüc (constart heatl to isotherms.l (constant tempe¡atur€). Wiü isothemal operation, the stuffia8 absorbg and gives up heat, maintaining a constant box tsmpemtule ánd ¡educing souad velocity. This shofiens the wavelengths. In theory you can add about 40% to box vólume by stuffing; in pmctice, about 20% is the limit. To stuff a box, cut the damping matedal into súall pieces and loosely ñll üe box. Don't forc€ etrough mateúal into üe box to comprcss it.


ConstructioñRules

39

what kind of damping material should you use?Fibe¡8]assis prob' ably üe most used becauseit is widely available and its cha¡acteristics a¡e well known. The acousticalglad€ comes in packagesof one square yard of material 1 inch [2.54 cm) üick. Acoustical g¡ade is p¡efe¡¡ed becauseit stays in place better than looser kinds. Ask for Radio Shack CaL No. 42-1082.You ca¡ substitute other materials such as polyester batting, rug und€rla]'rnent,or old rags;however,beware ofdense mateals üat would significantly changebox volu]n€. As a rule of üumb, small closed-boxspeakerenclosu¡esshould be filled with damping matelial. For this purpose, as p¡eviously mentioned, polyester fibe¡ or Acousta-stuf is the prcferred choice. Fiberglass,when used too close to üe driver. can get into the voice coil and . Rul€ 4: Install Drive$ from Outside the Box. ln earlier days, the twical high-fidelity speakerwas bolted to the rear of üe speakerboard (Fig. 3-171.In some €nclosures,that is the only practical way to mount a drive¡; but, if üe panel is very thick, such mounting produces a cavity at the hont of the spesler that can color the sound. The shary edge

Fieurc3-16- Doni lorye¡d¿nptnqn¿redal


40

Sp€akeFEoxConstructron

JI

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@ no* i*, au" r"na

fx'íi'f[í) ,:l'ii,'ri:::'ff#t"*' i:i:;'íí.rfr'":¿#I:;,::;iff board.A is undesiabte

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4' FinishingYourSP€akeB tweele$ . Rule 5l Choosean Open Grille Clotb Don t cover supec

llil:i f,lH':lxTl""$J"'i"1:::*"*il*'*;Íil::1i,""ff ;;;rikH,'l;'lJ',1ü'",i1'""'"t'T"1 vou can hear any change iD üe sound.w

l*l"l,"m,y;ilxi:);Jili["J11"3"1""5 YOURSPEAKERS IINISHINC onüe depeods cabinets Thekindof finishyouputonyour.spealer

''*"*¡ih"#i:il'#l'* m;*Iiki{"n:1fu "it'.'i"fl',?H!i'"tl!!iTi"'::"*::t#T jj ltf iH?,,S3;!'ry:*"Tif:HÍ,fl l,,x,Ifl Lf::;il".,l,:;:

ü!:T"',Tlriii1'Tii"S-:T'f, i'.r,:t?i:.t'tr5*i" on tshow üardon iisliSS',".;;;arks fl:l;J':i'":ili,:llli

;m:¡,i:**l**:iHiTi,tlTllI*'.3loJiil;'," *t¡ii

parts a'ü" ""i"t v"u wa¡rt,üen cáatit with a mixtue of th¡ee

ii"*"i"rr-¿' p- ,"'p:lli:,;I*",l'qlffill H:fff:;lJtil;

füit3Jx,H*É,Ti.:!T-:1ilh:s.{*d":

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;*:iTJü:ü!l *::"*:"lit":ll":;'ü'il",lii:T::1":il::ffi to thewood identical is that iJ """. "i"ii'i l" " pr"ceof wasÉmaterial us6d in the cabinet

"":l1i*xt"ll*'¿::.=!lr.y*f:¡;"g "r." "i,¿1"-r'¡ ü:üili;,:'il,:"";i" fixli'ilTil"1'T"","H;lT;'i

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42 Speaker-BoxConstruction HINfS MOREBOX-CONSTRUCTION

#ysit*lT'#*'""-ill;lr.;;i:k1ti.frr

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c¡ossover slstems wiü crossovernetworls lhe

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Closed-Box Speaker Systems you cAN ttAKE, aoTH tN A cLosEDBox Is THEsMpLEsTsptAi(ERENCLoSLTRE principle and in construction (Fig.4-11.One ofits greatvirtues is that it fo€ives minor mistakes in box design. The¡e is just one significant design problem to solve: What cubic volume is appropriate for the speake¡?

closED-BOXBASTCS An unbaffled speaker'srcsonanceis det€mined by üe mass of the moving parts and üe complianceofthe suspension.When üe speakeris installed in a closed box, the pressure of the trapped ah in üe box rest¡icts cone movement to some extsnt. In effect,a speakerin a box is stiffer üarl in free air, and this stiff¡ess mises the frequency of resonance. The smallerüebox, the more it¡aises the resonanceftequenct'(Fig.a-21. A certarnvoli]me ofair by its€lfhas no specialvalue of stiffness;that property dependson üe siz€ of th€ piston coupied to it. The atuin a box ¡esists the mov€ment of a larye piston mo¡e than that of a small one. Prcssu¡eis measuredin force per unit a¡ea,such as pounds per square iDch {lbs/sq in) in üe Bnglish system, dynes per squarecentimeter or Newtons pe¡ squaremeter in the metric system,Thus, üe presBuremusi be gr€ateragairst a lalge speakercone than it is againsta small cone. As the la¡ge cone moves, it chang€süe pressurc much more than a small cone when it moves. In fact, üe compliance ofbox air varies inversely with the square of a driver's cone area.Becausea 10'inch spealer has about four times the a¡eaofa s-inch speaker(or in met cterms,a25-cm ddve¡ has four tirnes üe cone ar€a of a 13'cm speaker),any given box volume would have 16 times as much compliance for the smaller speakff as the la¡ger one. O¡, statedin terms of stiffness,the box would

!{sn


44

Sysiems Closed-BoxSPeaker

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||hen *dQ\ra'¡es


How lo Fit BorVoLumeto Speaker

SPEAKER HOWTO FITBOXVOLUMETO Many audio fans buy a woofer, üen choose a box volume for it Someti;es the problem is reversedwhen the cubic spaceoccupied by srareosDpakprs;usl be re\UiLredto d renain presellrgure The problem in üe second.it u¿lionis lrndinglhP righr \pealer tor d 8i\ en box After studying Fig. 4-2, some speakerbuilders might be iempted to install the speakei described there in a la¡ger box than those shown' believing that they would obtain a deeperbass response.A 1árgerbox mishl p;duc. a wP¿l bossrespon.ebe,"us" ot o\"rdampinS dnd the lhedirin beLau\P s p " a l e r sp o w e r - h a n d l i nagb i l i l \ $ o u l d b Pr P d u L c d can be too A box over the cone rhe larse¡ box would have less cont¡ol pe¡formance. small, ¡üt it also can be too big lor optimum To prcdict a speaker'sbass performance in boxes of various cubic volume, tlüee specificationsmust be consider€d Theseare; . f"-the speaker'slree ail rcsonancehequency . Qrs-the speaker'sresonancemagnification at f" . V^s-the speaker'scompliance statedin te¡ms of the air volume with an equivalent compliance fo¡ that sp€ake¡ with these thrce vaiues ]'ou can predict, for a¡¡' suitable box volume, the following chamcteristics: . fcB-the hequency of üe closed-boxspeakerresonance,also called svstemrcsonance . QcE-the speaker'sresonancemagnification in the closedbox . fr-the ftequency at whichthe bassresponseis down 3 dB. called the cut-off frcqliency To see how diffe¡ent values of Q affect the hequency responseof closed box speakers,look at Fig. 4-3. As you can seefrom the $aph, the higher the Q, the grcater the hump in bassrespons€nea¡ systeú resonÁce. The graph in Fig.4-3 was based on the assumption that each cuffe representsa closed_boxspeakersystem with a syst€mresonance at 50 Hz. If w€ are compadrg curyes for a sirgle speaker,the frequency ofresonancewould be different for each vaiue of Q, rviü low values of Q associatedwiü a low resonance(Fig. 4-4) lt shows that the low Q svstemscan yield a better bass responsethan is suggestedby Fig 4_3 iou ca¡ calculate the vaiue of f, for any frequency ofresonance and Q bv' usine the data in Table 4-1 Moit engineersaim for QcBthat is betw€en 0 7, the value wiih the flattest resp;nse culve, and about 1 or slightly greater'A few aim for an overdamnádsvstemof below 0.7 Somerock music fans like a highe¡ Q


syslems SPeaker .16 Closed-Box

rcsponse'assuninían equalfrcquencvor Fig e 4's. How svnenQ affec9towiequenc/

'f;!;.iÍi:".{',"f i'"itrü't'""*i:ifi ::"':*'{:'"#',f;ffi üffi i*ill:"';;**Hl'n"ttll+:ft*f:*:t","fi [; áleanbass,keepit well below 1 ¡or coml

n*:jr*ilH'"''# l{;:trli*iili*t**+t"'

:*!t,Tif 1!':lliii:if 1i",""*xlgr".;.,:.""i::u#; .o*'o*''"*' s'J":i",.".,i'liii""'ii,o"",.lllu il,:,nÍ ¿T.


to speaker 4/ Howlo FjtBoxVolume f"=30Hz Q=0a VAS= 10 cu. ft. (283 liters) SüDDosevou want to accentuatethe bass slightiy, so you choose a final Q ;f ab;ut 1.2. The lirst step is to find üe desired ratio of closed box QiQcB) to the f¡ee air Q This ca¡ be solved by the following: Qc¡/Q=12l0a=3 This ratio also will b€ the ratio of the closed-boxresonanceto heeair resonance,so üe system will have a resonanceÍieqnency of about 90 Hz. However, anoth;¡ important considerationis the basscut-offf¡equency. Looki¡B at Table 4-1 you find that with a Q of 1 2, üe basscut-

t 4

fr I 7I

a"\ \

o.7'

/ 0.5

7

/

d8

T

// ----......:> f

rctated As the Fisue 4-4. How Qs, thenumbe¡ on the ctnes, and tow¡requencv respon'seof a s¡nqledt¡vet arc cutue response peak ¡n the b;^ rctune ts rcducetl, lhe hequenq of rcsonanrc and Q arc raised Thjs causesa


48

SYstems closed'8oxSpeaker

Factor

't*+*+'Pru:;f

Freqr¡encJ fable 4-1. Cut-otf

Q1 SysteñQ 0.5 0.6 o.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

úonance Freqt¡encY

1.00 0.90 0.81 0.79 o.76 0.74 o.72 o.71 0.70

**r,,ng,gf ¡*$fl,\ili:lryfii,x .IHiüi1"*t'r*,;*H.r" f, lli:;ffiifi

-t"-i-llu¡'l'ry,1ilH'f*+

$**ew*t$t

ft (35 41) (283l)/s = 1 25 cu Ve = 10 cu ft values you get ftom you to check the permits that lleÉ is a {o!úula Fig 4-a: A=(Qrs/C¡'?-1 (w¡e¡e Á is V^"/V"l


Howto FitBoxVolume ro Speaker 49 24 23 22 21 20 ¡9 t3 17

I

I I I

I

15 tl t2 l1 t0 9 I

I

I

I

1 I I

o.o11 0.041 0.045 o.o48 o.050 0.053 0,05ó 0.059 0.063 0.067 0.071 0.o77 0.083 0.091 0.r0 0.l3 014

5 l 2

0.20 0.25 0.31 0.50 l 0

A 1 +

2 ac3/Ors

3

Figurc4-s. Ctosed-boxdesjgnchañ. In the prcceding example, where a e.s of 1.2 is desiled, A=11.2/0.4),_ 1 =8 Il for thF sdmespealer.¡ou de, ide you want ro exptorethe kind of , o¿ssra¡BFyou would Betu ith d eG ot 0.7.the e s,e., ralio would ba 1.7s. Looking at FiB. 4-s, it appearsthar the V,yV" raiiá is about z, so: VB= 10 cu. ft. (283il/2, o¡ 5 cu. ft. (141.5l) Wiü a bo\ volumeof 5 .u. li.. the sysreñ re.ondncF !r ould be abour _^ . 5J t-lz.becáuse thF re¡onanLe ratjo is aboutpquallo thc e rario. or t.75,


50

system'5 ClosedBoxSPeaker

that with a Q of 0 7 the bass 30 Hz. Refe¡ ng back to Table 4-1' you see

ji:m*.T,xT::';"t#;.''ff ü1"'i'"!:;:j{i ;:::r#H:¡ilT

I'ix"g::l*=*lt*.t"xl",li'",ffift $j"*l';fj"i""1.ffi hmuencl signalsbut tlte conewill move ", "*n damas"at hish powerlevels' ;;;;;;t;n:;"

SUMMARY CTOSED-BOX outliDedis basedon a theorelical prcviously Thedesignprocedure

:'":il*1* fl ;iiliü:ü "4{11 I.T:,'.t"iilii'tr* can Asvou or1¿14 :fi ;i,: "a.ado iffit:::J:'li],x'""':*'::":i^1fi

"J,",,$jli:;H#I"fi .T$i,,:l"* :H*T:lk"""""Ji3ilf .":"J:ff . Box loo large = weak bass r"duced powef handling . Box too small. boomy bass reduced bassrange

shipv'/'il_":.'y::li:1':::i'"'i:il"" :il*'":3::i aspeaker tfyouhave-

ur,111l-"-pl*:¡";P-":iI:ffi pr"c'#.";.i"*!.-;"i,:"'"d 10m€rcl^t¡e:o^:: 'i¿á ra+eenouch i" on" ü:]'#;il ";;;;i;;oi" rver t]¡e holes in üe Lafons areaof your \pe€ker'Hold your sP*l"t l ,",,^ .^"" "^.,n¿chñ6mv

o''"rlth:!T:i:T9:"P""Y: *i,¡''""u ii.';;;;.""i.á lT,"';,¿;-. "-r".e"'q'ey¡11'1'l:'l'J: i'""iil;.'il;;.;;;:i "l"rt'"'i'lI 'l: 111:'i:1\:"'+l: #i".ii!."'?" ;:: i::?h'":;iii TT:",'::Jhi ;;;',;;'i"?e,"""v'i1t'"'"1.fl:l:¡:'.0:::"1""::'i

ilTJ::'iiiff

"ct'".pt"'::,::lT:P*f -b"l;H il::: ilT,J'ili,;ñ,.1-isniÁi*ttv.""'*r :"'i'I"1",:t",::l ""Y: *' """ i""o, ;;;;";,J'" ilF :T 3 in chapte¡ th" const'u"tionrules ;1"";"1;;il;;i.'"t-ov".


-

*5

Ported-Box SYstems Speaker

iirÍ;[r!i.',i:$fr {t**t\$.;'1,¿ svsrems ;;sot'por'd ilJffil:,:,Ti$.l'Jl:ül;'.'"Li,'.".fi';;,; ENCTOSURES KINDSOF PORTED

"*t_ i,** * llili: *i ",.{,*ilrii;,i.ir,llll}##

::" +:jf;'ril;*:+*li*1,*

*¡ftr*#*+üu*

rili*ifrt*l{*ltif*

ff*f;:i:n"É"xhTil'3 ::il.li'"'^ifl:t,


52

Ported-8o,(Sp€akerSystems

F¡gne 5-l-

Kinds of potled enclosu.es with act¡on at rctonance.

F4tne 5-2. A double-chanberrcÍlex. Ihe volumeof the larset chañbet is twice th¿tof ¡heothe¡one.A pon tuba h¿veúe tdf:,e dinensio¡ts.


Kindsof PodedEnclosurc, 53 makes üe duct shorter than One requirement is to select a po alea that

:is"i::l lffi:T?ffi.]ff h:r"'¡"'*":l'"":::l;1.*:i:";"ffi

*ilr"*lti:ml*li:,::lv."".H,Tittrti# i" deotb. tbeductca¡bebentt'.. * t '^ifj",l,\s;";,;i,;á:ffi*',

-;:*;""1""9¡¡;_l¡;i3["iffi lH:s[:*r'ls"]iff i.,T; i::#'[: *:*.m:r";.::**"$"ii;ixrilif.j;'Jt'":i" '"':"1íhtx ins charts, i¡cluding the ones shown in

:liir"l"*jlti,{tl"n"r,:*t""",ruffi ;"1;

""""nun",::f*,i"J.iil:T:ll1$",".1il:1"i,'"i*,'1,1 tubli* "T*:{#r"il#}t;:S:.i.::i u*ái"¡" r"," p."i.such ;;;; ü *:lii}g:::::l:"*ililllll'i1'1"á":,.;

-die d,,c, p.ope.

i#t*:1":*i*1""*;ff ffi"T-'$;;'"'i#ti:*"-:¿' :'l::T"'; ::'**i':{:;;*:tltil:li:v,:::"r"";lñ#ii;ffi -essarv.elbows are available '" one hom t* '" r"stall a duct is to makea rFctangula¡ ?.il"t il is easierto mare me inch [1.27cm) material For this hnd ol ducl

i:f;$"-J"H *i"*,,'n'xm**¡l#':xiHJ$""'P-i'#f

o l d É 1e5\th¿n I in'he' Bi5 ts ',:i,-i.-ni fieu¡e 5-3. Ho^ ¡o |''t¿lt ¿ ton|du't A vtonB the(cnteL e th'ouqh dist¿n' É the t",et¡.t he d;' | "¡*"*


54

PoftedBox SPeakersystems

...

:...:

'H;,';;, #';::Y' :,3ki,:::' !;::,' :.!h';!'; :i;:i :#x:;nriffY; boardhDm the requircd vou murt subtr€rI üe th rLknes"of the spealer rind

i";^i;;;,;;;;'

tbis durt Instdrr ';;;iol., t.nsú orú'eoaa-on

of duct with screwsor naiLsanclgLue

fti i::T,ili5i':.Tl'i::?ff """{*xl*i**'H'"J,^l'-ff

*rffi; *i*l+*:r,;*#J,*n*x#:f"* il:i:J",::':,*j :'JJjil:tT:f ;#h'j:t:;1*fi l"*",:"'*lfi iJ""-".i"t

to reflectedsoundin üe upper i"lll u" i"ore susceptible

:lil;: ih1i r;:rl':li:"'"i;:: lliÍLill*i;* ffiji :triJ;sJ

;+i"r; rrr,x"i¡iir{lii?iiüfti"'.;*klü: w*lr s*:#:ri i##*"""ff :r'?i#T*:x{'""lllr}i"i::f

$*'*"tr'.*i",t*i1#.'T$ifil;dx*fJ;ii]


How BoxVotumeAliecrsResoonse J5 d¡-onecone. A passivs ¡adiator should have high comptiance and a¡ effectivecone areatwice that ofthe woofer. An old speaier can be used as^apassiveradiatoriI it hdsa hiBhlycompliantsus¡ension.just lnock ot¡ lne magnetand rcmoveüe polc piece.To tune il. usemodelinqcls\ as a tempora¡y weight; then use silicone-rubbersealerto glue an áqual weight ofwashe$ orrings ofsolde¡to the co[e. place thesámassesnea¡ the center ofthe diaphragm in a balanced pattern.

HOWTUNINC AFFECTS REsPONSE A p¡operlytured po¡ted-boxsystemcan havea very flat bass

responsecurve fight do\a.nlo cut-ol,f{Fig.s-5).tt lhc box ii tun"d roo high. therewill be a hump in rh" frequen"yresponsc:if il is tuned too low the bass!r ill beuFak tn thp ddls of the classicbassreflex,lie inslructionsfor builders of ported boxesalways sratedthsr thp box snouldbe rune(llo lhe frce-airrpsondncc ofthe speal<er, Now we know lhat lhe optimum tuninS f¡equcncyrdries tor differpnl.peakersfoom ftequenLiesmuch lower rhdn the free-¿ü resonanceio hequcnri"s abovethe ftee"air ¡esonance.It atl depends on the e ofthe speáler For example.a spFakerwith d high Q will pedl at resonaoce. wh j¡e a spFalerwili d ¡o\ Q will ha\e a reducedresponse at ¡ts freoucncvof resoo€nce. To pra\ Fntboom in ¿ high e sppdkei.a porteabox strouláU. tuned lo a hequencywell bclow r"son¿nce.Houever.a lpeale¡ with ¿ low Qcan be overdampedat resonanceunless the box is tuned to a he_ quency highe¡ ihan no¡mal.

HOW 8OXVOTUMEAFFTCTS RESPONSE Thc effeclot box volume on d poned sysremjs much rhe sameas ^ ror a Llosecl-t¡ox spealer:small boxpsraisethe e a¡d increascboomi_

+t FiEurc 5-5. freguency-rcsponse cuNes of a bassreflex speaket||hen the enctosute is tuned to a frcquency that is (A)too h¡Eh,(B) too low and (C) opt¡nrun.


Ported-8oxSpeakerSysrerns

ness morc tha¡ large boxes. This indicates a large box for speakers with a high Q. If a small box must be used, üen the speakershould have a low Q wiü high damping. Such te¡ms as high and low Q or large and smsll boxes don't mean much without having numbers attached to them. For po¡ted box speakels, you can conside¡ speak€rswith a Q of below 0.38 to be low and those wiü Qs signiñca¡tly above that point to be high. Speáke$ designed specifically for use in ported box€s have signiffcantly lowsr Qs üan those desigred for closed-boxuse. Wheü€I a box iB large or small for a given speake¡ d€pends on that spealer's compliance, or Va. A box that seems la¡ge in your liviDg room might be small acoustically for a large woofer. A high-complia¡ce 1s-inch (38-cm) drive! for example, can have a V^s of from 20 to g0 cubic feet (500 to 850 liters), so a 6-cubic-foot (170-U floor model speákercabinet, which would seem ve¡y large to most families, would be acoustically small for such a woofe¡. Tho same floor cabinet would be acoustically v€ry large for an 8-inch {20-cml driver wiü a V^s of 1 cubic foot (28 l). The ra¡ks of high-fidelity spesle¡s inclüde drive¡s wiü specificaüons üat range from one ext¡eme to the other a¡d beyond. These speciñcations mlrst be conside¡ed if you intend to design a ported box for you¡ spsaker.

THITTE PORTED-BOX DES¡CN A.N. Thiele,an Austalian engineer,has brcughta high degee of order to the fo¡merly haphazardtask of designinga pofted box for your Bpeaker. To usethe Thielemeüod, threekinds of ddve¡ specifrcations muBtbe consideEd: . f"-üe drive¡'s ftequency of ftee-air rcsonance . Qrs-the ddver'e ¡esonancemagnification at f" . V^s-üe driver'scompliance,statedin te¡msof the air volumethat hassn equivalentcompliancefor that üiver Using these tluee values you can determine üe ür€e critical aspectsof a portedbox. Thesea¡e: 1. Box volume{V¡) 2. Box resonance ftequoncy(f6) 3, Systemcut-offfrequency(I3) Here we will follow a simplifreddesignprocess,using chaftsto obtainfactorsthat,wh€n multiplied by d¡ive¡ specificatiotrs, give optimum values(1and 2). Ifyou wantto pursuethe designp¡ocessfir¡üer, you can use the pocket-calculator designmethodin Appendix A. O¡, for a more sophisticatedapproach,thereis the computerp¡ogram,i¡) Appendix C. If you have accessto an IBM-compatiblecomputer,you


Ihiele Ported-BoxDeslgn

57

can quicklydesign a¡ optimum encloslue for you¡ speaker.you also can explo¡e how cha¡ges in box volume or tuning affect pe¡formance. The computerwill q!icklv mal€ s graph of rh; frequency.r".ponse curvetor eacrlt¡oxvotumeor tuning condition üsr you Fnter Here is how you can do üe iob with nothing mo¡e tha¡ pencil €nd paper Supposeyou want to design an enclosurc for a 6%-in;h woofe¡ with these specifications: f"=50H2 Q=oa VAs= 1.5 cubic feet, or 42.5 liters The first step is to find the optimum volume for the FDclosure. Refeffing to Fig. 5,-6and following a verlicsl linc upward from 0.4 to ¡¡e cune. then Ieñ to the maJgin,il appearstbat the \o¡ume fscto¡is

1.0

I

I

2.O

0.1

0.2

03

0.4

0.5

Fisurc5.6. vot¿nc t¿c,o,, ;;;;;:":;: F,ñd,hepo,nt on ¡he( u,w that.oíptpandsto theQoftou,pe¿l./ thenn'otpto¡hetphñal7inroBet¡hctotune factar.Muk¡ply yout speake/svñ by the votumefac¡ot to get th; opt¡nun


5a

Ported-8oxSpeakerSylems

about 1.1. Multiplyitrg 1.1 x 1.8 cubic feet (or 42.4 t), the woofer,s Vas, gives a suggestedvolume of 1.65 cubic feet or 46.2 liters. To.getthe coneLttuniog frequpncylfBl.go ro Fig.5-7. ASainlocate . rne pornt on r¡e curve d¡¡ecuyabovethe e value{0.4land moveto the len.magrn to get the tuning tactor,which app€arsto be about 0.q6. Multiplying 0.s6 / 50,H2(f")givesan F¡ of +8 Hz. This completesrhe bas¡c.desiBn. except for rhe dimcnsionsot the box and pórt, tur it would be helpf\rl to know the bass cut-off ñequency, fr. To determine the cut-off.locatethe VslVr. ratio.I I in tlriscase,alónethe baseof Fis. 5-8. The¡ move up to the ¿urve a¡d lefl ro üe margin ro get t¡" basi ¡ange factor. In this case,it appeárs to be about 0.95. Thé cut-off frequency will be 0.95 x s0 Hz (f") o¡ 47.s Hz f.). The chans in Figs.5-6 ard S-Zare desigredto give Lheoptjmum . values for your woofer Ifyou want tobuild an enclo;re t}lat is smaller than optimum, you car estimate how much it would limit the bass range by refe¡¡ing to Fig. 5-8. Fo¡ d€tails on how changesin volurne or tunitrg tequency affect the performanceof you¡ desig;, use the calculator or computer programs in Chapter Z or the ones described in AppendicesA and C. ODe adva¡tageof the compurefprogramLDp is t¡al rt shows a graph t¡at indicátes the sbspe o[ úre responsecurve for each change you make.

OBTAININCTHEBOXAND PORTDIMENSIONS You can lind the internal dimensions of the box l¡om Tables 3-1 and 3-2, in Chapter 3. If you obtained rhe suggestedvolume in cubic leet, you must multiply it by 1728 to convert to cubic inches. In the previous case,1.65 cubic feet, the volume in cubic inches, tu¡ns out tA be about 2850 cubic inches. If you ale using the metric system of mea_ surement, use Table 3-2. To convert cubic feet to liters. multiDlv the volume in cubic feetby 28.3. As a rule ot üumb, wben designinga reflexenclosure,use lhe next highecvolume in the charteven úough l0 percentovervolumealready is provided. Becaus€ol unpredictabl€ internal losses,po¡ted svstem; often ¡eed some exBa volume to reachlbe performanceievel suggested by üeory Ifyou go to Table 3-1 and uE€the next higher cubic viiume, 3000 cubic inches, the suggesteddimensions of the enclosu¡ea¡e gX x 14%x 24. Theseintemal dimensions allowavolume ofabout 3300 cubic inches, oralmost 16 percent aboveüe recommended2850 cubic inches. Ihe ñ¡al step in box dcs¡gni! to find the comectporl s¡ea and . length lo lune th€ box to 48 Hz. To do thal. consult the port design chafs in Tables 5-1 to 5-4 for English measürementsor 5-S to S-8 f;¡ met¡ic. These cha¡ts are new and somewhat differcnt from those published in ea¡lier editioN of üis book. They show shorter lenstÉs of pipe becaurethey are basedon rpal.world boies with damping mal€rial on üe walls. Ea¡lier chafs werc basedon bare boxes.John Hintington


Obtainin8the Box and PortDimensions 59

,2.0 1.9 1.8

1.5

\ \ \ \

Ll

\

1.2 Turiñg !,1 1.0 0.9

0,7 0.6

0.1

_-_------.

0.2

0.3

0.4

0.5

0.ó

aR

Fieu¡e5-7. funinAr¿.totch¿rt Fir't|Pt thPtumnBurto! hd rn¿lchetthe Q ol t$on¿n(cb\ thFtuntngÍ¿'tol );ur sPedle'thPnnuttipt\¡hékequent\ot Íee'¿¡ fequency l¡t) to Eetthecoffect tuned'box use¿lüe po¡t dimensions fo¡mulas from LDP to develop the sp¡eadsheet charts in Figs. 5_1to 5-8. Ths goal is t; find a port that is la4e €nough ir area to avoid air noise aná yet isn't too long for the box. As a rule of thumb, the port area "houldbeát teast rüat of-üe cone (Tables5-9 and 5'10). For a 6%-inch,


60

S\5tems Pofted_Bo'SPe¿Ler

t.5

3,0

2.5 2.0 \

1I

L5

1.0

0.5

0.5

10

t.5

2.O

2.5

#:-!!;!if"*.*;[r'n i{:rii:;|iiiilii:-i:iiif 21 or 17-cm,woo{e¡ the cone arca is about

squareinches or 14^5square

*i:t*:t;"*::ulnn.::+':t;*:[H.:i::'.1;xl-"'ii,!:"'#ili"i!!:i-!:_1ilin::*fi: ::,."xi $;:',:".';:l

* lff*.li"i#r:f;r,iiltrfu1ti:*:trur¡i ñr"il;r".ru:+sr[i,::!ütll:fi+H¡i,

[iflii:;f,$#'""r:'r*td'i#l:sri\i= I

ll}*:llL':;:tTiJ;"'"",1'5"'.i'#i'lxlH:"i:t"ffil"i# '^",".ik;,1-::lllT, ii:'};i:;tr^*:if;ill:li"1i1']i3T"lill;


Obrainingthe Box and port D¡mensions 6t

Table5-1. Duct Lengthfor porlsof 1.S-inchDiameter. Tubetengths(lnches) D = 1.50in. TunedFrequencies Volume (cu. ft.) 20 0.25 0.50 4.75 r.00 1.25 r.50 1.75 2-OA 2.50 2 . 75 3.00 1.50 4.00

25

8.41 5.91 7.79 4.44 6.24 3.45 4.31 3.14 2.72 2.37 2.07 1.82 1.40

2_22 1.47 1.20 0.9A A.79

30 8 82 5.38 3.66 2.63 r.94 I ,15 1.08

40 6.08 3.55 2.29 1.51 ',t.03

50

60

70

7.67 5.9) 3.66 2.29 4.31 3 . 0 9 2 . 2 2 1 . 0 8 2 . 37 1 . 5 6 0 . 9 8 1 . 1 0 0.79 0.82

80

90

1AA

0.79

100

Sp¡€¿dsh@t by lohn Hunhngbn

ideal volume will be 46.2 lirers and rhe tunin8 f¡equancywi]] be 48 Hz. Tables-6 showstubc Iengthsfor porrswith a d ismelerot 5 cent¡meters. rné nFaiestcubic !otuma is 4S litars.ll is apparentthat d s.centimeter lube rs smaller than neressan:the lcngth i; too shod to ljsr on the Lharl.L;oinglo the ne\t lafgprdiame¡Dr. Tabie 5-2, the c6¡¡s6¡¡.rrth appearsto be belween4.0 ¡ enlimete¡s.lor 50 Hz, dnd 6.7 cenrimer;s. for 45 Hz. The best procedure in this caseis to trv seve¡altubes. from 7 r pn¡imelersdown lo 4. a¡d u5eIhF one th,r sound. best. l f 1 o r h a r " l p s t e q u i p m p n t v. o u . h o u l d r c s l y o u r w o o t e f s .a s . der ribed in Chaptej9. ihcn lc\t üe final pcrformani"of your porred box. The¡e are severalways to identify the iuned hequency of the box. One is to locateüe ftequency at which the woofer'simpedince droos to it. minimum vdlué.This poinl o,cur" bet\apenrhc iwin imoed;ce humps lhat ¿ip chdraLtpristiL ol reflpr svsrpm".lf you have no lest equipment,you canprobably locatethebox hequencywith a frequency_ test rcco¡d. S€t the enclosureon irs back and dust the woofer cone wiih poraderedr halk. Ar the poÍ tpqupncy lhp woofFrLonewiii aoooarto st"nd srill. tor Lontirmarionot tiF tcsl. holo a lrqhtedcandleoi,erthe pofl. At lhe Dort+pqucn.r. thp n"né tvill qui\ er iiol.ntlv.


62

systems Port€dBoxSPeaker

{or Portsof 2'inch Diameter' fable 5-2. Ouct Length fub€ Lengths(lnches) D = 2.00¡n. . TunedFrequencrcs (cu. fiJ 0.25 0.50 o.75 I 00 1.25 1.50 1.75 2.00 2.50 2.75 3.00 3.50 4.00

25

20

30

35

40 c1t

a.57 6.81 9.7g 5.55 8.12 460

45 615

70

50

60

460

7.17 4.74 259 I17

rü)

80

90

3-16

2.08 I 30

ooo i.áa ¡q'a ¿co Ioo z oa t ¡o 7 1 7 ¡.i., .l.lo I26 s 34 4.12 3)_4 l5Y

t.lq 2.l 2.49 1 44 1.85 0 95 1.37

¿.ao I ¡+

i.¡i 4.88 4.15 3.9O 3.16

2.40 1 06 2.06 0 8¿ 1.77 1.30

Sprea¿rhetbYlohn HÚnÚngon

#':{l,:?xil:T;*tlÉ""}iffir;"J{iH#lt{:li ^Lv = -^fB (2lvlh)

whele

** "* :::'"'I:::i::'"' =i:u"s':ltg:i ^I

inches or cm = chanqe i¡ vent length in

"qi:i':dt* *S]t$H¡k$ii",-"'**¡,'i-ffi


FineTu^lngby Ear

6.1

Table5-3. Duct Lencthfor Portsof 3-inchD¡ameter.

(lnches) TubeLengths D = 3.ooin. Tuned Frequencies Volume (cu.ft.) 20 1.00 1.25 1.50 2.00 2.25 2.50 3.00 3.50 4.00 4.50 5.00 6.00 8.00

30

40 9 . r3

4.61 7.32 6.29 4.74 241

7.32 6.17 5.26 4.51 3.88 2.90 2.16 1.59

8.89 7.41 6.91 5.49 4.43 3.60 1.94 r . 1 3 1.95

7.11 4.58 3.74 3_O7 2.51 2.05 1.33 0.79

8.61 6.29 4.74 2.81 2.16 1.61 1.22 0.87

60

70

80

90

100

6.17 4.43 2 . 1 6 0 . 7 9 4.34 2.94 I t l 1.12 1.95 ] 59 1.08

Spreadsh€tby loh¡ Hunrngtoñ

^L, = -[3 Hz) (2 x 4 inches/4s Hz) = (3) (0.1722inchesl = -0.53 inches This indicates you woüld remove about 'l inch ftom the length of the pipe, making it 3'l inches long. Notice that t¡is fo¡mula assignsa negative value to the charge. If the frequencyis too low. the desired change rvill be positive. This combination produces a negativesolution to the problem, requiring a subtracüon hom duct l€ngü. If üe frequency werc too high, the deshed changewould be negative,and üe two negatives would produce a positive solution, or addition to duct length. This formula tends to under€stimate the amount of DiDe that should bc removed.so you mighl hsve to maketwo cuts. FINETUNINC BY EAR The design processjust describedis basedon Thiele data; however, if you happen to find a chart of Thiele's o ginal alignments, you will


64

Sy'slems SP€at€r Ported'8o¡ of 4-inch Diafieter' Table 5-¿. Duct Length fot Ports

(lnches) fube Lengths D = 4.00in. TunedFrequencies

(€u.ft.)

20

40

30

25

1.00 t-50 2-00 2.50 3.00 3.50 9.21 4.00 7.74 4.50 6.57 5.00 4.A1 9.76 6.00 7.O0 7.79 3 . 5 5 8.00 6.32 2.60 10.00 4.26 1-28 12.00 2.88

a-23 6.48 5.17 4.15 3.34 2.12 1.24

60

80

70

9 2 1 5 . 1 7 2.74 9.76 6.a7 4.41 2 . 1 2 9 4 8 6.32 4.15 2.60 6.74 4.26 2.51 1.28 4.99 2.AA 1.44 3 70 1.90 2.74 1 . 1 6 1.99 1.39

90

100

1-lb

spr€a¿shetbYlóhn Hunhngbn

""'*:"ilil j:";)H:.:Iffi .lilTj:,'j::i::'# 3,x,.fi :i:l ;:f l"'.H,"".#;n:ii.*ixf :

*ji*:r;:***:lt:"tr;:'"":ffi

i''e¡].+i *l*:i*-l¡il1fl fltf*lidffi oo wear'or' *'" u""i"tighrrv

**l****s*u,e* ;"::d:,\#:Hü#*fnl"'ll"t

$di,:fHo,?:';"Jl'"ir""0,.fK""," üli:,:",:"fi


Ibe Boom-8ox Alignment 65 Table5-5. (Metric) Duct tength for Portsof 3.2-cent¡melerDiameter.

lube Lengths(Centimeters) D = 3.20cm luned Frequenc¡es Volum€ (l.iters) 3.2 5.OO 7.50 r0.00 15.00 20.00 25.00 30.00 35.00 40.00 4s.00 50.00 60.00 75.00 100.00 125.00 150.00

20

20.36 16.43 13.63 I r.52 9.89 8.58 6.62 4.65 2.69

25

30

21.93 15.65 I1.88 9.36 7.57 6.22 5.18 4.34 3.08

40

22.44 16.43 22.97 16.03 r r.52 14.25 9.62 6.62 9.89 6 . 4 1 4 . 1 6 7 . 2 7 4.49 2.69 5.52 3 . 2 1 4.24 2.29 3.34 2.62 2.03

45 20.07 15.65 12_31 9.36 8.43 6_22 4.56 3_08 2.62

60

70

80

90

100

't7.25 11.A2 8.30 5.89 4 . t 6 9.89 6.41 4.16 2.62 5.51 3.21 3.34

sp€adrheetby lohn Hrnringón.

THE BOOM-8OXALICNMENT w.t.J. Hoge describeda¡ alternative design fo¡ ported boxes that is cellsd,e fourth-order boom box \BB) alignmenf. Booñ óox vr'asa term particularlymistünedmodels.The term onceusedfo¡ reflexspeakers, is only pa¡tially appropriate he¡ebecaussof a typical peal near cut-off, but ü€ pesl usually is well controlled. It often Bive6th€ effect of good low-bassresponsewiüout beingsloppy. RobertBullock extendedHoge'sdesigninto a superfourth-order boombox(SBBJfo¡ speakers oflower Q. D¡ivercfor SBB.designsmust havoQs of 0.37or lower The amountofboom dependsmainly on the Q of üe driver.Driverswiü a Q of 0.45or lower areparticularlygood fo¡ this aligllment. Even those wiü Qs of 0.5 can be fitted into a boom box with a peal oflessthan 2 dB. In oüeway the boombox alignmentis like the classicbassreflexof many ys¿úsa8o.The box is tu¡ed to the f¡ee-airrcsonanceof the


E

6 E

t

E - 9

v- g

-E .n ,l

Fo 9

qq19.199.:

É

É E l;s

66

3' ¡ € 3


3Bq

sil6E .El : l

Y

sRKR{84

u:s:= :Ae

E t Y E ¿ E -a ü9

.EtE !9:

eqqi-\!fl1

¿

f99::*-

E a¡eiEH R99::5s95EAq d ; v i - @ < d

t

= REB cidiÓú qEb

: -9

Il ¡ E

E.q: ?REEE?B?E?;B 9a

;--

l É


ó8

PortedBox SPeakerSyfems

D¡ameier' Table5-8. (Metric)Duct Lengthfor Por{sof 1o-centimeter (Centimeters) TubeLengths = cm D 10.0O TunedFrequencies

3.2 5.00 7.50 10.00 15.00 20.00 25.00 10.00 35.00 40.00 45.00 50.00 60.00 75.00 100.00 125.00 r50.00

21.95 1A.76 21.10 13.96 24.08 1 5 . 0 4 9 . 1 7 1 5 . 5 6 8.78 4.38 1 9 . 4 5 1 0 . 4 5 5.02 14.54 7.O4 2 . 5 2

90

50

40

(Liters) 20

22.46 18.40 15.24 12.72 8.93 5.15

21.95 15.56 20.67 I1.30 16.29 8.26 r1.00 5.98 10.45 4.20 a.40 2.78 5.34 2.27

f00

20.67 18.40 13.00 2 1 . 3 0 13.96 8.93 5.34 13.47 7.97 4.20 a.7A 4_38 5.65 1.98 3.41

spre¡dsheeibY lohnHun¡nBon

cübic driv€r. The only d€cision yoü have to make is to sel€cta suitable volume. 'lhevalue fo á"ri"n a boom box. uoethe chan in fi8 5-l0 Locate Lay charl oI the ieft at üe scale vertical Ior lhe O oÑour wooferon the ruler rhe where A of line value rhe rhe charr and read ;;i";;"t"". will tind rhe correerralu" bv lollowing a verrical^line ;.;;t;;i., ol t¡e hom the ooinl wherF the ruler crosseslhe line to lhe bottom by enclosure lor volum€ vow ;;;i.'Y"'i """ calculatplre suggeslcd this formular

vs= (¡l (o (v^s) box lor !hc" Here is ar example.Suppose)ou w¿¡t to deli8n a boom a v^' or o4and a 6 -inch (17-cmjwo;ler describedea¡lier'Ithad Qo! Therefore: l sT v€tue ofabout i.i lo¡i" f""t (+l s t). rhe chart givesanA


the Boom_Bo\AliEnmenl

69

Mir¡mum Port Diameters'*tttt*I: fabl€ 5-9. SuS8ested Minimum Minimum Approxiñate PipeDiameter Speal€r Size (Sq.ln.) {!q. rn., (ln.) I 5 6.5 8 10 12 t5

21 30 50 80 133

1.15 1.8 3 4.3 7 11.5 19

1.5 2 2.5 3 5

Units' MinimumPortDiañeters'Metric Table5-10. SuSSested Minimum Miniñuñ Approximate Diameter Port Speaker PortArea cote Area (cm) Size (Sq.cm) (Sq.Cm) (cm) 10 13 17 21 24 30 38

50 85 145 240 350 500 900

12 21 35 50 70 130

3 3.9 5 6.7 9.5 12.9

VB= (187)(04) (1 5cu ft) or, with metr:icunits v"" = (1.87)(0 4) (425 t ) = i 12 cubicfeetor 31 7 liters

":bT".:-'];llti,fie oc.if \ouwereusinsm"¡: cut-ott B in Fis.s-s.rhe torüsero¡ il o"t th" ftequ"n"y of -.rú fo"¡that is aifollo*s'

f' = B(f")

I¡=Bx(


70

Ported-SotSPealetSYsterns

rA rf---] ^**'" ',n:,1:i:i:,'i^:t',;ii:!*",:":r?::;;::,: 1'::: ' Hov t-e. F¡qüe po¡ a'ea tÓtuu'' P!1'¡ "

^

sinplepon reduce

the B line crosseb you will find thal the ruler tn soing lo üe charl aDove at abo; r't5 For the wooler f3= (1.15)(50 Hz)

f.=115x50Hz = 57.5 Hz

$::: "*"':T:"*i";':'l"i :l;:''l):il [ffi#l*h:'"'Jl"J:':'1T REFLEX THEDOUBLE-CHAMBER.

**:l''t'*+n:;*",*dlí*:"""

mN;**$*l$*lt*""


Ihe Doubl€{hamber Reflex 71

^ 0.¿

1I o,5

"A ' or "8" > val(F of

oesisnÍo,boorn+*'¿'p'n^!!*,ll¡jrff;#;Jifi#E11::,::';':/¡':;:'; fis,'ftsro. ;nic¿ttines.potitionedso.itct6s¿stheten.mary-'';s=li""r.n"áu,=o.Abxv&¿ndÍ,--B\h. n to.alinp tlrc red]i.¿lline wfse he rutet ct6ses


SYrer¡s BoxSPeaker Porred

rcflexsvsten Fisure5-1t. A d.)uble'chamhet used ||erc sa elbo\|s box, ¡his fat to;E

stniiht PvC pipe poñ' weteroa

'fiirk""lg;3til:ilEi'lii}:i$i #l*iffi T#il#"**km:'x :l#ii::HiL'^","""H:*::,"ff rorcraritv' curve iiTli.,'J;¿."*.er H*:i:ti::lt"l$"1'f J'i,T'i

:#'*Til:i"T{"1""l,"¡i iTi{i* ¿il't;:iffi :"".:i""xT$f

i;:1"""",:"1HiH"","'.:;ü"*::"::1*lt'í'lk

j$ii*":il*:i*J*;$i*i:'É tl füt:{fl iI tnH"lfu n,k'i"'f'""*:rilr it*';lt;*i'*.t:, duce mmble


Howto Desiena Double-Chamber Reflex 73

----------------> F a¡leña Fígur 5-12. fheorct¡calconeexcursionol a wooferin thrcekindsof enclosutes:a closedfux, a sinple reflex,anda double-chanbetreflex.

HOW TO DESICNA DOUEI"E{HAMBERREFIEX You can apply Thiele data to ü€ double-chamber¡eflex a6wsll as to siDtle-chamberboxes.He¡s is how to do it in a four"stepprocedr¡re: . Chooseyour woofer. Use its specificationsto fitrd the optimum volume. Maks thb volume the total volumg of üe two chambe¡s.Or, you ca¡ reve$s üis procedure by setting th6 total volume, as desi¡ed,and choosüs a woofer to match rL.


74

Ported-BoxSpeakerSystems

..,j:rit:i:"{j,"".' h[?*LiH:i Rid';'"l'ff ffiil'i;j'li*

r:"-n*i*-$lilrr':{ri:3.'::'$i*{ih*

*t**g*i:-t*f{:ii,-'J'r$::',

n#xi*l*'n::,""l;l[i,""til]lljFútl"I.

*l*l*";rlÉ*u;ü1ffi::::";;l:ru"*"'ll"'T::#n o'L?i;

advantagesover singledoubt"-chamberreflex has .some

,l*'r,'ffi;rl**¡**l:;-t ü, }tü', Ji'fi *+*iÍ""#:'tr, li,*ü*'FI':'"t,""i$.il+ii:*'#:f ji"i,j,#T",:"*l,H#i*"'"*¡i' 'J":l'i'':"'$ ;l{}.;+.

liüHr$iffi',ilft*$iliffi ffRftq'i*"$j:.3r.TLlr":'"jT:: SYSfEM5 BAND-PASS

lir;H:"i;::llr*ff :i,:5:t:'ff i:{i"1,[{ii¡ ,n",u;:1ff

[;*'i.¿l;+t*+Er.*i*r*}


Band,hssSystems Z5 b€¡d-pssssystemcan be employedteiú s sjng¡ewooferor wirh .A mur|lprewooters,tl two ¿re used.one ca¡ bc wired lo eachchannelof a stcreo_ system.as shown in Fjg. 5- 13.Tbe ba¡d-passbo\ LUts otf l¡c hrBnerhBquencyoutput oIthe woote¡s.allowingyou to usethF syslpm as a subwoofer.It might be desirableto use a crossoverfilter to fufher block higher frequency sound ftom the band-pass svstem. Trial a¡d efror can $ride you with that. By_choosing different alignments, you catr alter üe Íiequency ran8e,flatnessof r€sponse.and efficiency of tbe system.The range of á band-pa5sspeal.er is deñned as the band of frequencies betwc;n the lwo points whe¡e üe responseis down 3 dB bom the level ar mid_ band. This r€nge is dets¡mi[ed by the location of the cent¡a] ftequency, set b-yfc ofthe closed box chamber,and the damping factor.The damp_ irg factor, S, cán range ftom 0.s (or even lower), to ó.2 in a twical sys_ tem. The lower values fo¡ S allow a wide¡ ¡ange, but wiü increasing rippleinüe responsecurve. At án S of0.Z, therásponseis üeoreticalti flat. Above 0.7 the ¡ange nanows mpidly. The voiume of the cornpart'm€nt in Iront of the woofer is detemined by the value of S and the chaEcteristics oI the driver. Jn addition to choosinga woofer and S, lhe remain¡nBdps¡sn . cho¡Lc, is thc system Q lQ¡) for tie d¡iver in the closed-boxiompalment.You cancalculatetie properclosedvolume justasyou wouli for 8nY Oürerc¡oseo-boxsvstem,

Figure 5-t3. Foutth-odet band-pass enctosures.when usedas a subwoofet, the Ieft and dght channel bass can be n¡xed in one enctoswe by using two woofe' ol with a double vo¡ce co¡l woofet


76

Ported-BoxSpeak€.syst€m, For the vohme of üe ported chambe¡ {VT}, uss this forEulal

vr = (z XsXersD,0x) a¡d for f¡ f" = {ac"} (fs/Qls) This system is easily designedwiü a computer program süch as LDP.With it you can quickly exploro how cbangesin S or systemQ will alter üe frequencyrangeof your syst€m.Afte¡ you ente¡ t¡e pa¡aBetels for your woofe! arld the desiied values for Q and S, LDP lists üe low and high frequencycut-offpoiats, the po¡t dimeDsions,and the rclative efñciency of üo system,comparedto ths output level of the woofer in a conventional box.


*10

Projects A MAIoR coA! IN sErEcrl,\c coNfoNENTs AND tN DlslGN cHolcEs FoR THES!

proiects was valüe. Any spealer system is a compromise between cost and difficulty of construction ve$us level ofpe¡formance. Alúost any spealer can be improved by specifying more exotic components' but vou can reach a point oI diminishirg ¡etuns. As in pasle¿itionsofthis bool. simple crosso!ernelworksare t¡e of the design"boires for crossor"rPans might sPcm ¡ule here.-Some appear to violate the ¡ules, it is becausethe final deciIf they unusual. was made by a listening test. case sion in each ofevery constructronstep here' descriptions find detaiied You won't expected that you follow the but it is discussed, arc Soecial o¡oblems 3 The photographsshould in Chapter outlined ptocedures cónst oction you questions have abo¡¡tp¡ocedüe' any to answer show enough detail asPectof any pmi is an impofant volume cabinet that Remember space,as long fit any special to dimensions change can usually ect. You be mainshould volume piPe, but cubic or a you a cube don't ms](e as fiBure. at the sDecified tained Varioussuistitr¡tjoni ca¡ be made,bul don t Pxpe(tro substitute another wooler fo¡ one of the po ed-box proiects and 8et optimüm perfoünance. Such substitution can ruin arry ported-boxproject unless the new woofer has similar specifications for Q and compliance The speake$ that specify L_pads on the tw€eters require carefirl adiuslmentof üose conÚols Start l¡e ddiuslmentproccssby luming th€ t\¡eetersll lhe wav do$rn,l¡an advdncathe conbol unlil the souDd ofthe tweeter is blenáed with üat ofthe woofer' Ifthe tweeter appea¡s to srandoul. you haveSoneloo far' Of it eveDonesiogingor speal_ ine sounds¿s il lhev ha! " a falserPethwh i"t le. you hdve8on" way too fa.riVoicerecording!are usuallyb€sl Iof t$Feteradiuslmentberauseit is easv to notice when t¡e human voice sounds natural o¡ unnatural' N;te that üe description of each project includes tbe values for maximum and minimum imP€dance These can be useful, pa¡ticularly


the value for low impedancs. Some speale¡ systemshave impedance lows clown to 3 o¡ 4 ohms. a¡d sometimeseven lower. Some amplifiers don't work well with such low impedance.Speakerswith highei mini_ mum impedance,and with a low ratio of high to low' are easie¡to drive a¡drork well with low-priced receiversoiamplifiers. N€t box cubic volume is also listed. tfyou don,t like the shape of thebox used in the p¡oject, you can use on; of the tables tn Chapie¡ :, oI t¡e computer program for box dimensions in Chapter 7 or in LDp to-design an enclosure of anoth€r shape with the specified cubic volum€.

PROIECT 1:A MtNt-SPEAKER . Low impeda¡ce: 8.8 ohms @ g0o Hz . High impedance: 2Z.Sohms @ 5.S kHz . Box volume: 0.07 cu. ft. (2 liters) The small tull-range Radio Shack speaker(Table10_1)used i¡lthis Droi_ ecl has b€en around for many years. A spealer in such a tiny box iar:t Deexpectedlo reproduce Iow bassor fill á¡ auditorium with sound. Fo¡

- *y rimitedspace €nvironment, it canoffervery

li::11::i?á:.

A cssuálgldncear rhe phorograpba lFigs.g_l and r0_1)mighrload vo\rlo concludetiat the spealeris instslledin a cube_shaped box. Not so. An inspection of Fig. 1O-2wiII show that üe intemai dim€nsions are in the ¡atio of 4:5:6, so no dimensiotr is a multiple of either of üe others. Becausethe box is so small, %-inch (1.3_c;) pl],.wood is ade_ quate.lbrthe sicrewdlls. For the speal€rboard and bacl, t-inch (1.9. cm, plywood or paf icle bodrdis a bettercboice.One unusualleatureis the way üe spealer board is set in the box. The cleats that hold the spaale"rboard a-reset ba_ckonly x inch lo.g5 cm), le ing the board pro_ lrude hom tle hont of the bor. you can usa any k_inch f0.95-;m) rounded molding,as shown in tbe phorographs. tó cover r}e spea_kei board-sraw edge.Nole thal lhe back is setin flush.

Table10-1. Components Usedin proiectt. Componenl R C L S

Descriplion 50 Ohm,1OWatt 814 N.P.tl€c. 0.1mH,20 gá. 4"full range,RadioShackt40-¡ 197


Project2: A Low-CostTwo WavSpeaker

to limsht¿Á fírru¡el0-I PrcFc¡| bo\ in 'on9fuctrcnNotPtheuacol ñoldin+ eáEe, ot plyvood -de. ¿ndprcie.tiñ] 9e¿retbo¿¡d The sDeaker'sperformanceis enhancedby a filter that contols.an jevel in the re8ion n"ar 5 kHz The raw spea-ker excessiveenergy vou "l'"iá¿ "l¡' ñárd until the filtei was added To male the filrer' corc' a 1-inch can either buY a 0.1-mH coil or wind your own lf you use magnel wrre ;-incb wide, try using 20 leet t6 l matets)of 2O-gauge jour homemaoe Even ifvou have no lesl equipment lou ca¡ parallel thP max'ng coil witl ar 8- oi lo-tlF Lapácilorand a 50 ohm resislor' final choice ofcapacitor bY ea¡. tests' The value ofihe resisior, 50 ohms' was dictated by listening in filte$ fl" "ái"nt"t"¿ u"lu" *as low€r, but the discussion of notch ChaDter6 explainswhy such a valuecan be righl can 8do" ih""" tpiul"tt *"." used without a Srille Il desircd lou a n vk i n do t l h i n$ i l l e L l o t h u s j n g a t h i n S r i l l e b o a r d s u c h a s a p ¡ e c e o l %-inch(0.3-cm)ha¡dboard

fwO-wAY SPEAKER PRolEcT2: A LOW-COST . Low imDedance:7.3 ohms @ 200 Hz . Hish im;edance: 35 ohms @ 2 kHz . Boi volume: 0.¡s cu. ft (10 litersl


(1.9)

1 " I i .9)

t* - ri ..1 i--' i . i l l 6',(15.2)

7'117.a)

r

i i i r i i il i!-.1 1 t 4 - U O . 2 |t !

]L

(0.e5)

s

*40-1197

Fígure tO-Z. Const uction plansfor prcject 1.

l':*fi i#iil'*+*t***i:t;ii*dylüi

r*Hfifil*#rflTlilt¡* iütri**!]iii''#:#il**m#l*+


Prciect2; A Low'CostTwo-WavSpeak€r 149 Tablel0-2.

Component

cr LI R1 L2 R2 C2 S1

s2

componenrsUsedin Proiect2.

Descr¡ptior 2.2UF,mylar 0.5 mH,20 ga. L-pad,I-Ohm 2.0 mH, 16 ga.oI better 10Ohm,l0 Watt 6.8pt N.PEl€c. ,/¡"domeMeetet PEl,from MCM, t53-440 61¿" woofer,MCM,551 170

After adding qsphalt lining to the inner wall6, the volume was further - _r€duced. the enclosure(Fig. 10-3)occupiesvery little spaces¡d canbe built tom any Una otz-io'ch (l s cmi material,suchas pl''wood or Pafiicle ¡o"ta. i¡" iote*at constructronis show¡ in tle phologmPhof Fi8 ióa. Aftertiat photowasmade l addeda layerofasphsllroofingmate' ri¡l to the iDlemalwalls gluing il in with construclionadhestve l s .eduiesthe ¡et voLtmeaiit, but filling the box wiü polyesterbatting can make up for any volume loss. After iostalliüg th€ back wiü coü_ stmction adiesive. i caulked üe joints sround the back by working through the woofel cut-out in üe hont pansl' M"oretime v/as sp€nt on crossove¡ tinkering than on any other sugaspedof the designof this proiect.Teslsand lonS-lermIistening-.ror' a two Problems se;t€dthat l¡e id€alcrossovershouldconecl tbr it'*á; *r"4. twical of all small speakersand some peaksin üe r"rooo"". I vi,otii¿ on t¡e pests ñrsl: tryioBvsriouspeákfilters Afr€r a n;mber of testsit becameevidentthat the addsdfilters would make fo. " "o.ol"* ..o".o*'er' Retumingto the goalsof simplicity s¡d low cost, a much easierpath seemedpossible That was lo incrBasethe valueofthe coll in üe low-passñlier to üe woofe¡ With an L-padon tbe üeeler, it pe¡mitteda Soodaudio balanc€ The exha largecboke coil doesdoubieduty: it shápesüe responseof tbe woofer'conecting ior the rising """pott"" ",,-i, ánd alÁo dspressing the peaks at üe uDper -"itt" end of ths woofer'srange. i.o".ou". network iJ a hybrid, part fhst-o¡der (woofer) and second-orde¡(twe€ter) I used a Erylar capacito¡ in üe tweeter leg' but an N.P.electrol]'tic will probably not show any obvjou6 degradationin souDd.For the ¿o acrosstl¡e tweeter circuit, note that 2o-gaugewi¡e is hoaw enough.Tfyou wiDd your own' smallerwire suchas 22 Sauge' l"n ú" u""a]¡o. r¡" toofer óoil.on üe othsrhand.theheavierüe wi re


150

Projects

s¿"(21.6) 7'117.81

-____+1 101"(26)

r_

If--

':1'o-*l

I ..","

' ). -'.-;-l ). r)- ' - ' " . 1;,,';) (

:,

,'-: É

t lo*"ry íu"¿ -N '

- , . _ '@ ¡ ¡l vi t€u8 tn€gr ' ,

3*'18.e)

,l

.-

Box

, ; , , , ' . ' itl , ; 1 'l ",..'--í;-¡:,-,1','.' s¡'1s.e)

I

PEI MCU *5$4

s2 rg gaor beder

10o< R2 f cz

*5s-1170 (17)

plansfot Ptoj*t 2. figüe 10-3. Cons¡ruction

üe better.Id€ally it shouldbe 16 gauge,or better,on a4 air-cor€coil, but 18gaugsis probsblygoodenough. MlNI-SPEAKER PROIICT3: A FORTED . LoveimDedance:6.2 ohms@330Hz . Hkb impeda¡ce:21 obms@88 Hz . Box volume:0.2 cu. ft. [5.7liters)


Project3:A PortedMini-Sf€aker

figurc l0-4. Boxe of P¡oject2 duing @nstructioñ.walls weredanped trygtuine p¡ecesof asphaltroolingon thm with consttuctionadhesive. This prciect uses ¿lrivers of higher cost than those specified for the first two projects (Tablo 10-3). These mini-speakers offer good imaging, probably due to t¡eir narrow boxes, as well as gleat clarity. Several young musicians aided in the final development of this Foject by participatinS in critical listening tests. Lat€r, when some listerers heard Erya's "Watemark" recording on these speak€$, they werc amazed at the full sound coming from such small boxas. The only caveat fu that they can't fill an auditorirm with the sound of a ¡ock band and no one should expect that from 4-inch (1o-cm) v,¡oofe¡s.

Table10-3. Components Usedin Proiccl3. Componenf

Descripl¡on

c1

4 pf, Myla¡ 4 Ohm, l0 Watt LI 0.4mH, 168a. R2 7.5 Ohm, 10 Watt c2 10 pF,N.P.Elec. S1 %" dome tweet€r,Vfa Dl9TD 05 52 4" Kevlarwoofe.,Audax HTI OoKO Driversavailabl€lrcm Meniscusor Parbtxpr6s

R1

:

t


752

Projectj

Small spsake¡ssre best usod on stands€¡d, ifyou wadt optimum imaging and depth of ñeld, thoy should be r^'ell away fiom üe rear wall. However üey can tre used on shelvesif you don't shovethem too close to üe wall and block the po¡t actio¡. A ¡€a¡ port hs! c€Itain advantages(Fig. 10-5). One vi¡tue is that the additio¡ of a cosmetic grille has ao effect on po¡t action. Thi6 catr be a p¡oblem fo¡ pofis installed on the speale¡ boa¡d. Even minor port blockegecan altor üe Q of the port, li¡¡itin8 its dampiüg on th6 woof$, Anoüer advsntag€of a rear po¡t is that a¡y midrange sound üat oscapesis lo6s likely to bs noticedby listene¡s.

f-7"(f7.8)-1

Noich

r- 8¡,,er.o)_--¡ l l ( 0 . 6 4 x 2 . s 4 )I r-/'(r7.8)--l I

I r- 5¡"(1a)-t| 1""t" ll

j-

,.=,=.=i I

iil

+ t ' 1 1 2 4i.36" (3.4)

__-T

.J

ii'i i.i.,i

f-

II

-1 7" 117.8'

ct

F1

i

*i"

I

tu*r.*3*¡ ¡'1r'8)

J

10pl =

Grill€llam€ dn trcrh plywood figue taLs,

Constuction plans fo¡ tuojed 3.

-


P r o l e clt: A P o n e dM i ¡ i S p e a k e r

',,ouo,o '', i':;';"',:;';,,,ili:ifri:.::'":,í::':'tr:,::; :::1;:;tr"i!,,!!;*,

'!:::ii::i::;::;'Jiír:':"::;,::!:?::;.!:f;f::::,::l: Figúe t9-7.

frcnt iteft) an(t rcar rte||s oi an attenúte rrpe af t)on íat p.ajeú 3 The


154 P.ojects Th€ port tr¡b€susedin üis p¡ojectweremadeto be üsedaselectical co¡duit tubjog.Sucbtubjngis lesssxpensivethanmostothert,?€s. bul bewareof dimensionslvariations.Althougbüe tubeswereplain)y marked "1%," when measued by a vemier caliper, üe diame¿r was 34.4mm, or about1.35inches.ü you build this p¡ojectand use t¡üe r.2s-inchlubing,try cuttin8r¡e tonSthbackto abour4.2Sinches. Tbe $ossovornetworkis a simple frrst-orderoetwork.The capac¡to¡ i¡ the tweeter leg is üe value you Eight exp€ctto use for a 36O¡-Hz crossover,but üe coil in the woofer ñlter night be chosenfor a lower hequency.Witb the impeda-nce equaüzerin the circuit, ths woofer.s 'mpedánceisapproximately6.5 ohms.At üat valu€.üe coi¡ suggested herc is, by fo|mula, riSht fo¡ about 2600Hz. A8aid, listening tesi;often dictate a value diffe¡eút from üat suggestedby formulas. A mylar capacitor is ¡ecomr¡ended fo¡ the tweeto¡ ñlter, and 16-gaugewire for the woofer coil. The resistor in the tweete¡ ci¡cuit can be replácedbv an L-padif you wa¡t a more flexiblecontrol on t¡e twester.I¡ that iase yoü might ñnd it necessaryto increasetho value of the capacitor in the tweelercircuil Fom 4 to 4.7 uF. - Noticein the photoofthe frnishedproject(Fig.10-6)that the g¡ille board is iDstalled by dúlling holes in the ñont edéeof the top anJbottom paneiand installingshortpiecesof dowel at üe top andbottomof the gdlle ftame to ñt üe holes. To makesu¡e the location of the dowels is ¡ight to match that of the holss, tempora¡ily nail the uncoveredgille frsme to the ftont of th€ enclosure and drill holes top and bottom throuShthe grille ftame ¡nro üe pa¡€l ed8es.piacesof t-in f0.64cm) I¡ble 10-4.

Component

cl LI

R1 R2

c2 L2

c3 R3 c4

pcofipoJlents

Used in

Descr¡pt¡or 4.0It Mylar 1.0mH,20ga. L-pad,I Ohm l0 ohm,5 wan lpF 1.0mH,16ga. 4.7yF I Ohm,10Wan 10pt N.PE¡ec. VifaD26fC-Os

52 wooleravailable fromMeniscus, olhe. €omponents fromMeniscus or hrts Express.


P¡oject3rA PorredM¡ni-Speakef r5j

cl_owel ffea suitable siz€ for üis job. Use dowel lengt-hsof ,1i¡ f1.9 cmJ ano dnrr the eclges.oftlle pa¡els to a depth ofjust over , in (t 27 cmJ. . As a nnaj touch, cove¡ üe bont of üe speal.erboard \a,irhfe¡r. ti.s also a _Boodidea to cove¡ tho intemal edge;f the grille ftan; ;iü-; stlip oIopen-Lell foam weat}lerstripping mateci¿llo prevenl rsflections oac1.r: rrr: remove rhp g¡it¡F framp thite list€ning. Írilers.put 9r.just rr you plan to tbesesmallspealerson a shell closeto a ¡ei sur, . race.you mr8ht .onside¡moving tle port fiom thp redr to the front.To oo úat. you can mále a ¡eclangular port at the bottom of the froot panet. You cán uso LDp to design "uch a porl tuned to ábout S5 Hz. One way rs ro use üre construction meüod shown in Fig. 10-7. Tty using '-in.11.9,cm) rails undF¡th€ pa_flition. a duct abourz inchesls.bBcmi maxrn8tne port arc€ . inch by 2 inches,or L9 by 5.08 cenrimet"r".it urar ts loo crowded lor (he drivers. you can male the slot oven nar-

¡--t+.l'ieoz1---

¡ |.-rzi"oz.rr1

-r

I I

I

9 É

a t

ei

at'ú al --l rd' (25'4) l'-

I é

1 tt gt

plyrcod 3"(1cm)rhick 2"

8n 10¡t

Íigurc 1O-8. Ptansíat proFc¡ 4

-


case'the rower. such as I inch (1.27cm) by 2%inches (7 3 cm) ln €ithe¡ at i""niii "iift" ¿""t "t'"tld be about 5 inches Note that' theo¡etically slightly to i"."? "t tf" slot gets narrower, the area must be ¡edüced of duct' length the same with fiequency -ui"t"i" ttt" *-Jt""ing PROIECT4: ANOTHTR PORTEDTWO'WAY SPEAKER . Low impedance:6 4 ohms @ 230 Hz . HiPh impedance:20 ohms @ 74 Hz . Boi volume: 0.6a cu ft. (18lite¡s) wooferand 3 Yearsago,mosl 'ompd' t \péaker'I onsisledof dn 8-inch Now the mosl populararrangPmenJ.seems tweeter'ina two-nav_.ystPm wiih a small dome tweeter' Using a (ri woofer ál ur-i""}' io L" " protects ,.ult"t ,"oof". p"t-its a higher crossoverfrequency' which ranSe the lwpeierfro; perlormingal d freouenL)belo\"/ils oplimum ol lhe srze This sv"lemrpod; an P\r eplion¿ll)robustbdcsre\pon\aior the wóofer. üe For this oioi"cl, Ihe ' hoi.e was a $ell knoltn Vila woofet j viia rw".r.t rhc D¿61G-05{TableI0-a) The specificaPr7i.v;;;; o'35' and a tions Íár the woofer show bassresonanceat 37 Hz' a Qrs of v . . " i r . z t " " . h . ( J 4 7 l i t e r s lA r L ¡ n l h f o u g h L D P s h o t { e d á n o p l i m u m u o i u m e o { O g , r , h ( 2 55 l i l p r \ ) .r u n e dt o 4 0 H z T h i s s y s l e ms h o u l o

\h¿pP ¡ hc. / 1/FOY ÍiEurc lT-c. P'oi t a 'PFJkar' (ub| is nat ¿tLcretl eatúne tt be u"ed si¿pes Gn

Etia oí LDP.Altenate


152 Prciect4:Another Po¡tedTwo'WavSpeaker

i'x.: ift::l:i:';;T1;'' ilTtri", :1il"":l;:ixil:i::"".11í:í il,r,,:l$rii*',:lill:t"i"l;m;x""T:"^::; '"i;i'il -rhis\ asdonebur'aherlinit,;;:.;; ;"t cu rr'rFis ro-8)

ii*:,'*;:t*n:*:1""t*:t'ru:1il:i;:;', " tiilXlt;.".t".

network'verv "",-ork forP¡oiect4 is a secon¿l-order ibook'íde"ignto¡ a-ohmdriversat 2400Hz' "rrni*"," " iiJ*tirorey """ii üe -'ata''je reflectionsto üe woofercone'I filled th€ "ir-*"* of Makesüreth€ toP "o"* ¡"fti"J t¡" .."fer with Acousta-stuf

wlil,;:1',:1"ffi^":""i;; ú,epoñopFnins áá-pr'e;"L¡¡" u"row

"r ""4 i"'i"i"¡¿" i*"""t1"J"""1t'l'ffJ:l""iffiH:,1'*:s;;:i ü;*ln*;"'JH":l$::i:l';1:J#:J""xll"'e^

y;"*il{ii*,'*,i:','"'ilm::'H'il^:;::

,il";n:i:'i''J,::1"H# í":::.1r*iÍ',:*liT:ixTffi hoiect 3.

Designing, building, and testing your own speaker system  

Designing, building, and testing your own speaker system

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