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DESIGNING FOR ACOUSTIC PERFORMANCE
AWD Jongens MSc (Elec) Eng. 

AWD Jongens heads the Acoustics Laboratory, UCT. Since 1971, he has conducted research in various fields of acoustics, including architectural and building acoustics, noise and vibration control, transportation noise and physical and acoustical properties of materials. He is a consultant to government departments, local authorities, industry and private bodies and is a member of national and international standards committees. 

A Sound Environment
Requires Sound Thinking
Our environment has a significant influence on our feeling of well-being and our ability to perform what we intend to do, be it to work productively, play well or obtain proper rest. Unfortunately, all too often, buildings and even whole townships, are built without sufficient thought being given to how different activities may impact on each other - especially where noise is concerned. Do we ever bother to think why our work is so stressful? Very often this is because we try to concentrate on what we are doing while battling against a background of noise. The more one needs to concentrate on an activity the less one can tolerate noise. So also with sleep. We subconsciously expect that the walls around us will insulate us from our surroundings. If not, the intruding noise impedes what we are doing and imposes a certain amount of stress upon us. We work or sleep less efficiently causing irritation or even frustration if we are not able to reduce its effect. Most of us are unaware of what it is that is troubling us and do not know what can be done to improve the situation. This is aggravated by the fact that a great deal of confusion exists regarding the role of sound absorbing and sound insulating materials. 

Sound Absorption
We would feel uncomfortable if we had to work or sleep in a room with lots of echoes. We would also find it a strain to follow a speech in a reverberant lecture hall. We thus reduce the reverberance of the room by lining the walls of the room with just the right amount of porous sound absorbing material to make us feel comfortable. The material needs to be porous so as to permit conversion of sound energy into heat by frictional rubbing of the air particles against the interior structure of the material and by vibratory motion of the material. This process of converting sound energy into heat is what we call sound absorption. Not all porous materials are good sound absorbers. The material needs to have a specific porosity and fine structure for optimum sound absorption. Various porous materials are available with differing effectiveness in absorbing sound. Certain sponge-like materials and in particular fibrous materials, have been found to be very good absorbers of sound.

However, the absorbent treatment needs to be very thick to absorb low-frequency sounds. For example, in large-screen cinemas such as the IMAX the multi-track surround-sound loudspeaker system is designed to completely take over your aural environment and give you the illusion of "being there". It is vital that the walls, floor and ceiling do not exist - acoustically - to avoid reminding you that you are sitting inside a room. This is particularly important at low audio frequencies and yet is the most difficult to achieve. Very thick sound-absorbing treatment to the walls and ceiling of the IMAX was necessary, incorporating the most efficient fibrous sound- absorbing material. The treatment varied in thickness from 200mm to over half a metre. 

Sound Insulation
Sound transmits through a barrier such as the walls, floor and ceiling of a room by setting the barrier into vibration. This vibration reradiates as sound into the space on the other side of the barrier. The amount of vibration depends on the loudness of the sound, its frequency content and the mass of the barrier. The heavier the barrier the greater the sound insulation. However, the lower the frequency of sound, the heavier the wall will need to be. In fact, very low frequency sounds such as produced by a disco readily pass through conventional single brick skin or cavity walls. The only way to stop such low frequency sounds is to build two separate walls spaced far apart interspaced with sound absorbing material. The loudness of sound is attenuated in three stages: first as it passes through the one skin, then the sound is attenuated further by being partially absorbed within the large cavity, before yet further attenuation as it passes through the second skin. This type of construction is used in concert halls and was incorporated at the IMAX to ensure that no exterior city noise leaks into the theatre even if a helicopter flies low overhead. 

Lightweight office partitions use the same principle of two skins, normally plasterboard, spaced approximately 60mm apart with soundabsorbing glassfibre placed between the skins. The same degree of sound insulation as that of a single brick wall can readily be achieved in this way. Because sound-absorbing materials are porous and normally very light they do not, on their own, make effective sound barriers. 

The proper design of the acoustics of a building, appropriate to its intended use, requires a knowledge of the human response to sound and of the sound absorption and sound- insulation properties of materials. Such information is obtained from tests conducted on such materials according to national and international standards in special facilities available at the Acoustics Laboratory at the University of Cape Town and at the South African Bureau of Standards in Pretoria.