Technical Report: Building Acoustics of Multipurpose Halls How can the designer ensure appropriate conditions for every type of use?
School of the Built Environment Course: Architectural Technology 4 Tutors: Robert Mason, Michael Lee, John Wood Code: TR2 001 2008-9 BSV10110
Author: Felix P. Kuester; Matriculation Number 08012601
Table of Content page
2. Literature Review
2.1 Physics of Sound
2.2 Perception and Measurement of Sound
2.3 Room Acoustics as Distinct from Building Acoustics
2.4 An Approach to Acoustic Design
2.5 The Reverberation Time as Essential Acoustic Quantity
2.6 The Sound Level in a Room
2.7 Room Acoustical Parameters
2.8 The Room Eigenmodes (Natural Modes)
2.9 Absorber Types
17-23 18-20 20-21 22 23
2.9.1 Porous Absorbers 2.9.2 Helmholtz Resonators 2.9.3 Micro-Perforated Absorbers 2.9.4 Panel Absorbers 2.10 Diffusor Types
2.10.1 Geometrically Structured Surfaces 2.10.2 Phase Grids of Helmholtz Resonators 2.10.3 Quadratic Residue Diffusors (QRD) 2.10.4 Micro-Perforated Diffusors (MDP) 2.11 Nuisance
25 26 27 28 28-30
2.11.1 Air- and Structure-Borne Sound Transfer 2.11.2 Duct-Borne Sound Transfer
3. Acoustical Design Study
3.1 Room Geometry
3.3 Eigenmodes and Reverberation Time
A List of Relevant Norms and Regulations
B Vertical Sections
C Reverberation / Use of Acoustic Boards
This report shall give an introduction to the physics of sound explaining how a sound wave is excited, propagated and finally absorbed. With the sounds varying properties at different ambient conditions in mind, an approach to room acoustics is made. The field of room acoustics seen in distinct to building acoustics often escapes the architectâ€™s notice. Acoustical requirements for the design of multi-purpose halls can be mutually exclusive. This makes a sound understanding of the basic acoustic parameters and their interaction essential. Accepted reverberation times and sound levels are stated in the standards. There are also established methods which allow the measuring of a number of seemingly subjective parameters e.g. the liveliness of a room or the spatial perception of it. The determination of such parameters is not always computed and there are numerous cases in which a scaled physical model can be more reliable. Finally, a calculation of the main resonance frequencies (the eigenmodes) and the typical reverberation time for the â€˜SoCoâ€™ hall proposal is made. Rest on this evidence; a feasible acoustic-design approach is described.
2. Literature Review
2.1 Physics of Sound
Sound is a travelling wave which is an oscillation of pressure that can be transmitted through a solid, liquid, or gas. The sound speed varies and is depending on the material and the ambient conditions - mainly temperature and pressure. In 20 째C (68 째F) air at the sea level, the speed of sound is approximately 343 metres per second (Houghton Mifflin Company 2006). Two other basic characteristics of a sound wave are frequency and amplitude. Frequency describes the number of oscillations per unit time. Its physical measure is HERTZ (Hz). Amplitude is a physical measure for the loudness of a sound (Sound Research Laboratories Ltd 1988). A sound with pressure changes at one frequency is a pure tone and has the frequency spectrum shown in Figure 1 where frequency is represented in Hz along the horizontal axis and amplitude along the vertical axis.
Figure 1: Line Spectrum of a Pure Tone
1000 2000 Frequency (Hz)
Most sounds aren’t pure tones but they show periodic waveforms. Periodic sounds are combinations of pure tones with various frequencies and amplitudes. Noises and complex sounds such as music or speech are rarely periodic. A typical waveform of such a broad band noise, as shown in Figure 2, has no pattern and is said to be continuous. All nonperiodic waveforms can be analyzed into sinusoids by the Fourier transform. Nevertheless, filters allow a simpler representation of such a continuous spectrum. The representation then shows all the sound energy, measured between a lower and an upper frequency.
Figure 2: Waveform of Broadband Noise
To standardise measurements, international octave and third octave frequency bands have been established at the centre frequencies that annotate Figure 3. The upper limit of an octave band with the centre frequency (f0) is defined as f0 x √2 and the lower limit is defined as f0 / √2. This makes the upper frequency being twice the lower frequency (Sound Research Laboratories Ltd 1988).
Figure 3: Continuous Octave Band Spectrum
500 1000 2000 Frequency (Hz)
2.2 Perception and Measurement of Sound
For humans, hearing is normally limited to frequencies between about 12 Hz and 20,000 Hz (20 kHz), although these limits are not definite. The upper limit generally decreases with age (Harry F Olson 1967). Also the human ear can detect sounds with a very wide range of amplitudes. Therefore, sound pressure is often measured as a level on a logarithmic decibel (dB) scale. The sound pressure level (SPL or Lp) is described as follows:
where pref is the reference sound pressure and prms is the average (root mean square) sound pressure being measured over time (C. L. Morfey 2001). As the human ear also does not have a flat spectral response, in the field of acoustics, the measure of a sound pressure level is often frequency weighted. As shown in Figure 4, there are four weighting networks which are a representation of the relative sensitivity of the ear to tones at different frequencies. The A network covers the pressure levels up to 55 dB, B-weighting ranges from 55 to 85 dB, and C-weighting is used for higher amplitudes 5
whereas the D-weighting is only used for an extreme sound pressure level beyond the absolute threshold of pain.
Figure 4: Weighting Networks
Frequency (Hz) on a logarithmic scale
In practice, the dBA level has become accepted measure for loudness independent of amplitude for most applications (Sound Research Laboratories Ltd 1988). As we all know from experience, sound sources have directivity e.g. a trumpet. On the other hand, it is known from several technical sound sources of interest that those radiate sound in all directions uniformly. This is if their size is small compared to the wavelength. That matter of fact is due to the different properties of sound under different ambient conditions and at different frequency levels. There are situations in which the propagation of sound can thus be described best through â€˜sound raysâ€™. If those hit a surface, depending on the impact angle and surface properties, they are reflected or they are absorbed or diffused. The ray pattern of acoustic waves can be described with geometric acoustic models. At lower frequencies or in a changed acoustic environment or the sound propagation from the same source has to be calculated according to the acoustic wave theory. The theory which was constructed by the Scottish physicist Maxwell allows the determination of 6
changes in frequency based on interferences or the amplification respectively the absorption of a sound due to resonances. Acoustic environments also have specific reverberation times. Their duration depends on the room volume, geometry and interior design. During a period of two seconds, for instance, the sound travels a distance of nearly 700 metres and consequently hits the surfaces of a given room several times. If the original sound source emits a short impulse, an echogram is obtained as shown in Figure 5.
Figure 5: Time series of reflections in a room enclosed by plane walls
The delay between the individual sound events is expressed by the different distances of the direct and the mirror sources to the observation point. For higher order reflections, the impulse becomes more and more blurred (M. Moeser 2004). Also all other room acoustical parameters can be determined from the room impulse response which is a specific form of echogram. The latter will be described in paragraph 2.4.5 in more detail. Still the intelligibility of speech and the stereophony of a room are mainly determined by reverberation times and room specific blurring. Therefore these two characteristics are the most important determinants for an appropriate acoustic design of a room.
In a multipurpose hall, like the one taken as a base for this report, there are different adequate reverberation times. They are a function of the kind-of-use; also they are often mutually exclusive. Nevertheless, there are means and technologies that allow a designer to reach appropriate conditions and meet the regulations. The most common products and technologies will be described thereafter in detail.
2.3 Room Acoustics as distinct from Building Acoustics
Building acoustics comprises all matters related to sound transmission and sound insulation between rooms, e.g. the determination of the sound reduction index of walls, windows, doors, and ceilings. The special field of room acoustics on the other hand, deals with the propagation of sound in rooms. The intelligibility of speech in a theatre or the clarity of music in a concert hall is investigated here. Our daily life is constantly influenced by room acoustics, e.g. in factory buildings, open-plan offices, cafeterias, and restaurants (Dr. J. Hunecke 2009). This report is focused on the field of room acoustics. It will touch building acoustic matters, whenever the two fields of study interfere. One of these interdisciplinary topics is the limitation of air-, structure- and duct-borne ambient noise in halls which is described in paragraph 2.11.
2. 4 An Approach to Acoustic Design
According to Glen Ballou (2005), the way sound behaves in a room can be broken up into roughly four different frequency zones: â€˘ The first zone is below the frequency that has a wavelength of twice the longest length of the room. In this zone sound behaves very much like changes in static air pressure. â€˘ Above that zone room resonances dominate because wavelengths are comparable to the dimensions of the room. This zone extends until the frequency is approximately 8
11,250 x âˆš (RT x 60 / V), with RT being the typical reverberation time and V the volume of the room. The resonances are dominated by the so-called room eigenmodes (or natural modes). These are three-dimensional standing waves within the room developing at very specific frequencies. Each eigenmode is associated with a different sound pressure distribution. A person moving around in a room where an eigenmode has been excited will register strongly fluctuating sound intensities. â€˘ The third region which extends approximately two octaves is a transition to the fourth zone. In a car it is centred at about 400 Hz, in an average living room at about 180 Hz, and in a concert hall at about 30 Hz. â€˘ In the fourth zone, sounds behave like rays of light bouncing around the room. The amount of energy a sound ray looses with each reflection depends on the nature of the reflecting surface.
2.5 The Reverberation Time as Essential Acoustic Quantity
As already mentioned, the most important physical quantity used to describe the acoustical properties of a room is its reverberation time. It is a measure for the reverberance of a room and is defined as the length of time required for the sound level in a room to drop by 60 dB. The duration of the reverberation time in a given room mainly depends on the sound absorption properties of its walls, floors ceilings and its furnishings as well as on the volume of the room. The reverberation time is frequency-dependent since stones, wood, carpets or textiles absorb sound to a different extent at varying frequencies. The appropriate reverberation time always depends on the use of the room. Several authors give their recommendations. The ideal reverberation time must have the same value at all frequencies from 30 to 12,000 Hz. Or, at least, it is acceptable to have a linear rising from 100 percent at 500 Hz to 150 percent down to 62 Hz. For broadcasting and recording studios and conference rooms, values under one second are frequently used. Requirements for other typical room types are as follows:
Auditoria: In the case of auditoria one has to decide whether they are only used to play music or for music and speech purposes alike, or solely for speech purposes. This distinction should be made very carefully as it determines the necessary reverberation time which must be longer for music than for speech. To overcome this problem some auditoria have been designed with revolving wall- or ceiling panels which provide an acoustically hard surface at one side and an absorbent surface on the other. By revolving the panels the reverberation time can be altered (Sound Research Laboratories Ltd 1988). Also it can be increased electronically by feeding a delayed signal through a series of loudspeakers at the correct volume in relation to the natural sound e.g. one of such systems known as ‘assisted resonance’ is installed in some of the lecture halls at Edinburgh Napier University. The requirements on lecture halls as distinct from auditoria are described in the section below.
Lecture halls, conference or meeting rooms and classrooms: In all these rooms a high level of speech intelligibility is required. In order to achieve optimum reverberation times, it is usually necessary to only partially construct suspended ceilings from absorbent material. The standards contain several examples illustrating the position of absorbent sections. The broad rule for these constructions is that the ceiling must be reflective in the middle part of the room. Most manufacturers of acoustic ceilings offer systems that allow the combination of sound absorbent and sound reflective elements to meet the requirements mentioned above.
Figure 6: The ‘Musikvereinssaal’ in Vienna
Dimensions: 48.8 x 19.1 m; height: 17.75 m. Capacity: 1744 seats
Chamber music halls, concert
Figure 7: Simulated Sound Pressure Level Mapping
halls, theatres, opera houses: These rooms are amongst the most
standards can only recommend preferred reverberation times. Existing concert halls with highly acknowledged
properties all have reverberation times ranging from 1.5 to 2.0 seconds. The Musikvereinssaal in Vienna
opened in 1870. It is still acknowledged as one of the best concert halls in the world. Its reverberation time lays at 2.0 s with the audience present. When designing chamber music halls, concert halls, theatres, and opera houses care must be taken as well of the fact that for the audience the acoustic quality should be more or less the same irrespective of their seats. This usually requires complex computer simulations to determine the ideal inclination of walls and ceilings as well as a favourable distribution of absorbing, reflecting and sound scattering surfaces. According to the market-leader for sound and vibration solutions, Brül & Kjær, there is currently only the software the Danish company Odeon reliable enough on frequencies clearly above the eigenmodes. Figure 7 shows an Odeon visualisation of the sound pressure level distribution in a given concert hall. For the calculation, the sound field is seen as finite number of particles, bouncing from wall to wall with the sound-source being located in the stage centre. Current findings show that especially the long-time neglected stage geometry now seems to be of mayor importance for the over-all performance of a music hall (M. Aretz; R. Orlowski 2009). •
Sports halls, pools: Here the main differentiation is based on the fact whether the sports hall or bath is designed for single lessons or multiple lessons. If several classes are to be instructed at the same time, a shorter reverberation time is required for the same volume. The sound level generated in the hall during normal usage will then be 11
considerably lower, and pupils will be able to understand their teachers without too much distraction caused by the other classes. The Planning Advice Note PAN 56 and the National Planning Policy Guideline NPPG 11 refer to the impact of noise from sporting activities. All expected noise emissions have to be taken into consideration by the designer, when choosing an approach to room acoustics in a sports facility. Health and Safety research and considerations by the education authorities regarding new schools has resulted in the introduction of legislation to comply with the acoustic requirements. Restrictive requirements are laid out in Building Bulletin BB93 and are part of the Building Regulations for the design of sports halls: •
Indoor Sports Hall <1.5 s
Gymnasium <1.5 s
Swimming Pool <2.0 s
Dance Studio <1.2 s
Multi-purpose Halls <0.8 - 1.2 s (Drama, P.E., assembly, occasional music)
RMP Acoustic Consultant C. Steels states that in practise, the above mentioned times are virtually impossible to meet as e.g. sports halls must generally have a hard (sound reflecting) wall surface up to a height of at least 3 metres. He thinks that a reverberation time which does not exceed two seconds as it is recommended by the Sport England Agency for all types of sports facilities is entirely sufficient. This value can be reached with a reasonable amount of acoustic absorbers being installed.
2.6 The Sound Level in a Room
The sound level is a measure for the loudness level of a sound source. In rooms the sound level is characterized by two different sound fields, the direct sound field and the diffuse sound field. The direct sound field is the sound field that would develop around a sound source if the room did not have any boundaries. With an increasing distance from the source, the sound level decreases continually. In case of omni directional sources that radiate the sound uniformly in all directions, the level decreases by 6 dB when doubling the distance. 12
The diffuse sound field is created by the fractions of sound that are reflected by the boundary surfaces of the room. They propagate haphazardly in the room and fill it evenly with sound energy. Consequently, the sound level in a diffuse sound field is the same irrespective of the position. As Figure 8 shows, both sound fields superimpose each other. Near the sound source the sound level is dominated by the direct sound field, and at greater distances the diffuse sound field is dominating. The distance from the sound source at which the level of the direct sound field is equal to the level of the diffuse sound field is called reverberation radius (Dr. J. Hunecke 2009).
Figure 8: Omni Directional Source
K: Direct sound field of an omni directional source D: Diffuse sound field rh: Reverberation radius
In practise, the sound field of halls is often tested using a physical model to a scale of 1:50. For an exact measure all room-components with their absorption properties as well as the emitted frequencies for the measurement have to be scaled down accordingly.
2.7 Room Acoustical Parameters
There are a large number of acoustical parameters for rooms. Those that Dr. J. Hunecke 2009 and M. Barron 1993 mention to be most important in practise will be presented here. The parameters purpose in general is to describe subjective sound impressions in an objective manner. All room acoustical parameters can be determined from the room impulse response. The room impulse response is the acoustic answer of a room excited by a short bang.
The so-called Dirac impulse shown in Figure 9 is in theory an infinitely sharp peak. Consequently, the acoustical parameters derived from the room impulse response are only valid for the chosen combinations of emitter and receiver positions.
Figure 9: Schematic Representation of a Dirac Impulse 0,3
0,2 0,1 0 -0,1 -2
• The Definition [Df] is a measure of the intelligibility of a speaker. It indicates how well a speaker can be understood by a listener in a particular seat of an auditorium. The Rapid Speech Transmission Index [RASTI] is a measure that indicates the quality of the transmission of speech. It is used with reference to public address systems as it takes into account the noise from external and internal sources. External sources are, for example, the background noises at a train station. Internal sources are, among other things, the noise of amplifiers and the harmonic distortion of loudspeakers. • Clarity is a comparable measure of the clarity of music. It is used for large rooms such as chamber music and concert halls. It is not suitable for the evaluation of the reproduction of music by loudspeakers in a recording studio or listening room. • The Lateral Fraction [LF] is related to the spatial perception of music in a concert hall and indicates how much the listener feels surrounded by the music. The Bass Ratio [BR] describes the warmth of the music.
2.8 The Room Eigenmodes (Natural Modes)
Room Eigenmodes are three-dimensional standing waves that can be excited at the characteristic eigenfrequencies of the room. Figure 10 shows a plane sound wave vertically incident on a rigid wall which is completely reflected. The incident and the reflected wave superimpose each other according to the principle of superposition to form a standing wave. Directly in front of the wall, which is supposed to be stationary, the air cannot move. Consequently, on the wall, the sound velocity of the standing wave must be zero. At the same time the sound pressure has a maximum value at this position. The first maximum of sound velocity is phase-shifted at a distance of a quarter of a wavelength from the wall.
Figure 10: Standing wave in front of a massive wall
p: sound pressure v: sound velocity λ: wavelength
Only the sound propagation at frequencies below a room-specific limiting frequency is determined by the room’s eigenmodes. Each eigenmode is associated with a different sound pressure distribution. A sound pressure receiver such as the human ear or a microphone moving around in a room that has been excited at such an eigenfrequency will register strongly fluctuating sound intensities (Glen Ballou 2005). The sound pressure distribution, as well as the frequencies at which these room resonances occur, depend, among other things, on the geometry and volume of the room. For rectangular rooms the eigenfrequencies can be calculated by means of a simple formula. For rooms with other shapes, however, time-consuming numerical methods must be applied.
Figure 11: Eigenmodes in a Cubic Room
Figure 11 shows a representation of the square value of the sound pressure at the walls of a rectangular room. Dark areas correspond to a high sound pressure or high loudness. To distinguish between the different eigenmodes a combination of three natural numbers (nx ny nz) that refer to the level and axis is used e.g. (1 2 1). In order to achieve good acoustics it is desirable that the eigenfrequencies are uniformly distributed, and that no accumulations occur near a particular frequency. If the room was a cube with edges of 2 m length many eigenfrequencies would coincide. The room eigenmodes (2 0 0), (0 2 0) and (0 0 2), for example, would all be excited at a frequency of 171.5 Hz. This coincidence of eigenfrequencies occurs whenever one dimension of a room is an integer multiple of another such as, e.g. in a room that is twice as wide as it is high. Favourable spatial proportions, on the other hand, would be (1) : (1.4) : (1.9) or (1) : (1.6) : (2.1). Dr. J. Hunecke (2009) and several other authors recommend considering the modal density as important criterion to lay out the proportions of a performance hall.
2.9 Absorber Types
The absorption of sound is mainly due to three different physical mechanisms that are all based on the conversion of sound energy into heat. In porous absorbers the oscillation of the air particles is slowed down by the porous or fibrous structure of the material. As a result, frictional heat is produced. Porous absorbers are textiles, carpets, foams, mineral wool, special acoustic plasters and porous stone materials. Edge absorbers are a special form of the porous absorbers. When arranged at the edges of a room they offer high efficiency at low frequencies. In Helmholtz resonators the air in the resonator opening is excited to strong oscillations at the resonance frequency. As in porous absorbers, frictional heat is produced as the oscillation of the air particles in the opening is slowed down due to friction. The main difficulty when designing Helmholtz resonators is to adjust the frictional resistance to the ideal value. Micro-perforated absorbers are another form of Helmholtz resonators. They are characterized by a multitude of tiny holes (with a radius of less than 1 mm) and low perforation (less than 4%). Since friction between the air particles and the walls of the holes is sufficiently high it is not necessary to fill the holes with additional porous material. Panel absorbers are yet another form of resonant oscillating systems. A panel with a closed surface is fixed in front of a volume of air in such a way that any airborne sound hitting its surface causes the panel to oscillate. At the eigenfrequencies of this oscillating system the amplitudes of the oscillation are particularly high. The oscillation of the panel is damped by the friction between the molecules of the panel material. So here the sound energy is first converted into the oscillation energy of the panel and only then into heat. To achieve optimum efficiency most panel absorbers require additional damping of the air space behind the panel by means of mineral wool or foam material. To be able to quantify the efficiency of absorbers, the absorption coefficient was defined. It indicates the portion of incident sound energy that is absorbed. The absorption coefficient for normal incidence, which plays an important part in research and development, assumes values between 0 and 1. In a laboratory setting that value is measured in the impedance tube (also called Kundt's tube). 17
In the actual room measures can only be taken to a lower limiting frequency of 100 Hz to 125 Hz. Below these frequencies, due to the wave nature of sound the position of the absorbers within the room is of essential importance. Currently there is no standardised method for the measurement of the absorption coefficient at low frequencies. Care must be taken when comparing different manufacturer’s products (M.W. Simons & J.R. Waters 2004).
2.9.1 Porous Absorbers
C.L Morphey (2001) describes a porous absorber as any kind of porous or fibrous material such as textiles, fleece, carpets, foams, mineral wool, cotton wool and special acoustic plasters. They all absorb sound energy as they damp the oscillation of the air particles by friction. Porous absorbers are most effective in slowing down air particles with a high sound velocity. When mounted directly onto the wall they therefore must be of a certain thickness in order to absorb sound waves down to a certain lower limiting frequency. If, however, the absorber is mounted at a distance from the wall, its thickness can be reduced accordingly. Manufacturers of acoustic ceilings take advantage of this effect. Besides the distance from the wall, the flow resistance of the material also is of high influence. Porous absorbers are available in different forms of products:
• Textiles: In room acoustics textiles play a part in form of curtains or people’s clothes. Curtains with a wall distance of about 10 centimetres are good absorbers down to a lower frequency of about 125 Hz. This, however, requires that they are not covered by an airtight synthetic coating. Very thin curtains that can be looked through hardly absorb any sound. Their flow resistance is too low.
• Fleece: Most manufacturers of perforated metal, wood or gypsum board ceilings use special acoustic fleece. This thin fabric offers an optimum flow resistance and fulfils the relevant fire protection requirements. If the perforated portion is high, the panel or board only serves as a mechanic carrier for the acoustic fleece. If the perforated portion is small, the panel or board and the air volume behind it act as a Helmholtz resonator. With most constructions the absorption coefficient can be increased by an additional layer of mineral wool. • Carpets: As the thickness of carpets hardly ever exceeds one centimetre, they only absorb high frequencies from about one kilohertz onwards. Carpets alone are therefore not sufficient to achieve good acoustics. Additional measures must be taken for the absorption of lower frequencies. • Foam: Foam is often used as upholstery in furniture. Since upholstered furniture is usually distributed over the room the diffusion of the sound field is increased. Foams especially produced for acoustic purposes are used as inserts in metal ceilings, as a filling for panel absorbers or as edge absorbers. A visually unobtrusive finish can be achieved by covering the foam with a kind of textile wallpaper. Figure 12: ‘Soundscape’ by Armstrong Industries Inc.
â€˘ Mineral Wool, Cotton Wool: Mineral wool in the form of pressed, stiff boards is often used in suspended acoustic ceilings. Systems, similar to Figure 12 are available with a variety of mounting systems and in any colours and finishes. In loose form mineral wool is frequently used as an additional layer on perforated panels and to improve sound insulation in lightweight interior walls. In this form it can be replaced by cotton wool. â€˘ Acoustic Plaster: The term acoustic plaster refers to special plasters that are applied in such a way that many interconnected cavities are formed. If only a thin layer of acoustic plaster is applied, it is only effective for high frequencies, similar to carpets. If, however, the plaster is applied to a sound-permeable carrier material mounted at a certain distance from the wall, a good absorption coefficient can be achieved for low frequencies as well. In this way it is possible to install joint-less acoustic ceilings that look like a normal plastered ceiling.
2.9.2 Helmholtz Resonators
Figure 13: Principle of the Helmholtz resonator
Helmholtz resonators are acoustic systems consisting of an oscillating air plug connected to an air volume as the illustration Figure 13 shows.
There are many different forms of Helmholtz resonators: an empty wine bottle, the corpus of a string instrument, bass reflex enclosures of loudspeakers and wall linings made from perforated wood or gypsum boards. The empty wine bottle is best suited to explain the geometry and operation of the Helmholtz resonator in a simple way: The air in the bottleneck forms a so-called air plug, and the air contained in the remainder of the bottle
is the connected air volume. The air plug has an acoustic mass which results from its geometry and the specific density of the air. It rests on the elastic cushion formed by the air contained in the remainder of the bottle. Together they form an oscillating system with a specific resonance frequency that can be easily excited, as is well known, by blowing across the opening of the bottle (Dr. J. Hunecke 2009). Helmholtz resonators are often used to amplify sound. With string instruments, for example, the sound energy emitted by the vibration of the strings alone would by no means be sufficient. Only when the strings are connected to the corpus with its openings sufficient loudness can be achieved. In order to prevent a Helmholtz resonator from amplifying sound and make it absorb sound, the air oscillating in the opening must be slowed down by friction. This is usually achieved by means of a thin fleece material glued to the rear of the opening, sometimes with an additional layer of mineral wool or foam.
â€˘ Perforated Linings of Walls and Ceilings: Probably the most common application of the Helmholtz resonator are suspended ceilings or wall linings made from perforated metal, wood or gypsum board. The air contained in the numerous holes of the boards is oscillating in front of the air volume enclosed between the boards and the ceiling or wall. These systems are tuned in such a way that several resonance frequencies lie next to each other and absorption over a broad frequency band can be achieved. â€˘ Bass Absorbers: Helmholtz resonators can also be used to damp individual eigenmodes of a room which develop at low frequencies. To attain a low resonance frequency, a relatively large volume with only one or a few openings is needed.
2.9.3 Micro-Perforated Absorbers
As Figure 14 illustrates, micro-perforated absorbers consist of a perforated plate with a multitude of tiny holes arranged in front of an enclosed air volume with a rigid back panel. When excited by a sound wave the mass of air in the holes oscillates in front of the elastic air volume. Thus we are dealing with an acoustic mass-spring system operating on the principle of the Helmholtz resonator.
Figure 14: Micro-Perforated Absorber
Figure 15: Velocity Profile in Comparison
Left side: In a normal Helmholtz resonator. Right side: In a micro-perforated absorber.
Micro-perforated absorbers are effective over a relatively broad band of frequencies. The perforated portion of their surface only adds up to a few percent, and no additional frictional resistance is required. Absorption is entirely due to the viscous friction of the air in the holes. It originates in the respective acoustical boundary layer, which is in the area where the velocity profile changes. The holes of micro-perforated absorbers are so small that this boundary layer extends over the entire cross-section of the holes. The principle is shown in Figure 15, where a micro perforated absorber is compared with a usual Helmholtz resonator. The thermal conductivity of the plate material is important, because a significant part of the frictional heat generated in the air is carried off by the plate. Micro-perforated absorbers can be made from different materials, including transparent acrylic glass.
2.9.4 Panel Absorbers
Panel absorbers are yet another form of resonant oscillating mass-spring systems. A panel absorber consists of a flat panel made of wood, metal, gypsum board or plastic material that is arranged in front of an enclosed air volume. The air volume is partly or completely filled with mineral wool or foam. Such a system has several resonance frequencies that can be excited by airborne sound.
Figure 16: Panel Absorber Types
The left oft the panel absorbers, shown in Figure 16 has the entire surface of the panel glued to a foam layer, whereas the panel absorber on the right hand side is laterally clamped. Its two lowest natural oscillations are shown in colour. If the panel is glued over its entire surface area to a foam layer located in front of a rigid wall, the panel as a whole is allowed to oscillate on the spring formed by the foam layer. The resonance frequency of the panel absorber can be calculated from the mass of the panel and the stiffness of the spring formed by the foam layer. The heavier the panel and the softer the foam layer, the lower is the resonance frequency. If, however, the panel is clamped in any way, it will no longer be able to move as a whole but will only be able to perform bending oscillations. These are slowed down by the friction between the molecules of the panel material. So the sound energy is first converted into oscillation energy and only then into heat. Often the air space behind the panel requires additional damping by means of mineral wool or foam. By skilful adjustment many of the possible resonances can be arranged next to each other and damped in the best possible way to create a bass absorber effective over a comparatively broad frequency band.
2.10 Diffusor Types
Further improvement of the acoustic properties of a room can be achieved if the natural diffusion of the sound field is increased by means of special wall structures. Even in ancient baroque concert halls balconies and walls were decorated with rich ornamentation, rosettes, and figurines to serve this purpose. With modern and plainer architectural styles came the necessity to design special diffusers. The use of diffusers plays an important part, not only in large halls, but also in small rooms such as recording studios or practice rooms for musicians. The structure of a diffuser is designed to deflect as much of the reflected sound energy as possible from the direction of geometric reflection and distribute it into other directions. Furthermore care is taken to uniformly cover a wide angular range (M. Moeser 2004).
Figure 17: Typical Scattering Diagram of a Diffusor
Basically, diffuse sound reflections can be achieved by two means: That is either by threedimensional structures such as the rosettes mentioned above, or by surfaces with locally reacting wall impedance. Wall impedance is the ratio of sound pressure to sound velocity at the surface of the wall. At every joint between two materials with a different acoustic effect, a part of the incident wave is scattered diffusely as shown in Figure 17. Since scattering is a wave phenomenon, the dimensions of three-dimensional structures must be within the order of magnitude of the wavelength of the sound to be scattered. Semi-cylinders, prisms or similar forms can be used to construct special geometrically structured surfaces.
For surfaces with locally reacting wall impedance the rule is that the wall impedance must change sufficiently within the frequency range considered. Phase grids of Helmholtz resonators are a good example. These are surfaces with periodically adjoining Helmholtz resonators. Maximum length diffusers and quadratic residue diffusers belong to the so-called Schroeder diffusers. Their design is based on special number sequences. Individual wells which completely reflect sound and whose depth varies according to the number sequences are used to form structures with very good scattering characteristics. Furthermore there are micro-perforated diffusers where the rigid wells of the Schroeder diffusers are replaced by stripes of micro-perforated resonators (Dr. J. Hunecke 2009).
2.10.1 Geometrically Structured Surfaces
Geometrical structures like those shown as section in Figure 18 reflect incident sound diffusely if their dimensions are in the order of magnitude of the soundâ€™s wavelength. The small sticks of wood contained in ingrain wallpaper, for example, have a thickness of about one millimetre. A wavelength of one millimetre corresponds to a frequency of 340 kHz, so diffuse reflection will only take place in the ultrasonic frequency range. Figure 18: Geometrical Structures for the Generation of Diffuse Reflections
gs: structure period
In the audible frequency range, however, the ingrain wallpaper has the same effect as a plane, rigid wall, which reflects an incident wave geometrically.
2.10.2 Phase Grids of Helmholtz Resonators
Periodic structures with varying wall impedance are called phase grids. Each phase grid has a specific reflection factor. The absolute value of it corresponds to the amplitude of the reflected wave related to the amplitude of the incident wave. So if the absolute value of the reflection factor is 1 this means that the incident wave is completely reflected. An absolute value of 0, on the other hand, means that the incident wave is completely absorbed. Being reflected, the wave experiences a so-called ‘phase jump’ that can range between 0 and 180 degrees.
Figure 19: Phase grids of Helmholtz resonators
Same perforation, varying cavity depths.
Varying perforation, same cavity depth. x gs: structure period
To achieve a maximum scattering, the surface absorption should be low and the phase shift maximised. These conditions can be realized using Helmholtz resonators which are tuned accordingly. The lesser a Helmholtz resonator is damped, the larger is the frequency range over which this phase shift occurs. At the same time, the resonator’s absorption capacity constantly decreases. The necessary great phase difference of adjacent wall areas can be realized by means of two adjoining resonators that are tuned to different frequencies as shown in Figure 19 (E. Meyer; H. Kuttruff; F. Rischbieter 1962).
2.10.3 Quadratic Residue Diffusors (QRD)
Quadratic residue diffusors belong to the so-called pseudo-stochastic diffusors. They are also called Schroeder diffusors, after their inventor. They consist of individual wells of different depths separated by a thin wall as shown in Figure 20. One-dimensional diffusors scatter sound only in the plane perpendicular to the wells, twodimensional diffusors scatter sound incident from any direction.
Figure 20: A One- and a Two-Dimensional QRD Array
Typically one of the prime numbers 7, 11, or 13 is used to determine the relation between the well-depths. Also, the prime number determines the length after which the sequence repeats periodically and thus also the number of wells the diffusor consists of.
2.10.4 Micro-Perforated Diffusors (MPD)
Micro-perforated diffusors (MPD) consist of individual adjoining stripe-shaped microperforated resonators as shown in Figure 21. Figure 21: MDP Array
These are tuned to different resonance frequencies. In this way adjoining areas with different wall impedance are created. Incident waves are scattered diffusely, as is the case with phase grids of Helmholtz resonators.
Even in an internally optimized acoustical room there may be nuisance caused by external sources. The noise that reaches a given room from the exterior is usually seen as a matter of building acoustics, whereas the room acoustics deals with the propagation of sound in rooms. Only the latter is discussed in detail in this report. Nevertheless, there are three sources of nuisance in a building that have to be taken into account because they can highly influence the acoustic quality of a room:
2.11.1 Air- and Structure-Borne Sound Transfer
Vibrations in walls and ceilings can be excited internally e.g. by walking on a floor or operating a machine or they can have external sources e.g. traffic. The vibrating building structure can then excite the surrounding air in other rooms and radiate sound.
Figure 22: Direct and Flanking Sound Transmission
The air-borne sound in a room e.g. speech or machinery also represents an exciting force with respect to the surrounding walls and ceilings. Vibrations are also created in the structure. The transfer path can be described as ‘from air to structure and back to air’. As shown in Figure 22, both mentioned forms off excitation have in common that the sound is not necessarily transported via the ‘direct’ path. In addition to the direct transfer path through the partition wall or the ceiling between two levels there are many other paths, so-called flanking paths. The partition wall for instance can have such a high sound insulation, that a flanking path is dominating the sound transfer. Therefore a further acoustic improvement of the partition wall will not achieve any better result in the total sound insulation. Measures to avoid sound-bridging are e.g. cavity stops, floating floors with elastic sealing around the flooring. Particular attention must be given to the pipes and ducts which pass walls or the ceiling. The applicable regulations on sound transmission in buildings can be found in the Building Regulations Scotland 2004 Section 5, respectively Part E of the Building Regulations as mentioned in the Appendices.
2.11.2 Duct-Borne Sound Transfer
One significant source of noise within a ventilation system is the fan. If the system’s natural attenuation is insufficient then the designer will need to consider installing additional damping. According to Sound Research Laboratories Ltd (1988) the designer has to pay particular attention to the following aspects: • Fan noise: This data should be available from the manufacturer as octave band SPLs. • System attenuation: The natural attenuation provided by duct runs, bends, branches and terminations. • Room atmosphere corrections: matching in respect of directivity, distance, room volume and reverberation time. • Sound reducing techniques: e.g. unitary attenuators, duct linings, plenum chambers or modifications of the system design Beside the fan the flow of air through the ductwork system can cause significant noise as well. Flow generated noise is the result of air flow turbulence which is usually concentrated into a discrete region. The most likely sources that can cause such turbulences are: • Flow over objects e.g. grilles • Constriction noise e.g. orifice outlets, changes of section • Jet noise as the air finally emerges from a duct • Boundary layer turbulence, as the air passes over apparently smooth surfaces • Flow around corners In the above mentioned cases the noise emission increases as square value of the air speed. As a rule it is favourable to choose large sections whenever possible, but to avoid unnecessary changes in sections at the same time.
3. Acoustical Design Study
3.1 Room Geometry
Subject to the acoustical design study is a multipurpose hall which is part of the ‘SoCoLeisure Centre’ design proposal, developed by F. Kuester (2009). The multi-purpose hall is located in the ground-floor resp. basement of the proposed leisure facility. The hall shall be used either as two badminton courts or as fringe venue for both theatrical performances and concerts. Three different sections through the hall are attached in Appendix B.
Figure 23: 2-Point Perspective (towards North-East)
Length: 32.30m Width (variable): 12.00m (9.00m) Height (floor to ceiling): 8.50m Gross floor area: 379.6m² Gross Volume: approx. 3200m³ Seating Capacity: 105 (15 fixed) Figure 24: 2-Point Perspective (towards South-West) As Figure 23 and 24 illustrate, the multi-purpose hall is designed in a rectangular shape. There are three shafts in the corners; two being staircases and one elevator shaft. Inbetween, there is a balcony with fixed seating. Also the additional area of 70 square metres behind the 31
roll-doors can be used differently, depending on the function of the hall e.g. as storage space or as cloakroom. The main access to the hall is through the entrance lobby on Southbridge level, using the staircase on the backside of the building. Artists can use the dressing rooms, located on level 1 and then access the hall by using the second staircase in front of the building. A third fire exit directly guides to the Street. In order to meet the fire regulations, to guarantee a good view for each guest and not least to achieve sufficient acoustic quality, the room-space which used for theatrical performances and concerts is limited to the back-half of the hall. It is separated through a partition curtain-wall. The front section of the hall can be adequately used as foyer or temporary restaurant area.
The Design suggests a for a multipurpose hall typical hardwood deck sprung floor with a minimum of 53 percent shock absorption. Cheaper solutions are available with PU-, PVC-, and linoleum-decking. Those have low impact-sound reduction qualities and also these are less wanted by athletes. The total wall area of 425 square metres is covered with acoustic wall panels which should have a minimum sound absorbing coefficient of 0.85. Also, the panels have to be robust enough to withstand impact from objects such as indoor footballs. This usually means that up to a height of three metres no absorbers and speakers can be installed. Exceptions are special flat-panel-speaker systems e.g. the Pioneer SFL 1 and for absorption, RMP consultant C. Steel recommends the Lambri Soundtube acoustic panels. These also have a relatively high a mid-band absorption rate which could particularly help to meet the regulations because those only consider frequencies, ranging from 500 Hz to 2kHz. The sidewalls of the hall are slightly spread (approx. 1째) thus flutter-echoes are not likely to develop. The ceiling could be hanged with a slope to guide the direct reflections to the listener. As there must be an absorbing material used for the ceiling in order to achieve a sufficient low reverberation time, this solution seems not feasible.
To the front and partly towards Southbridge, there is a glazed faĂ§ade with a total area of 250 square metres. As the front section of the hall is only to be used as foyer for performances or for sports use it is sufficient to incline the glazing backwards as it is already part of the design. This guides the sound to the absorbing ceiling and is not causing any direct reflections, so no additional absorber material as e.g. curtains is denied. The ceiling consists of a suspended acoustic board system with more than 90 centimetres absorbing space volume above. There are many possible board types available. Only it is important that the fixation prevents ball-impacts being able to lift the boards. Most manufacturers offer such systems. To avoid acoustical nuisance by the
Figure 25: Reverberation Time of the Hall
ventilation the air speed is kept to a minimum. Fresh air is guided in through the ducts along the walls and the used air is then exhausted through ventilation ducts running behind the suspended ceiling.
3.3 Eigenmodes and Reverberation Time
With the room proportions being (1) : (1.4) : (2 x 1.9) there are no combinations of the modes nx ny and nz thinkable in which those could be at the same frequency. The exact reverberation time depends on the specific material used for the ceiling and as wall cover. As Figure 25 shows, in an exemplary calculation with a given manufacturerâ€™s assortment, a sufficient reverberation time of 1.2 seconds (freq. band: 500Hz to 2Hz) according to BB93 and DIN 18041 can be achieved. The full document is attached in Appendix C. 33
Sound in buildings is frequently seen as an issue about ‘how to avoid noise nuisance’. This is in disregard of the fact that good acoustics are generally one of the key qualities of a building standing equally beside its design, the arrangement of rooms or the functionality of the services. The acoustic design of a room is recommended to closely match to the function of the room. In such a location, visitors will recognize a certain degree of tonal personality in distinct to an otherwise acoustically ‘dead’ or annoying room. To achieve that, there are various scientific parameters which extend way beyond normal regulations and on which decisions can be based. Multipurpose halls are likely to be caught between two stools, but still there numerous solutions allowing sufficient compromises or even to profoundly change a room’s acoustics as a function of its use e.g. electronically or by the use of revolvable absorbers. The ‘SoCo’-hall proposal features a room geometry which promotes agreeable acoustics and that limits the amount of expensive absorber material that has to be attached to the walls and onto the ceiling. Appendix C shows an exemplary setting.
• M. Aretz; R. Orlowski (2009) Sound Strength and Reverberation Time in Small Concert Halls , Achen, Department of Applied Acoustics, RWTH • B. S. Arup; R. Harris; J. Newton; L. Strand (2009) ‘Acoustic Planning of the Main Hall’, DETAIL Magazine Vol 3 pp208 et seq. • M. Barron (1993) Auditorium Acoustics and Architectural Design, London, E&FN Spon Ltd • G. Ballou (2005) Handbook for Sound Engineers, Oxford, USA, Howards Sams Editors • L. Beranek (1954) Acoustics, Columbus, McGraw Hill Books Inc • DETR (1991) Building Regulations: Approved Document E, London, Stationery Office Books • Dr. J. Hunecke (2009) Room Acoustics, Retrieved March 31st 2009 from www.hunecke.de • Houghton Mifflin Company (2006) The American Heritage Dictionary of the English Language, Fourth Edition, Boston, B&T Ltd • M. Moeser (2004) Engineering Acoustics, Berlin, D, Springer Verlag • Sound Research Laboratories Ltd (1988) Noise Control in Building Services, Oxford, UK, Pergamon Press • C. L. Morfey (2001) Dictionary of Acoustics, San Diego, Academic Press • J. Meyer (1995) Akustik und musikalische Aufführungspraxis, Frankfurt am Main, Verlag Erwin Bochinsky • E. Meyer; H. Kuttruff; F. Rischbieter (1962) ‘Messung der Schallstreuung an Flächen mit periodisch wechselnder Impedanz’, Acustica Vol 12 pp334-341 • H. F. Olson (1967) Music, Physics and Engineering, Dover, Dover Publication Inc • M.W. Simons & J.R. Waters (2004) Sound Control in Buildings, Oxford, UK, Blackwell Publishing Ltd. 35
5.2 Figures and Tables
• Figure 1: Graph; based on Sound Research Laboratories Ltd (1988); no copyrights • Figure 2: Graph; based on Sound Research Laboratories Ltd (1988); no copyrights • Figure 3: Graph; based on Sound Research Laboratories Ltd (1988); no copyrights • Figure 4: Graph; found on wikipedia.org (2009); no copyrights • Figure 5: Graph; based on M. Moeser (2004); no copyrights • Figure 6: Photograph; found on www.panoramafotos.net (2009); some rights reserved • Figure 7: Animation; found on www.odeon.dk (2009); some rights reserved • Figure 8: Graph; based on Dr. J Hunecke (2009); no copyrights • Figure 9: Graph; based on www.wikipedia.org (2009); no copyrights • Figure 10: Graph; based on Dr. J Hunecke (2009); no copyrights • Figure 11: Animation; found on Dr. J Hunecke (2009); some rights reserved • Figure 12: Sketch; found on www.armstrong.com (2009); some rights reserved • Figure 13: Sketch; based on M. Moeser (2004); no copyrights • Figure 14: Graph; based on Dr. J Hunecke (2009); no copyrights • Figure 15: Graph; based on Dr. J Hunecke (2009); no copyrights • Figure 16: Graph; based on Dr. J Hunecke (2009); no copyrights • Figure 17: Graph; M. Moeser (2004); some rights reserved • Figure 18: Sketch; based on Dr. J Hunecke (2009); no copyrights • Figure 19: Sketch; based on E. Meyer; H. Kuttruff; F. Rischbieter (1962); no copyrights • Figure 20: Sketch; found on Dr. J Hunecke (2009); some rights reserved • Figure 21: Sketch; found on Dr. J Hunecke (2009); some rights reserved • Figure 22: Sketch; found on www.alfwarnock.info (2009), some rights reserved
• Figure 23: 3D Drawing; own Work • Figure 24: 3D Drawing; own Work • Figure 25: Graph; Heradesign Ceilings Division (2009), some rights reserved
Appendix A - List of Relevant Norms and Regulations:
• BS EN ISO 354:2003 Acoustics – Measurement of absorption in a reverberation room • BS EN ISO 3382:2000 Acoustics – Measurement of the reverberation time of rooms with reference to other acoustical parameters • BS EN ISO 11654:1997 Acoustics – Sound absorbers for use in buildings – Rating of sound absorption • BS EN 61260:1996 Electro acoustics – Octave band and fractional octave band filters • BSR 2004 Section 5 - Resisting sound transmission to dwellings using appropriate constructions • BR 2000, Approved Document E – Resistance to passage of sound • Planning Advice Note: PAN 56 Planning and Noise - Provides advice on good practice and other relevant information • National Planning Policy Guideline NPPG 11: Sport, Physical Recreation and open Space – Noise from sport facilities • BS 5821-3:1984 ISO 717/3:1982 - Methods for rating the sound insulation in buildings and of building elements. • BS 8233:1999 Sound insulation and noise reduction for buildings – Code of practice • BS EN ISO 140-(1-8):1995 - Measurement of sound insulation in buildings • Building Bulletin 93 (2003) - Acoustic performance targets for the design of new school buildings • ASTM-C423-90a: Acoustics – Standard test method for sound absorption coefficients by the reverberation room method
Appendix B â€“ Vertical Sections 1) Vertical Section, facing Cowgate
2) Vertical Section, facing Southbridge
3) Vertical Section, to the East
Appendix C – Reverberation Time / Use of Acoustic Boards
Roof: Hera Design ‘Plano’ MW 40 h 305 mm (or similar) Wall: 60% of the wall surface ‘Deluxe’ MW 40 height 40 mm (or sim
This report was created using the computation service accessible free of charge under www.heradesign.at
23.4.2009 No information contract: Use of the computation service does not result in any contract, including any information contract, between the user of the computation service and the provider of the computation service or the companies offering the products the computation service is used for.
Disclaimer of liability: Although the computation service was carefully programmed, and the product data were carefully compiled, neither the provider of the computation service nor the companies offering the products the computation service is used for shall be liable for any loss or damage resulting from the use of the computation service. In particular liability for any loss or damage (e.g. design errors) caused by the use of any results, data, or information taken from the computation service shall be disclaimed.
Usage: according to:
auditorium (music and speech) DIN 18041 (may 2004)
Volume: Bare ceiling: Bare floor: Floor covering: Walls: Windows:
3226.6 m³ 379.6 379.6 379.6 0.0 425.0 0.0 250.0
m² m² m² m² m² m² m²
massive construction float. compos. floor parquet, laminate massive construction no curtains or blinds
105 3 0
spectators (fabric upholstery) musicians -
379.6 m² Heradesign plano 25, MW 40, h = 305 mm 225.0 m² Heradesign deluxe, MW 40, h = 40 mm
Noise Reduction ∆L [dB]
– Average Absorption Coeff. α
1.0 0.8 0.6 0.4 0.2 0.0 Frequency [Hz] Noise reduction through abs. and furn. Without abs., without furn., without peop. With abs., with furn., without peop. With abs., with furn., with peop. For chosen usage the DIN 18041 defines only requirements for the reverberation time!
3.2 3.0 2.8 2.6 2.4
Reverberation Time T [s]
2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 125
Frequency [Hz] Without abs., without furn., without peop. With abs., with furn., without peop. With abs., with furn., with peop. Tolerance limits for TSoll = 1.41 s For auditorium (music and speech): DIN 18041 requirements are met! Notice must be taken of the standard regarding placement of absorbers!
Product: Material: Dimensions: Mounting: Fire protection: Notes:
Heradesign plano 25, MW 40, h = 305 mm magnesite-bound wood wool with 40 mm mineral wool layer 600 mm x 600 (1200) mm Abs. Coeff. valid for depth of suspension h = 305 mm B - s1, d0 (EN 13501-1) Smooth visible surface, Standard colouring: White, Natural tone 13 (beige) Other colouring: RAL, NCS, StoColor
Absorption Coefficient Îą p
1.2 1.0 0.8 0.6 0.4 0.2 0.0 125
Product: Material: Dimensions: Mounting: Fire protection: Notes:
Heradesign deluxe, MW 40, h = 40 mm magnesite-bound wood wool w. real wood veneer on HPL, 40 mm mineral w. 600 mm x 600 (1200) mm Abs. Coeff. valid for depth of suspension h = 40 mm choice of 9 standard veneers (fir, maple, American walnut, birch, beech, European oak, European ash, pine, American cherry wood) in shifted wire/half wire
Absorption Coefficient Îą p
1.2 1.0 0.8 0.6 0.4 0.2 0.0 125