Friday, April 15, 2011

ACOUSTICAL DESIGN OF A SMALL AUDITORIUM

AR-461: BUILDING SCIENCE

By:

RAVINDAR KUMAR
Assistant Professor
Department of Architecture and Planning
NED University of Engineering and Technology
Karachi
2011

LECTURE NO. 17
TOPIC:                         ACOUSTICAL DESIGN OF A SMALL AUDITORIUM

INTRODUCTION:[1]
"The design of various types of auditoriums (theatres, lecture halls, churches, concert halls, opera houses, and cinemas) has become a complex problem in contemporary architectural practice because, in addition to aesthetic, functional, technical, artistic and economical requirements, an auditorium often has to accommodate an unprecedentedly large audience. Furthermore, present standards often mean that the same space must be used for a number of different functions and the capacity of the room must be adjustable to momentary needs."

"These are serious requirements and it must be remembered that, when an audience enters an auditorium, they have every right to expect comfort, safety, pleasant surroundings, good illumination, proper viewing and good sound." L.L. Doelle, Environmental Acoustics

OUTLINE OF ACOUSTIC REQUIREMENTS FOR GOOD SOUND:
  • There should be adequate loudness in every part of the auditorium, especially in remote seats.
  • The sound energy should be uniformly distributed within the room.
  • Optimum reverberation characteristics should be provided in the auditorium to facilitate whatever function is required.
  • The room should be free from acoustical defects (distinct echoes, flutter echoes, picket fence echo, sound shadowing, room resonance, sound concentrations and excessive reverberation).
  • Background noise and vibration should be sufficiently excluded in order not to interfere in any way with the function of the enclosure.

ADEQUATE LOUDNESS
The problems of providing adequate loudness result mainly from the inverse square law and excessive absorption by the audience attenuating the direct sound before it reaches the listener. This sound loss can be minimised in a number of ways.

  • The auditorium should be shaped so that the audience is as close to the sound source as possible. In larger auditoria the use of a balcony brings more seats closer to the sound source.
  • The sound source should be raised as much as is feasible in order to secure a free flow of direct sound to every listener.
  • The floor on which the audience sits should be properly raked as sound is more readily absorbed when it travels at grazing incidence over the audience. As a general rule, however, the gradient along aisles of sloped auditoria should not be more than 1:8 in the interests of safety. The audience floor of theatres for live performance, especially open or arena stages should be stepped [Tutorial: How to step the floor].
  • The sound source should be closely and abundantly surrounded by large sound-reflective surfaces in order to increase the sound energy received by the audience. It must be remembered that the dimensions of the reflecting surfaces must be comparable with the sound waves to be reflected. In addition, the reflectors should be positioned in such a way that the time-delay between the direct and reflected sound is as short as possible, preferably not exceeding 30 msec and definitely not more that 80 msec [Tutorial: Positioning reflectors].
  • The floor area and volume of the auditorium should be kept at a reasonable minimum, thus shortening the sound paths. The following table details recommended Volume-per-seat values for various auditoria.

Type of Auditorium   
  Minimum 
  Optimum 
  Maximum 
Rooms for Speech
2.3
3.1
4.3
Concert Halls 
6.2
7.8
10.8
Opera Houses
4.5
5.7
7.4
Catholic Churches 
5.7
8.5
12.0
Other Churches
5.1
7.2
9.1
Multipurpose Halls
5.1
7.1
8.5
Cinemas
2.8
3.5
5.6

RECOMMENDED VOLUME-PER-SEAT VALUES (m3) FOR AUDITORIA:
  • Parallelism between opposite reflective walls, particularly those close to the source, should be avoided in order to eliminate undesirable back reflection and flutter echo.
  • The audience should only be placed in areas which are advantageous to both viewing and listening - generally the two are in agreement. Excessively wide seating areas should be avoided. In addition, aisles should preferably be located at the sides of the auditoria where viewing is restricted, not down the centre where viewing and listening is most favourable.
  • In addition to reflectors directing sound towards the audience, additional reflectors are often needed to reflect sound back at performers to enhance their ability to hear what is happening on stage.
Whilst these measures will adequately improve the loudness in small to medium auditoriums, they will not perform miracles. In large auditoria, and especially outdoors, speech levels will often still be too low for satisfactory hearing. In these cases, the installation of sound-amplification equipment is nearly always necessary.
DIFFUSION OF SOUND:
In order to provide a high degree of sound diffusion within in an enclosure an abundant supply of surface irregularities, such as exposed structural elements, coffered ceilings, serrated enclosures, protruding boxes, sculptured surface decorations, and deep window reveals must be provided. These must also be of various sizes in order to affect all frequencies.

For reasons of economy and aesthetics, particularly in small rooms, the application of surface irregularities is often difficult. In such cases random distributions of absorbing material of the alternate application of reflective-absorptive materials provide an alternate, though less effective means.

The use of dedicated diffusers is particularly important in concert halls, opera houses, radio/recording studios and rehearsal rooms. The beneficial effect of acoustic diffusers upon the sound quality of auditoria is often quite marked as they retain the sound energy within the room whilst negating echo and standing wave effects.

CONTROL OF REVERBERATION:
All orators, actors, musicians and especially singers, will expect a reasonable reverberation time so they do not sound too stark or dry. The optimum reverberation times of a room implies the following;
  • Favourable RT vs. frequency characteristics.
  • An advantageous ratio of reflected to direct sound reaching the audience and
  • A steady, smooth growth and decay of sound energy.
Once the RT has been decided upon, reverberation control consists of establishing the total room absorption and ensuring it is adequately provided for. This means selecting the correct acoustical finishes for room surfaces as well as considering the effects of occupation and fittings. To calculate this at the earliest stages of design the Sabine formula is usually used.

A look at this formula clearly shows that the larger the room, the longer the reverberation time and the greater the absorption required. Thus, the RT can be changed within the same auditorium by enlarging or reducing its volume (e.g.: raising or lowering the ceiling, using more balconies, etc). Since the absorption of materials varies with frequency, so too will the RT. It is therefore essential to specify and calculate the RT for a number of frequencies throughout the audible range. It is often the case that the RT at low frequencies is most troublesome as this is the area at which porous absorbers are least effective. Thus, panel absorbers and bass traps may have to be considered.

In almost every large auditorium, the audience provides most of the absorption (0.45 Sabine per person). If you rely upon this and the attendance rates sometimes vary, then the listening conditions may suffer. The most effective (and most expensive) compensation for low attendance is to use upholstered seating with the same overall absorption coefficient as a single person. Some auditoria use absorber on the bottom of fold-up seating. Thus, when occupied, the absorption effect is reduced as it faces the floor whilst unoccupied if is folded up and faces the stage. As a general rule, sound-absorbing materials should be placed on those surfaces most likely to produce acoustic defects. This means that the rear wall should be treated first and then those portions of the side walls furthest from the source. There is no justification for putting absorber at the front of an auditorium or along the middle portion of the ceiling as the primary function of these areas is to reflect sound back to the audience.

ELIMINATION OF DEFECTS:
The basic defects attributable to room geometry have been touched in a previous lecture and consist of echoes, sound concentrations, sound shadowing, distortions, coupled spaces and room resonance.

1. ECHOES:
These are probably the most serious and most common defect. They occur when sound is reflected off a boundary with sufficient magnitude and delay to be perceived as another sound, distinct from the direct sound. As a rule, if the delay is greater than 1/25 sec (14m) for speech and 1/12 sec (34m) for music then that reflection will be a problem.

Solution: Either alter the geometry of the offending surface or apply absorber or diffusion. (Flutter and picket fence echoes)

2. SOUND CONCENTRATION:
Sometime referred to as 'hot-spots', these are caused by focussed reflections off concave surfaces. The intensity of the sound at the focus point is unnaturally high and always occurs at the expense of other listening areas.

Solution: Treat with absorber or diffusers, better still; redesign it to focus the sound outside or above the enclosure.

3. SOUND SHADOWING:
Most noticeable under a balcony, it is basically the situation where a significant portion of the reflected sound is blocked by a protrusion that itself doesn't contribute to the reflected component. In general, avoid balconies with a depth exceeding twice their height as they will cause problems for the rear-most seats beneath them.

Solution: Redesign the protruding surface to provide reflected sound to the affected seats or get rid of the protrusion.

4. DISTORTIONS:
These occur as a result of wildly varying absorption coefficients at different frequencies. This applies an undesirable change in the quality and tone colouration (of frequency distortions) to sound within the enclosure.

Solution: Balance the absorption coefficients of acoustical finishes over the whole audible range.

5. COUPLED SPACES:
When an auditorium is connected to an adjacent space which has a substantially different RT, the two rooms will form a coupled space. As long as the airflow is unrestricted between the two spaces, the decay of the most reverberant space will be noticeable within the least reverberant. This will be particularly disturbing to those closest to the interconnection.

Solution: Add some form of acoustic separation (a screen or a door) or match the RT of both rooms.

6. ROOM RESONANCE:
Room resonance is similar to distortions in that it causes an undesirable tone colouration, however, room resonance results from particularly emphasised standing waves, usually within smaller rooms. This is a significant concern when designing control rooms and recording studios.

Solution: Apply subtle changes in overall shape of the room or find out which surfaces are contributing and use large sound diffusers.

CONFLICTING REQUIREMENTS FOR SPEECH AND MUSIC:

SPEECH
  • The acoustics of a space designed for speech must primarily ensure definition and intelligibility, remembering, of course, that understanding in the speech communication process depends as much upon gesture and facial movements as it does on vocal projection.
  • The audience's expectations regarding the actual quality of the speech signal is not too critical, as long as the speaker's voice and accent are recognizable and the vocal information is understandable.
MUSIC
  • Music audiences, on the other hand, have inherited quite a developed expectation of particular sound qualities for various styles and eras of music.
  • Whilst definition is a prerequisite for speech, excessive clarity in music gives the subjective impression of brittleness or dryness. In addition, it accentuates unwanted bowing or fret noise, making the musicians’ job even more difficult.
  • Another difference is that music can consist of a great range of frequencies (20 Hz to 20 kHz) whilst speech is basically a narrow band signal (500 Hz to 4 kHz).



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