1/13

ACOUSTICAL DESIGN

2/13

Primary Structure of roomsVolume is application dependent;

Speech: 4 cu.m / person Concerts: 18 cu.m / person Parallel surafces to be avoided, to prevent flutter echo Rear walls of the room should not cause any reflection

on the stage, casuing echo

3/13

AUDITORIUM DESIGN: BASICSSize of auditorium: area of at least 0.5 sq.m/spectator Length of rows: maximum of 16 seats per aisle

25 seats/aisle is permissible if 1-side exit door of 1m is provided per 3-4 rows

Exit, escape routes: 1m wide per 150 people (min width: 0.8m)

Volume of room: according to acoustic requirement:- Playhouses : 4-5 cu.m/spectator Opera: 6-8 cu.m/spectator

4/13

5/13

AUDITORIUM DESIGN: General Design Considerations

1.VISUAL

2.VENTILATION

3.ACOUSTICAL

a.seating

b.stage

c. room shape

d.room walls

Open Stage Theatre

Box Set Stage

Restoration Apron Stage

Theatre in the Round

8/13

CONTROLLING RT RT =  0.161 V/A

Where:

R T = reverberation time in second

V = room volume in cubic meters

A = total room absorption

(A = a1A1 + a2A2 + a3A3 + a4A4 + …)

A: all surface areas

a: respective absorption coefficient

The optimum reverberation time of an auditorium is dependent on the use for which it is designed.

`

9/13

Some Absorption Coefficients

Frequency (Hz)

Material 125 250 500 1000 2000 4000

Concrete/brick 0.01 0.01 0.02 0.02 0.02 0.03

Glass 0.19 0.08 0.06 0.04 0.03 0.02

Plasterboard 0.20 0.15 0.10 0.08 0.04 0.02

Plywood 0.45 0.25 0.13 0.11 0.10 0.09

Carpet 0.10 0.20 0.30 0.35 0.50 0.60

Curtains 0.05 0.12 0.25 0.35 0.40 0.45

Acoustical board 0.25 0.45 0.80 0.90 0.90 0.90

10/13

AUDITORIUM DESIGN

11/13

AUDITORIUM DESIGN

Classic auditorium design: Elevated central speaker, balconies, coffered ceiling and stage curtain

12/13

AUDITORIUM DESIGN The auditorium is so shaped that the audience is as

close to the sound source as possible. A fan shaped auditorium with a balcony is

desirable to ensure a free flow of direct sound waves to listeners.

In an auditorium with cushioned seats and a sound absorbing rear wall for echo control, the average ceiling height H is usually H=20T where T is the mid frequency reverberation time in seconds.

Seating geometry is arranged to give all the audience good sight lines and at the shortest distance from the stage.

13/13

Steps for architectural acoustics within a space

1. Watch out for SOUND REFLECTIONS. Straight surfaces reflect sounds back into the central space making sound clarity muddy.

2. Select ACOUSTICAL TREATMENT carefully. Different materials absorb sound frequencies differently. Make sure your acoustical treatments are absorbing the right sound frequencies.

3. Diminish ECHOES when necessary. Be aware that sounds traveling within 30 milliseconds of each other are perceived without echo. Sounds traveling after the 30 millisecond threshold become echoes of the original sound.

4. Don’t let other building systems get in the way. NOISE CONTROL is important to keep in check as other building systems (like HVAC systems) operate. Keep such clashing noises to a minimum.

14/13

Steps for architectural acoustics within a space

5. Keep objects or other OBSTRUCTIONS out of the way. Objects that obstruct a sound path can block high frequency sounds. (Low frequency sounds can bend around objects.)

6. Get good PATTERN CONTROL. Make sure sound systems for a room get good sound coverage. This will prevent feed-back and other sound distortions.

7. For out-of-the-way listening areas get DISTRIBUTED SOUND SYSTEMS. Such “delay-fill” speakers operate with an electronic delay so the sound matches and is synchronized.

Room Acoustics Types of Auditorium:

Can be classified as follows –1) For Speech 2) For Music3) Multipurpose

Room Acoustics Behaviour of sound in an Enclosed Space :

Various phenomena which may occur are - 1) Attenuation due to distance 2) Audience absorption of direct sound 3) Surface absorption of direct and reflected sound 4) Reflection from re- entrant angle 5) Dispersion at modelled surface 6) Edge diffraction 7) Sound Shadow 8) Primary Reflection 9) Panel resonance 10) Inter- reflection, standing waves and reverberation 11) Sound Transmission

Acoustics for Speech :For Speech

1) Nature of Source of sound and its location is first to be considered.2) Unamplified speech sound normally Range from about 30 dB (whispering)

to about 60 dB (Lecture voice) when measured at a distance of 3m.3) Understanding depends upon the clear reception of a rapid sequence of

discreet sound, some are of which very short duration.

Power + Clarity = Intelligibility (able to be understood)

Power

Distance From Speaker

Directional Relationship to Speaker

Audience absorption of Direct Sound

Reinforcement by reflectors

Reinforcement by Loud Speakers

Sound Shadows

Clarity

Delayed Reflection

Echoes

Intrusive Noise

Ambient Noise Duplication of sound by Loudspekers

Reverberation

ELIMINATION OF DEFECTORSThe defects are:

• Echoes• Sound concentration• Sound shadow• Distortions• Coupled spaces• Room resonance

ECHOThese are probably the most serious and most common defect.

SOLUTION: Either alter the geometry of the offending surface or apply absorber or diffusion.

SOUND CONCENTRATION

Sometime 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.

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

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 rear-most seats beneath them.

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

DISTORTIONSThese occur as a result of widely varying absorption coefficients at different frequencies. This applies an undesirable change in the quality and tone coloration (of frequency distortions) to sound within the enclosure.

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

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.

ROOM RESONANCERoom resonance is similar to distortions in that it causes an undesirable tone coloration, 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.

AUDITORIUM DESIGN

The important factors which influence the acoustical design of an auditorium are:

• The volume.• The shape and • The sound absorption.

VOLUME OF THE AUDITORIUM• The volume of the auditorium should be in proportion to the intensity of the sound that is expected to be generated in the hall. In deciding the volume of the hall, its height plays a significant role than its length or breadth. The ratio of the height of ceiling to the width of the hall or room should be 2:3. This is on account of the fact that a small increase in height increases the volume considerably. The volume required for musical concert halls is larger than that required for halls to be used for speech alone. In case, however, the auditorium is to be used for both musical concerts as well as speech, the volume of the auditorium should be chosen as to have a value intermediate between the two. The following data may be used as a rough guide for deciding the volume of an auditorium:• For public lecture halls = 2.8 to 3.7 cu.m. per person• For cinema theatres = 3.7 to 4.2 cu.m. per person• For concert halls = 4.2 t 5.6 cu.m. per person

It has been recommended that the volume of an auditorium in cubic meters should not be greater than 5.7 times the number of seats provided in the hall.