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Research & Development > eJournal > Novel Method of Determination of Absorption Coefficient

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3. Background to the acoustics of enclosures.

Acoustic properties of an enclosure* are typified on its ability to absorb and reflect sound. The most important quantifiable parameter in this context is known as the absorption coefficient, symbol 'a'. The conventional method of obtaining 'a' is by the measurement of the Reverberation Time (RT). This is the time in seconds it takes for an impulsive noise emitted by controlled experiment diminishes by 60dB.

The range of a varies from 0.1 for the average room to 0.9 for special low echo rooms. The theoretical range of a varies from 0 to 1. The lower its value the livelier the room the higher the value, the more sound will be absorbed by the surroundings.

A definition of a can be given. It is the ratio of sound energy impinging on to a wall Ii minus the sound energy absorbed by the wall la divided by the sound energy impinging on to the wall. As an example say 100 dB of noise energy reached a wall and it absorbed no energy, the value of CE would be zero. i.e. a = (100-100)/100 = 0. This room would be lively and diffuse sound equally over its entire volume. Conversely, if it absorbed all of the energy of 100 dB then the value of a will be 1. This room would be anechoic or devoid of echo and appear dead to sound reflections. Most rooms have values of a which lie between 0. 1 and about 0.3.

The conventional method of obtaining the acoustic properties of an enclosure has as a major requirement the measurement of reverberation time (RT). The procedure to obtain this is elaborate, time consuming and expensive in equipment. The procedure is given in this chapter. Having thus measured the RT, a can be obtained from the following formula which combines the RT, the room volume V, its surface area and m The formula was first empirically produced by Sabine and is now established in all standard acoustics textbooks and included in research papers for example in a research paper by McNulty[4], the expression is given as:

where S_ja_j is the surface area and the corresponding absorption coefficient of the jth surface of the material of the enclosure which has W different surfaces. These surfaces may be wood, curtains floor paneling etc.

In this paper, a novel method is introduced of obtaining the absorption coefficient of a room and avoiding a frontal attack on the need to measure the reverberation time as described in the last paragraph. This is achieved by predicting the decay of sound for a set of absorption coefficients, which cover their full range from 0.1 to 1 typical of the enclosure under consideration. A family of decay curves therefore, is obtained for the enclosure, thereafter the decay of sound is measured under controlled conditions and the resulting decay curve matched to one of the family of absorption coefficients. The resultant matched curve corresponds to the particular value of absorption coefficient.

The technique has been evaluated successfully in the main auditorium at 'The Institute of Technology Tun Hussein Onn' (ITTHO). This hall has been investigated for acoustic properties McNulty and Ghazali [4]. during an 18?month period, with consistent repeatable measurement during twelve separate investigations in a report commissioned by the Ministry of Education of Malaysia. The report summarizes extensive measurements of reverberation time and other acoustic properties.

Results comparing reverberation time with the measurements made in this paper is substantiated against independent investigations taken by [5] and [6] and also the extensive results by McNulty and Ghazali [4] all the above reports are independent research and exhibit different aspects of the ITTHO hall acoustics. In RT measurements a small difference of about 0.7 seconds exists between [4], [5] and [6]. The aim of this paper is therefore to establish a novel and repeatable method of determining the RT of an enclosure by assessing the sound decay in it.

It is postulated that this technique is more advantageous for the determination of acoustic properties than conventional methods in two main points. Firstly, the need to measure with pistol shots or similar devices is obviated, and secondly a survey of the noise decay in the enclosure is obtained, which is a requirement of the acoustic designation of a room in any case. This latter measurement is useful in planning speech reinforcement systems in auditoria.

Figure (1) shows in a block diagram the comparison of the two methods. The conventional method requires much in the way of calibration and adjustment of the system to ensure a 60 dB drop for the RT. It has also more sophisticated equipment. The records must be compiled and processed for the absorption coefficient. On the other hand the novel method postulated here, is direct, simple and endorsement of its efficacy is highlighted by the consistency of the results with that of the detailed traditional method. The direct and simple approach elicited in the Figure shows that all that is required is the decay of noise in the room and a family of curves of decay in which the absorption coefficient can be read off directly.