Tuesday, August 4, 2009

Acoustics of Auditorium

ACOUSTICS OF BUILDING
 
We know that a lamp which gives brighter illumination of surfaces in a room if the walls and ceilings are sufficiently light in colour, because of larger reflection. A similar effect shall occur in sound waves also that a given source will produce a greater average intensity when the absorption is low. 

If the sound emitted from the source is stopped, the sound will not stop instantaneously, but instead the receiver will continue to pickup the successive reflections until they are completely weak. The louder the original sound, the longer this process takes place. 

This gradual dying of the sound wave is known as reverberation and it is most important factor to have good acoustics in buildings. There are other factors also which affects the acoustics of a building and they are loudness, resonance, reflections, echelon effect extraneous effect and focusing due to walls and ceilings.

DESIGN PROCEDURE FOR AN AUDITORIUM WITH GOOD ACOUSTICS

Site consideration

Plan the auditorium on quiet exposure, far away from high ways, flight path, and railway stations.

Location within the building

Use corridors and quiet buffer spaces to isoloate the auditorium. Treat corridors and lobbies with sound absorbing materials. All doors should be solid and bracketed around their entire perimeter.

Purpose

Depending on the purpose a good frequency response sound system will be required.

Volume

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.

Reverberation time

Sabine’s formula for reverberation time is used for suitable acoustic treatment. Where ‘V’ is the volume of the hall, ‘S’ is the surface area and ‘a’ is the absorbing coefficient.

Ceiling

Central area of the ceiling should be sound reflecting. The perimeter and rear to be provided with sound absorbing materials like acoustic tiles.

Sidewalls

Sidewalls should be sound reflecting and diffusing with as many irregularities as possible. For example, making doorway wider at one side of the wall keeping windows etc., and the back wall is treated with deep sound absorbing finish.

Floor

All aisles are carpeted except in front of the stage to make full noise control. Fabric upholstered seats are used. Absorptive and cushioned seats will give stable reverberation.

Balconies

Use balconies to increase seating capacity and to reduce the distance to the farthest row of seats.

Sound reinforcement system

In large halls a sound amplification system to reinforce the sound to a weak source in a large room is required. In addition there should be adequate loudness in every part of the auditorium uniform distribution (diffusion) of sound energy in the room. The hall should be free from echoes, long delayed reflections, flatter echoes, sound concentrations, distortions, and sound shadow and room resonance.

Seats

To make the hearing conditions satisfactory when the room is full or partly full, upholstered seats with absorbing material at the bottom are used, so that the absence or presence of audience does not affect the reverberation time.

FACTORS AFFECTING THE ACOUSTICS OF THE BUILDING

Reverberation is one of the important factors that affect the acoustics of a building. Besides reverberation there are other factors like loudness, focusing, echelon effect, extraneous noise and resonance.

Loudness

Suppose 1000 persons can hear the speech of a person in an auditorium, but there will not be any uniform sound distribution. So to ensure uniform distribution of sound intensity in the hall electrically amplified loudspeakers are used. These speakers are kept in different places in the auditorium and are kept at a higher than the speaker’s head. Amplifiers shall make the low frequency tones more prominent and hence the amplification has to be kept low. 

Focusing

The presence of cylindrical or spherical surface on the wall or the ceiling gives rise to undesirable focusing. In hall, the observer receives sound waves from the speaker along the direct path and the observer also receives the sound waves after reflection from the ceiling. 

Thus the intensity of sound received by the observer is comparatively higher than other positions in the auditorium. It may also happen that the direct and the reflected waves are in the opposite phases. This results in minimum intensity of sound at the observer. Further the direct and the reflected waves may from a stationary wave patterns. This causes uneven distribution of sound intensity.

Echelon effect

If there is regular structure similar to a flight of stairs or asset of railways in the hall, the sound produced in front of such a structure may produce a musical note due to regular successive echoes of sound reaching the observer. Such an effect is called echelon effect. If the frequency of this note is within the audible range, the listeners will hear only this note prominently. To avoid echelon effect, the staircase must have to be covered with carpets.

Extraneous noise

The extraneous noise may be due to the sound received form outside the auditorium and the sound produced by fans inside the auditorium. The external sound cannot be completely eliminated but can be minimized by using double or triple windows and doors. Proper attention must be given to maximum permissible speed of time and the rate of air circulation in the room. The air conditioning pipes should be covered with corks and insulated acoustically form the main building.

Resonance

The acoustics of a building may also be affected by resonance. If the resonance is of any audio frequency note the intensity of the note will be entirely different from the intensity desired. Resonance is inversely proportional to the square root of volume of the hall. So if the hall is of large size the resonance frequency is much below the audible frequency limit and harmful effect due to resonance will not be affected

REVERBERATION TIME (RT)

In designing an auditorium to its acoustics demands, the control of reverberation time is the most important factor. Due to multiple reflections of sound waves from the ceiling, floor, walls of the enclosures, the sound waves persist for a certain time even after the sound has stopped. This persistence of sound is called the reverberation time. 

The reverberation time is defined as the time the sound takes to fall by 60 dB when the source is stopped. The reverberation time is also defined as the time taken by the sound energy to fall to 10-6 of its initial value. 

The optimum value of the reverberation time varies according to the different types of sounds like music, speeches, drama, cinema, lecture etc., If the reverberation exceeds its optimum value, then the speech will be completely unintelligible, and if the reverberation is lower than the optimum value the auditorium will be somewhat dead. 

So the auditorium must be designed in such a way that it could have the optimum reverberation time.To have optimum reverberation time the auditorium must be furnished with good acoustical finishes, occupants, room furniture, curtains etc., Since the audience provide most of the absorption, the auditorium is vacant or partially vacant the hearing condition will not be satisfactory. So to make the hearing conditions satisfactory, when the hall is full or partially full, upholstered seats with the bottom of the seats with absorbing materials are provided so that the absence or presence of audience does not affect the reverberation time. 

In an auditorium reverberation time can also be maintained by eliminating unwanted echoes, focusing effects of curved surfaces, flatter echoes etc., Echoes, long delayed reflections and flatter echoes can be prevented by fixing sound absorbing materials upon the defect producing reflective surfaces. Large unbroken concave surfaces should be avoided or treated with good sound absorbing materials.

DETERMINATION OF REVERBERATION TIME

The reverberation time can be determined experimentally by exciting the hall with a sound source. The decay of intensity sound waves is recorded on a logarithmic scale on level recorder.

The excitation source may be a pistol shot or a wide band sound noise generated with or without a filter. The receiver apparatus consists of a microphone, a frequency analyzer and a level recorder. The loud speaker is placed in one of the corners of the room. The microphone must be placed away from the loudspeaker and not too close to the reflecting surface. The graph is drawn between the decay of sound intensity in logarithmic scale and the time taken. From the graph we could be able to determine the optimum reverberation time for the auditorium.

SOUND ABSORPTION

Sound absorption is a process in which sound energy is converted partly into heat and partly in to mechanical vibrations of the material. Carpets, suspended space absorbers and interchangeable absorption panels in rooms and buildings can absorb unwanted sound.

The sound absorption coefficient of a material of a material is defined as the decimal fraction of perfect absorption. Suppose if the sound absorption coefficient=0.6, then it means that there is 60% absorption. It is the efficiency of a material in absorbing sound energy at a specified frequency and varies with the angel of incidence and the thickness of the material. An open space is taken to the standard of unity absorption coefficient.



2 comments:

Anonymous said...

super

vallabh said...

too.. long but cool!!!