What do gymnasiums, indoor swimming pools, prisons, music and class rooms, water and sewage plants, tunnels, and mass transit terminals all have in common? They are all noisy!

Large volume areas with hard reflective surfaces may be functional, but when you introduce people and their activities the resulting noise is contained by the walls and the resulting condition is called reverberation.

All construction materials have some absorptive qualities, many like concrete, glass and steel are low on the absorption scale. Noise energy is easily reflected off of these materials within a confined space to produce an echo or reverberation effect. If, for example a construction surface such as concrete was perfectly flat, hard, and non-yielding, then all the acoustic energy (noise) would be reflected and the absorption coefficient would be zero.

On the other hand if there was an open window in the structure, the noise would not reflect at all, but would continue in an uninterrupted wave pattern until the energy was dissipated. The Absorption Coefficient would be 1.0. Neither case is practical, because all surfaces are somewhat porous and do yield under pressure, and an open window is not a practical solution to noise control problems.

Adding additional absorbing materials will serve the purpose of reducing the reverberant noise, but remember that the noise radiating from a source must first come to the absorbing material before it can be dissipated. Reverberation time is the duration in seconds before the acoustic energy has dissipated 60dB. Without getting theoretical, it follows that reverberant noise depends on the amount of energy initially generated, the volume of the room in which it occurs, and the amount of absorption that takes place at the boundaries of the room (A living room with wall-to-wall carpeting and drapes is "softer" than a bare room.

The addition of soft absorptive materials in addition to the normal construction will reduce the effect of reverberation and noise, by reducing the reflection of the sound energy within the confines of the room. Materials such as mineral wool, thick fabrics, carpets, some woods, and people can be placed in a room to reduce reflective noise. Studies of various materials have resulted in a list of common construction materials and their corresponding Coefficient of Absorption.

The proper placement of acoustic absorbing materials to capture the reverberant noise is important. Noise radiates from its source much like a growing bubble until it strikes a reflective surface in a room. Most noise sources i.e. people, machines, music etc. are near the flat surface and will radiate upwards and outwards. Placement of noise absorbing materials on the upper walls is important to reduce the initial reflected and radiated energy.

The reverberation of any room can be accurately calculated knowing the volume of the room and the building materials on the ceiling, floor, and walls. The calculations can be repeated with the proposed addition of acoustic absorbing materials added to the upper walls and ceiling. The resulting new lower reverberation time and reduction in noise levels can be established before the materials are applied to the room. This "what if" calculation is helpful to the architect and the user to establish the rooms acoustics before retrofit.

Sabines

A sabine is the term used to express 100% sound absorption per square foot of surface. The amount of sabines is simply the sound absorption coefficient multiplied by each square foot of surface. A plaster wall 10’ wide by 10’ high having a sound absorption coefficient of 0.02 could then be stated as having a total absorption of 2.0 sabines. For convenience in industrial and architectural noise control problems, the absorption coefficient at 500 cycles per second, (Hz) is often used. An absorbing panel 30" wide by 8’0" long, or 20 sqare feet of area which has a coefficient of 1.55 at 500 cycles per second could be called 31 sabines. To be a bit more conservative, we use an average of coefficients in the 250, 500, 1000, and 2000 cycles per second bands. This would reduce the same absorbing panels to approximately 25.0 sabines.

Through study and experimentation it has been concluded that reverberation time in seconds is equal to 0.05 times the volume of the room divided by the total number of sabines in the room.

T=.05V / A

A = sabines
T = reverberation time in seconds
V = volume of the room

 

Reverberation time will reduce as the number of sabines is increased. The change in noise levels of the reverberant field (far field) is expressed as noise reduction and is equal to 10 times the log. Of the final amount of sabines divided by the original amount of sabines:

N.R. = 10log A2 / A1

N.R. = Noise Reduction
A2 = Sound absorption after acoustic treatment (sabines)
A1 = Sound Absorption after treatment (sabines)

 

The noise reduction of the far field will be 3.0dB if we double the number of sabines by adding extra absorption. It will be 6.0dB if we quadruple the amount of existing sabines, and 10.0dB if we add ten times the sabines.

In conclusion, the addition of absorbing materials to a room or facility with hard surfaces will reduce the reverberation and noise levels. This reduction can be accurately calculated and asist the builder to best determine the acoustic results. There are a number of commercial noise absorbing products available, and one should consult a directory such as Sweets (Section 09500) for their source.

Conclusion


In conclusion, the addition of absorbing materials to a room or facility with hard surfaces will reduce the reverberation and noise levels. This reduction can be accurately calculated and asist the builder to best determine the acoustic results. There are a number of commercial noise absorbing products available, and one should consult a directory such as Sweets (Section 09500) for their source.

 

 

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