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    The  Loudspeaker  Study   Chris  Nottoli   Melissa  King,  Joshua  Roberts,  Javier  Forero,  Frank  Minella   Columbia  College  Chicago                 Acoustical  Testing  I   Dr.  Dominique  Chéenne,  Dr.  Lauren  Ronsse   October  16th  2013  

       


Table  of  Contents   Abstract:  ..................................................................................................................................................  3   Introduction:  ..........................................................................................................................................  3   Materials:  ................................................................................................................................................  4   Frequency  Response:  ..........................................................................................................................  4   Crossover:  ...............................................................................................................................................  8   Polar  Directivity:  ..................................................................................................................................  9   Conclusions:  ........................................................................................................................................  10   Additional  Tests:  ................................................................................................................................  11   Introduction:  ..................................................................................................................................................  11   Damping  Results:  ..........................................................................................................................................  11   Comparison  Results:  ....................................................................................................................................  12  

Appendix  A:  .........................................................................................................................................  14                    

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Abstract:       The  frequency  response,  crossover  frequency,  and  polar  directivity  of  an  Event   TR8XL  speaker  were  analyzed  inside  Columbia  College  Chicago’s  anechoic  chamber  using   TEF  20.  The  frequency  response  of  the  speaker  was  also  analyzed  in  a  non-­‐anechoic   environment.    The  response  was  relatively  linear,  showing  many  irregularities  throughout   the  frequency  spectrum.  When  compared  with  the  test  conducted  outside  of  the  anechoic   chamber,  the  irregularities  were  still  present.  The  crossover  point  occurred  at  1898  Hz  in   contradiction  to  where  the  magnitudes  of  the  drivers  crossed  at  2059  Hz.  The  polar   directivity  varied  throughout  the  frequencies  where  at  lower  frequencies  the  speaker  was   omnidirectional  and  became  directional  at  higher  frequencies.  

Introduction:       The  loudspeaker  analysis  was  conducted  at  Columbia  College  Chicago,  using  the   anechoic  chamber  located  in  LL01  with  TEF  6.0  software.  Josh  Roberts,  Melissa  King,  Javier   Forero,  Mike  Minella,  and  the  author  of  this  report  collaborated  on  this  study.    The   objective  was  to  analyze  the  frequency  response  both  inside  and  outside  of  the  anechoic   chamber,  the  crossover  frequency,  and  polar  directivity.  Two  additional  tests  were   conducted  to  determine  the  frequency  response  as  mass  was  added  to  the  subwoofer  and  a   comparison  with  a  Shure  SM-­‐63  microphone.        

 

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Materials:     •

Speaker:  Event  TR8XL;  QSC-­‐K8  

Microphone:  Electro-­‐Voice  RE-­‐55;  Shure  SM-­‐63  

TEF  6.0  

Outline  electronic  turntable  

Computer  with  TEF  20  and  Outline  interface  

Anechoic  Chamber  13’  10”  X  10’  2”  X  8’  2”      

Frequency  Response:       This  initial  speaker  used  in  this  analysis  was  a  QSC-­‐K8.  Preliminary  steps  were   taken  before  the  frequency  tests  were  conducted.  The  Electro-­‐Voice  microphone  was   calibrated  with  TEF  to  .00099  V/Pascal.  A  test  to  determine  if  the  microphone  was   receiving  signal  from  the  speaker  was  then  conducted.  As  seen  in  Appendix  A,  Fig.  A-­‐1,  a   magnitude  of  less  than  0  decibels  was  being  obtained  before  the  direct  sound  had  reached   the  microphone.  Upon  observing  the  frequency  response  in  Appendix  A,  Fig.  A-­‐2,  there  was   no  valid  signal  received  by  the  microphone.  Fig.  1  shows  the  signal  flow  followed  to  correct   the  absence  of  a  proper  signal.  The  equipment  rack  patch  bay  had  been  altered  since  last   used  and  was  corrected.  This  correction  can  be  found  in  Appendix  A,  Fig.  A-­‐3  along  with  all   other  patched  connections  used  within  these  tests.    

 

 

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Fig.  1:  The  output  of  the  signal  flow  from  TEF  is  displayed  on  the  right.  The  input  signal  is  displayed  on  the   left.    

   

A  time  response,  found  in  Appendix  A,  Fig.  A-­‐7,  was  conducted  to  again  determine  if  

the  microphone  obtained  proper  signal.  Tested  at  2  feet,  the  direct  sound  arrived  at   3.81ms.  The  arrival  of  direct  sound  was  expected  at  2ms.  The  1.8ms  of  late  arrival  of  the   acquired  to  the  expected  direct  sound  explains  the  speaker  change.  This  late  arrival  time   occurs  because  the  QSC  speaker  stays  on  standby  until  a  signal  is  present  to  determine  if   the  signal  can  potentially  destroy  the  speaker  by  DC  power.  A  time  response  was  conducted   on  an  Event  TR8XL  at  2  feet.  The  signal  reached  the  microphone  after  1.65ms  ensuring  that   the  results  being  received  were  accurate  as  seen  in  Fig.  2.    

 

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Fig.  2:  Time  response  on  the  Event  TR8XL  speaker  at  2  feet.  Direct  sound  reached  the  microphone  at  1.65ms.    

 

 

 

In  obtaining  the  frequency  response,  the  microphone  was  repositioned  1  meter  in  

 

front  of  the  speaker  as  seen  in  Appendix  A,  Fig.  A-­‐8.  Time  resolution  was  sacrificed  since   the  test  was  conducted  inside  an  anechoic  chamber  allowing  for  a  15  minutes  sweep  from   20–20,000  Hz.  The  graph  of  the  frequency  response  can  be  found  in  Fig.  3.  The  response  of   the  speaker  was  relatively  linear  and  had  many  irregularities  especially  at  225  Hz,  360  Hz,   and  550  Hz.  This  could  be  due  to  the  size  of  the  speaker  and  the  tendency  of  the  low   frequency  driver  to  produce  an  inaccurate  response.  The  speaker  also  rolls  off  at  17,000  Hz   indicating  it  cannot  accurately  reproduce  the  full  frequency  spectrum.  There  was  also  a   noticeable  drop  in  magnitude  between  2700  Hz  and  5500  Hz  occurring  after  the  crossover   point.  The  differences  between  phases  of  the  two  drivers  may  be  the  cause  of  this   phenomenon.  

 

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Fig.  3:  The  frequency  response  swept  20Hz-­‐  20,000Hz  over  a  15-­‐minute  period.    The  frequency  resolution   used  in  the  test  was  11.0  Hz  with  a  2Hz  bandwidth.    

 

 

The  frequency  response  of  this  speaker  was  also  tested  in  a  non-­‐anechoic  

environment,  found  in  Appendix  A,  Fig.  A-­‐9.  Time  resolution  could  not  be  sacrificed  so  that   reflections  off  the  nearest  objects  would  not  be  captured  in  the  response.  The  microphone   was  placed  1  foot  from  the  speaker  so  the  time  resolution  will  only  allow  the  direct  sound   from  the  speaker  to  reach  the  microphone  with  minimum  reflections  from  surrounding   objects.  Therefore,  ten  tests  were  conducted  in  octaves  from  20Hz-­‐40Hz,  39Hz  -­‐80Hz,  etc.   to  maximize  the  best  possible  time  resolution.  The  graph  in  Fig.  4  shows  the  frequency   response  obtained.  Since  time  resolution  could  not  be  sacrificed,  frequency  resolution  was   very  difficult  to  achieve.  However,  many  characteristics  of  Fig.  3  were  still  present.      

 

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Fig.  4:  Frequency  response  of  an  Event  TR8XL  in  a  non-­‐anechoic  room.  Breaks  in  response  are  due  to  the   start/stop  of  each  test.    An  overlap  of  1Hz  at  the  start  of  each  test  was  used  to  overlay  from  the  previous  test.  

 

  The  inaccuracies  of  the  low  frequency  driver  were  still  present.  The  crossover   frequency  can  be  pointed  through  a  dip  in  the  magnitude.  The  poor  response  of  the  high   frequency  driver  is  also  evident  as  it  rolls  off  at  7,000Hz.  The  discontinuities  that  occur   throughout  the  graph  are  due  to  the  10  individual  tests.  It  can  be  concluded  that  Fig.  4  was   an  excellent  representation  to  how  the  speaker  reproduced  sound  in  a  non-­‐perfect  space.  

Crossover:      

The  initial  crossover  point  was  hypothesized  to  occur  between  2,000-­‐3,000  Hz  

where  the  magnitude  in  Fig.  3  drops  roughly  3dB.  Tested  in  the  anechoic  chamber,  the  low   frequency  driver  was  covered  with  absorptive  material.  The  microphone  was  then  placed   directly  in  front  of  the  high  frequency  driver.  Another  test  was  conducted  covering  the  high   frequency  driver  and  placing  the  microphone  directly  in  front  of  the  low  frequency  driver.   Results  could  be  found  in  Appendix  A,  Fig.  A-­‐11.  An  additional  test  was  conducted  from   1,000-­‐4,000  Hz,  placing  the  microphone  directly  between  the  high  and  low  frequency   drivers  due  to  the  lack  of  interactions  of  phase  information  obtained  from  Appendix  A,  Fig.    

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A-­‐12.  As  seen  in  Fig.  5,  the  crossover  occurs  at  1898  Hz  where  the  phase  is  88  degrees  out   of  phase.    

Fig.  5:  The  phase  test  was  conducted  between  1,000  Hz  and  4,000  Hz  for  600s  with  a  bandwidth   of  2.2  Hz.  Note  the  30  dB  drop  in  magnitude  at  1898  Hz.  As  the  phase  is  88  degrees  out  of  phase   cancelation  occurs,  which  caused  the  significant  drop  on  magnitude.    

 

 

Additionally,  the  acoustic  and  electrical  phases  were  not  at  the  same  frequency.  The   high  frequency  driver  crossed  at  2059  Hz.  This  may  also  have  occurred  because  the  sound   was  bleeding  through  the  absorptive  material,  skewing  the  data  received.  

Polar  Directivity:       The  expected  directivity  of  the  loudspeaker  was  hypothesized  to  be  omnidirectional   at  lower  frequencies  and  become  more  directional  as  frequency  increases.  Measured  at  1   meter  from  the  microphone  in  the  anechoic  chamber,  the  loudspeaker  was  centered  on  an   Outline  electronic  turntable.  The  Outline  interface  was  set  to  turn  every  5  degrees  with   relation  to  the  parameters  in  Appendix  A,  Fig.  A-­‐13  along  with  the  speaker’s  polar   directivity  in  A-­‐14.    

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A  3.5-­‐minute  sweep  time  with  small  interval  changes  were  used  to  capture  as  much  

detail  about  the  directivity  as  possible.    Below  32  Hz,  the  directivity  was  rugged  looking,   which  can  be  explained  either  by  poor  low  frequency  reproduction,  any  resonance  of  the   speaker  itself,  or  both.  The  speaker  was  in  fact  omnidirectional  between  32-­‐50  HZ  meaning   the  wavelength  of  these  frequencies  diffracted  around  the  back  of  the  speaker  housing.  The   speaker  then  became  subcardioid  between  56-­‐250  Hz.  At  500  Hz,  the  frequencies  become   smaller  than  the  speaker  where  directional  characteristics  began  to  appear.  

Conclusions:         The  frequency  response  of  the  TR8XL  loudspeaker  showed  a  relatively  linear   response  from  700  Hz-­‐  2,700  Hz.  With  a  roll  off  before  100  Hz  and  after  17,000  Hz,  the   speaker  cannot  accurately  reproduce  these  frequencies.  However,  most  of  those   frequencies  are  inaudible  to  most  humans  and  can  be  overlooked.  The  response  in  a  non-­‐ anechoic  room  showed  similar  characteristics  to  the  test  conducted  in  the  anechoic   chamber,  but  showed  how  it  actually  reproduced  in  a  room  it  would  most  likely  be  used  in.   Further  testing  of  the  frequency  response  in  a  studio  control  room  would  give  better   insight  as  to  how  it  may  affect  the  outcome  of  a  recording.    

The  crossover  was  found  to  occur  at  1898  Hz  where  the  drivers  were  88  degrees  out  

of  phase.  This  crossover  point  contradicts  where  the  two  magnitudes  crossed  when  testing   individual  drivers.  Additional  tests  could  be  taken  to  determine  how  the  speaker’s  port   hole  affects  the  outcome  of  all  tests  in  this  report.  Also,  using  more  absorptive  material  and   eliminating  any  bleed  through  of  sound  could  be  tested  to  determine  if  the  crossover  in   magnitude  of  each  driver  shifts  closer  to  the  electrical  crossover.    

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Finally,  the  polar  directivity  was  found  to  be  omnidirectional  below  56  Hz,  

subcardioid  between  56-­‐250  Hz,  and  an  increase  in  directivity  as  frequency  increased.   Further  testing  into  the  single  hypercardioid  response  at  280  Hz  could  be  investigated  to   determine  if  a  possible  resonance  of  the  table  or  speaker  caused  this  to  occur.  

Additional  Tests:     Introduction:       As  an  extra  credit  study,  the  frequency  response  of  the  Event  TR8XL  was  tested   adding  eight  coins,  more  specifically  quarters,  to  the  low  frequency  speaker  to  evaluate  the   effects  of  damping.  Each  quarter  was  taped  around  the  speaker  evenly  to  ensure  that  extra   mass  affected  the  whole  speaker  as  seen  in  Appendix  A,  Fig.  A-­‐15.  The  frequency  response   of  the  speaker  with  and  without  quarters  would  be  compared.  In  addition,  the  Shure  SM-­‐63   microphone  was  compared  with  the  Electro-­‐Voice  RE-­‐55  expecting  to  obtain  similar  results   since  both  are  dynamic.  Quarters  were  removed  from  this  test,  since  damping  was  not  the   concern.   Damping  Results:       The  results  of  damping  were  as  expected  seeing  a  3-­‐6dB  decrease  in  magnitude  up   until  700  Hz.  By  adding  more  mass,  the  speaker,  seen  in  Appendix  A,  Fig.  A-­‐15,  would  have   had  to  exert  more  force  in  order  to  produce  the  same  frequencies  at  the  same  magnitude   explaining  this  occurrence.  As  the  frequency  increased,  the  effects  of  damping  were  not  as  

 

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noticeable  since  higher  frequencies  stopped  relaying  on  the  size  of  the  cone  taking  less   force  to  produce  the  frequencies  as  seen  in  Fig.  6.    

Fig.  6:  Damping  with  Quarter.  This  test  foucused  in  between  20  Hz  and  1,000Hz  with  a  two  minute   sweep.  Note  that  the  peaks  at  60hz,  120  Hz,  180  Hz,  etc.  are  due  to  the  vibrations  of  the  quarters  on   the  cone.  As  the  crossover  nears  and  the  high  frequency  driver  begins,  these  peaks  become  less   prominent.  

 

   

Comparison  Results:       The  frequency  response  of  the  Electro-­‐Voice  was  used  as  a  reference  so  that  a   comparison  with  the  Shure  SM-­‐63  can  be  analyzed.  Many  similar  characteristics  were   found  between  the  two  microphones;  for  example,  the  drop  in  magnitude  between  2,700-­‐ 5,500  Hz  was  still  present,  as  was  the  roll  off  at  17,000  Hz.  The  differences  in  the  frequency   response  can  be  due  to  the  sensitivity  of  the  microphones,  the  size  of  the  diaphragm,  and   calibration.  Overall,  the  Electro-­‐Voice  continued  to  produce  more  accurate  results  that  can   be  easily  interpreted.    

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Fig.  7:  The  comparison  between  Electro-­‐Voice  and  Shure  SM-­‐63.  Sweep  time  of  60  s.  and  a  167   Hz  bandwidth  were  used  to  get  a  general  sense  of  the  differences.                                                          

 

 

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Appendix  A:  

 

                Fig.  A-­‐1:  Time  response  test  conduced  at  2’2”.  The  decibel  level  before  the  direct  sound  had  obtained  decibels   below  0  dB.  In  addition,  the  direct  sound  was  captured  at  3.66ms.  This  “late”  arrival  of  direct  sound  was  to  be   investigated.      

 

Fig.  A-­‐2:  Frequency  response  related  to  Fig.  A-­‐1,  showing  no  true  signal  being  received.        

Fig.  A-­‐3:  Patch  Bay  connection  at  equipment  rack.  The  signal  from  TEF  goes  through  station  two   output  3  to  anechoic  input  3,  which  in  return  gets  set  to  the  speaker.  Anechoic  1  is  the  signal                       received  from  the  microphone,  which  gets  sent  to  station  2  output  1  to  TEF  (mic  A).    

 

 

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Fig.  A-­‐4:  TEF  20  connection;  BNC  line  out  to  station  two  XLR  input  3(Fig.  A-­‐5)  and  mic  A  input  from  station   two  output  1.      

Fig.  A-­‐5:  Station  two  patch  bay  mentioned  in  Fig.  A-­‐4.    

 

 

  Fig.  A-­‐6:  Chamber  snake.  Channel  1  is  receives  signal   from  the  microphone.  Channel  3  is  the  output  signal  to   the  speaker.    

Fig.  A-­‐7:  Time  response  after  patch  bay  correction.  Direct  sound  reaches  the  microphone  at  3.81ms.    

 

                                       

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Fig.  A-­‐8:  General  set  up  for  loudspeaker  tests  in  the  anechoic   chamber.      

  Fig.  9:  Set  up  for  analyzing  the  frequency  response  of   the  loudspeaker  in  a  non-­‐anechoic  environment.   Microphone  was  placed  1ft  away  from  loudspeaker.  

 

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Fig.  A-­‐10:  Parameters  for  frequency  response  in  a  non-­‐anechoic  room.  A  0.88ms  delay  was  included  in  each   test  to  account  for  the  1  foot  distance  between  the  speaker  and  the  microphone.      

 

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Fig.  A-­‐11:  Crossover  with  absorption  covering  each  driver.  The  expected  crossover  was  thought  to  occur   where  both  magnitude  cross.  Note  that  the  high  frequency  driver  appears  to  be  producing  low  frequency   tones.  This  is  due  to  the  bleed  through  of  the  low  frequency  driver,  which  had  an  effect  to  where  the   magnitudes  crossed.  

Fig.  A-­‐12:  Crossover  phase  related  to  A-­‐11.  

                               

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Fig.  A-­‐13:  Polar  directivity  parameters.              

 

 

 

 

 

 

 

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Fig.  A-­‐14:  Polar  Directivity  graph.      

 

  Fig.  A-­‐15:  Eight  quarters  were  added  to  the  speaker   to  determine  the  effect  of  damping  as  mass  is  added.  

 

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The Loudspeaker Study