Acoustical defects

The perfect acoustical condition in a room or auditorium are obtained when an average sound rise to a suitable intensity with

a. no distortion of the original sound

b. die (decay) out quickly.

The following acoustical defects have to be remove or minimize in order to achieve a perfect acoustic in an enclosure.

A. Echoes

· Echoes cause disturbance and unpleasant hearing.

· An echo is sound repetition produced when a reflected sound waves from surface of wall, floor, ceiling coming from the same source reaches the ear just when a direct sound is already heard.

· The formation of echo normally happen when

a. the time lay between the 2 sounds is about 1/17 of a second.

b. the reflecting surfaces are situated at a distance greater than 15m.

c. the shape of the reflecting surface is smooth and curved.

· Echo can be overcome by using absorbent material (rough and porous) to disperse energy of the echo.

B. Reverberation

· Reverberation is the continuation of sounds caused by multiple reflections between the surfaces of an enclosure.

· Reverberation is not the same as an echo. The reverberative reflections are heard as an extension of the original sound.

· Hard surfaced rooms will have a longer reverberation time than rooms finished with sound absorbing materials.

· The reverberant sound combines with the direct sound to produce a continuing ‘ reverberant field ’.

· A room totally without reverberation is termed ‘ anechoic ’ and is achieved by using special absorption techniques at each surface.

Reverberation Time

· Reverberation time is the time taken for a sound to decay or diminish by 60dB (decibel) from its original level.

· The time taken for this decay in a room depends upon the following factors:

1. Areas of exposed surfaces
2. Sound absorption at the surfaces
3. Distances between the surfaces
4. Frequency of the sound

· Reverberation time is an important index for describing the acoustical quality of an enclosure.

· The correct reverberation time in between this two limit is called ‘optimum time of reverberation’.

· The ideal reverberation time depend on:

1. Size of an enclosure or room

Recommended Optimum Reverberation Time for various building.

Building	Reverberation (seconds)
Cinema / Theatre	1.3 – 1.5
Church	1.8 – 3.0
Conference	1.0 -1.5
Music Concert Hall	1.0 -1.5
Parliament / Assembly Hall	1.5 – 2.0

1. Types of activity

i. Speech: 0.5 – 1 seconds reverberation time

Short reverberation times are necessary for clarity of speech otherwise the continuing presence of reverberant sound will cause the speech to be blurred.

ii. Music: 1 – 2 seconds reverberation time

Longer reverberation times are considered to enhance the quality of music otherwise short reverberation will cause the sound ‘dry’ or ‘dead’.

Sabine Formula

Sabine Formula give predictions of reverberation time for rooms,

Reverberation Time, t = 0.16 V = 0.16 V

A S x ά

Where t = reverberation time (s)

V = volume of the room (m³)

A = total absorption of room surfaces (m² sabin)

= Σ (surface area x absorption coefficient)

= S x ά

S = total room surface area m²

ά = mean absorption coefficient of room surfaces

C. Sound foci

· There is a possibility for reflected sound rays to meet at a point called sound focus when sounds are reflected by a concave shape interior surface or dome ceiling of an enclosure.

· This causes a concentration effect for the reflected echoes and consequently a sound of large intensity at this spot.

· The spot of unusual loudness or intensity is known as sound foci.

· Remedy:

a. Absorbing materials should be used on reflecting surface.

b. Ensure the design throughout the interior hall has no concentration of reflected sound.

D. Dead Spot / Sound

· This is a side effect of the sound foci.

· Dead spot is the deficiency of reflected sound rays where these spot of low sound intensity causing unsatisfactory hearing to the audience.

· Remedy:

a. Use suitable diffuser to enable uniform sound distribution throughout the enclosure.

E. Exterior Noise

· This defect is caused due to poor insulation and partly due to poor planning.

· The exterior noise is carried inside the enclosure through the loose doors, windows and ventilation.

· Remedy:

a. This penetration of outdoor noise inside the enclosure can be corrected by properly planning the building location of the building with respect to the road.

b. Providing and adequate insulation for various components of the enclosure.

Room acoustic

· In domestic circumstances the internal acoustics of rooms are not usually critical, but in larger spaces such as lecture rooms, conference and music rooms or in assembly halls and auditoria the satisfactory use of space depends on satisfactory acoustic conditions.

· This is a problem of considerable complexity in auditoria and expert consultants will normally be employed to advice on the problem.

· There are the planning and shape of the room to give good path of sounds between the speakers or performers to the audience and the rate at which a sound dies away.

Direct sound

· Ideally all the occupants of a space should be able to hear by means of a direct path of sound between the speakers or performers and each individual occupant.

· In conference venues, council chambers or boardrooms, where any occupant may be called upon to speak, this is achieved by layout of seating so that all the occupants can see one another clearly.

· A long narrow board table will be much less satisfactory than circular arrangement.

· In rooms where there is a large passive audience, it is possible to improve the direct path of sound by raising the speaker and also by raking the seating so that each member of the audience is less obstructed by those in front.

· Sound reflected once from a space can, if it is directed in the right direction, useful augment the direct sound.

Shaping an auditorium to improve the path of sound from source to audience.

	Slope of ceiling directs sound to audience		Slope of ceiling directs sound to audience at back

Rake of seating improves direct paths to individual members of the audience  

· It is important that the extra distance which first reflections have to travel over the direct path should not be too great.

· If the additional distance is over 20m, the reflected sound will arrive at the listener noticeably later than the direct sound and give rise to an echo.

· Care is therefore needed in disposing reflecting surfaces to ensure intelligible reinforcement of sound without echo.

· In general reflecting surfaces should be flat, curved surfaces may give rise to sharp concentrations of sound in some places and lack of reinforcement elsewhere.

· In the same way that properly disposed reflecting surfaces can assist in distributing sound, inappropriate surfaces will have an undesirable effect.

· Cross-beams projecting below a general ceiling level are very undesirable since they prevent part of the ceiling from reflecting sound to the audience.

· Reflecting surfaces do not necessarily have to be part of the main fabric of the building.

· In very large halls it is not unusual to hang a reflector over the speaker’s position.

· The Building Research Establishment recommends as a minimum thickness and weight a surface of 25mm plasterboard, painted on the side used for reflection.

Hall Shapes

Generally, there are 3 types of shapes describe as an important feature for good acoustics of an auditorium:

a. Rectangular

Koerner Hall in Toronto, Canada

· A rectangular plan or ‘shoebox’ is the traditional shape of many successful older concert halls.

· The sound waves tend to establish themselves along the length of the hall and all listeners receive a strong component of direct sound.

· Reflectors can be used to direct sound to the rear of the hall and absorbers used to prevent unwanted reflections.

· Traditional ratios of dimensions for height, width and length are about 2: 3: 5.

b. Wide fan

Harrogate International Centre, UK

· A fan shape or short wide hall allows more people in the audience to be near the source of sound and gives them better views of the stage.

· Wide curves were used for the seating of ancient outdoor amphitheatres can give an acceptable acoustics.

c. Horseshoe

Reverso Squadra, Singapore

· A horseshoe shape is common in traditional opera houses where the tall concave shape at the back of the hall is broken up by tiers of seats and boxes.

· The audience and furnishings (seats) in these tiers also act as an absorber.

Дата добавления: 2015-11-14