Definitive study of acoustic absorption measurement - large reverberation chamber

This thesis report focuses on the Large Reverberation Chamber test in a team thesis project. This thesis project identifies the energy absorption properties of different acoustic materials by taking measurements through a few methods, namely the large reverberation room tests, the small reverberation room test and the impedance tube test. The objective of this thesis project is to set up all the various techniques of measurement and to compare the results obtained from the three different methods. A large reverberation chamber is a room with hard surfaces that reflect sound. The sound from a noise source is reflected many times to produce a non-directional or diffused sound field within the chamber. The sound power emitted from the noise source can be determined by measuring the sound level in the chamber corresponding to the characteristics of the room such as its volume, surface area and objects in the room. The reverberation time of the chamber can be calculated and be used later to obtain the coefficient of absorption for any test sample mounted in the room. The Australian Standards (1988) explained that when a sound source operates in an enclosed space, the level to which the reverberant sound builds up and the subsequent decay of the reverberant sound when the sound source is stopped, are governed by the sound-absorbing characteristics of the boundary surfaces and objects within the space. The Reverberation Time, T60  is the time taken for the sound intensity or sound pressure level to fall by 60 dB (Porges, 1977). In this project, the large reverberation room tests were carried out at ACRAN, a noise control and air conditioning company to obtain the reverberation time for a few acoustic materials. The reverberation time is in turn used to obtain the sound absorption coefficient. The large reverberation chamber measurement method was easily available and had been widely used to predict reverberant sound absorption coefficients. As early as 1950, comparison of results obtained using the reverberation method has been shown to agree closely to the acoustic impedance measurements (London). Moreover, Goldsmith (1999) claimed that the ‘reverberation chamber development will produce a test technique that is robust (i.e. highly repeatable, highly reproducible, and technically thorough).’ Due to the significant reliability of the large reverberation room test, this method and the impedance tube method are used concurrently to compare the absorption coefficients obtained by the recently developed small reverberation chamber method. The final outcome of this thesis project is to compare the sound absorption coefficient of acoustic materials using the Large Reverberation Room method, Impedance Tube Method and Small Reverberation Room method; and if the comparison of results shows little discrepancies, therefore confirming the validity of the Small Reverberation Room method. This thesis research revealed that the results obtained using the small reverberation room method shows large discrepancies in comparison to the results obtained using the large reverberation room method and the impedance tube method. Further research should be carried out by using different recording and analyzing software to confirm the validity of the small reverberation room method.