Auditorium design guidelines

“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 unprecedented large audience.”

Requirements for Good Acoustics

The following points are fundamental to the provision of good acoustics within a modern auditorium

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.

Guidelines for adequate loudnessThe 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.

Guidelines for adequate loudness

As a general rule, however, the gradient along aisles of sloped auditoria should not be more than 1:8 in the interests of safety. 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 msecand definitely not more than 80 msec.

Recommended Volume-per-SeatValues (m³) for Auditoria.

TYPE OF AUDITORIUM min opt max

for Speech 2.3m³ 3.1m³ 4.3m³Concert Halls 6.2m³ 7.8m³ 10.8m³Opera Houses 4.5m³ 5.7m³ 7.4m³Catholic Churches 5.7m³ 8.5m³ 12.0m³Other Churches 5.1m³ 7.2m³ 9.1m³Multipurpose Halls 5.1m³ 7.1m³ 8.5m³Cinemas 2.8m³ 3.5m³ 5.6m³

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, offered in small rooms, the application of surface irregularities is often difficult.In such cases a random distribution of absorbing material of the alternate application of reflective-absorptive materials provide an alternate, though less effective means. ceilings, serrated enclosures,protruding boxes, sculptured surface decorations, and deep window reveals must be provided.

Control of ReverberationRT = (0.161V)/A

where:

V = the volume of the enclosure (m³) and

A = the total absorption within the enclosure (Sabine).

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 auditoriumby enlarging or reducing it's volume (ex: raising or lowering the ceiling, using more balconies, etc). 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.

Control of Reverberation

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. when occupied, the absorption effect is reduced as it faces the floor whilst unoccupied if is folded up and faces the stage.

Better Placement of Reflectors:

The basic concern is the availability of direct sound to all the audience at the same time. To maintain this reflectors are best used.

Also to correctly placing these will enhance the indirect rays to reach the public at the earliest.

Elimination of Defects

The basic defects attributable to room geometry:echoes sound concentrations, sound shadowing, distortions, coupled spaces and room resonance.

SOUND CONCENTRATONSometime referred to as 'hot-spots', these are

caused by focused 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.

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.

SOUND SHADOWINGMost 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 rearmost seats beneath them.

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

Coupled SpacesWhen 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.

ROOM RESONANCERoom resonance is similar to distortions in

that it causes an undesirable tone colouration, however, room resonance results from particularly emphasized 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.

ELECTRO ACOUSTICSSystem SpecificationsAny such sound amplification system is expected to

meet the following criteria:It should properly transmit a wide range of frequencies (30 - 12000 Hz) in order to maintain the correct balance between fundamentals and harmonic frequencies. It should possess a high dynamic range, reproducing sounds from a whisper to a shout without distortion. It should remain undetected. The illusion of a natural sound source should be preserved in spite of the use of amplification equipment. It should be free from disturbing echoes and feedback.

TYPES OF SYSTEM ARRANGEMENTS

There are three types of arrangements CentralDistributed stereophonic

Centrally located

Also known as a high level system, this is essentially a single cluster of loudspeakers located near the source. Such a system gives maximum realism as the amplified sound, whilst increasing loudness and clarity, is still associated with the original source.

Distributed system

Basically a number of loudspeakers spaced throughout the auditorium. This is also known as a low level system as each individual speaker operates at a low amplification level to service only a small part of the whole audience.

stereophonic

Two or more loudspeaker clusters at strategic positions within the auditorium. Such systems are used when there are a number of different sources to be amplified or the source is quite mobile. By using two or more microphones, each connected to their own cluster of speakers, the spatial relationship between the sources is preserved in the amplified sound.

Guidelines for speaker placement

In placing loudspeakers, it must be remembered that:Every listener must have a sight line to the particular speaker designed to supply them with amplified sound.

A loudspeaker cluster (particularly the central type) may require a great deal of space. Concealed loudspeakers must be hidden behind sound-transparent materials such as thin cloth or gauze, never behind a reflective panel.

Loudspeakers should always radiate sound energy wholly at the audience, not at any sound-reflecting surfaces.

Loudspeakers should never be directed at the pickup microphone as this will inevitable result in a feedback loop.

Guidelines for speaker placement