The Team’s approach into the Definitive Study of Acoustic Absorption Measurement involved three methods; the Acoustic Impedance Tube, the Large Reverberation Room and the Small Reverberation Chamber. The particular interest and primary focus of this study is to obtain the sound absorption coefficient, α of the various acoustic materials that were tested and compare the results between the various methods involved.
There are currently two methods in measuring the energy absorption properties of acoustic materials; the impedance tube (standing wave ratio or two-microphone method) and the large reverberation room. The objective of the Small Reverberation Chamber is to explore the feasibility of performing acoustic absorption measurements in a smaller, cheaper and mobile environment with the aim of obtaining suitably close sound absorption coefficients which are comparable to those obtained from a standard large reverberation room.
The large reverberation room requires space and specific construction and the high cost incurred in acquiring such a facility provides a considerable limitation. The small reverberation chamber however, is cheap in construction materials and does not require heavy and precision construction and is easily dismantled and assembled for transportation purposes to the test site for setup and experimentation.
The chamber plywood planks were transported from the University of Queensland’s Mechanical Engineering workshop to the Acoustics lab for setup. The plywood planks were joined to the chamber base with 90° angle brackets. The sides and roof of the chamber were held together by self-tapping screws. Three-quarters of the roof were fixed to the sides to provide added stability and support, leaving a removable opening for access into the chamber for setting up test equipment.
Three materials were tested, Soundguard Sound Sorber/PU Foam and CSR Bradford ‘Supertel’ and ‘Ultratel’ Glasswool. The test results were compared to those obtained from the large reverberation room at ACRAN, a noise control and air-conditioning company at Richlands and the acoustic impedance tube which was housed in the Acoustics lab in the Division of Mechanical Engineering.
The tests were performed at one-third octave bandwidth frequencies ranging from 100Hz to 5000Hz. The reverberation time, T60 of the test materials in the small chamber was found to be substantially shorter as compared to the large reverberation room as was expected from previous studies. The Bradford Glasswool displayed a shorter reverberation time than the Sound sorber foam implying that the former had better absorption properties than the latter. The sound absorption coefficients were calculated to be reasonably similar at low frequencies of up to 250Hz in comparison to the acoustic impedance tube, the large reverberation room and the data from the material suppliers. However, the coefficients were found to be varying significantly at the medium and high frequencies. This shortcoming could be due to a number of reasons such as the intended use of the software and its application, and experimentation setup. The GNU Octave Band program adopted in this investigation was found to be limited by the smallest reverberation time measured and this affected the resulting sound absorption coefficient which was found to be significantly varying from the material supplier’s test data.
It was initially intended to compare both the GNU Octave Band and Maximum Length Sequence System Analyser (MLSSA) programs. However, the PC card for the MLSSA was not functioning and the decision was made to focus on the analysis of the test results based on the GNU Octave Band program which was used for the first time in small chamber testing instead. The results from the GNU program were varying from of the MLSSA program. As such, this opens up a possibility of a further investigation in the near future to verify the validity of the MLSSA program which was adopted in a previous study and which gave reasonably satisfactory results; and to propose the use of an upgraded version of the MLSSA program in comparing the sound absorption properties of acoustic materials found in a small chamber.