The frame around a test specimen and its influence on the measured sound absorption coefficient

For most practical cases of reverberation room measurements of sound absorption coefficient according to ISO 354:2003, the standard requires the edges of the test specimen to be covered by a reflective frame. The purpose of this requirement is to keep the edges of the test specimen from absorbing sound. While this statement is consistent with practical measurement results at medium to high frequencies, it has been shown that the low frequency performance of a high-absorbing test specimen worsens when a frame is not in place. To further investigate this phenomenon, a series of reverberation room measurements have been carried out on several variations of a porous absorber test specimen. The goal of the measurements has been to determine whether the size and dimensions of the test specimen affect the test results, and if any such affections are dependent on the presence of a frame. Additionally, the sound absorption area of the frame itself has been measured to elucidate whether the frame is likely to give a significant contribution to the total sound absorption area of a framed test specimen. The results will be presented and discussed in this paper.