To comply with ISO 354:2003 during laboratory testing of sound absorbers, the edges of the test specimen shall be covered by a frame. The Standard states that the frame and the test specimen shall be of equal height. This is usually unproblematic, but exceptions occur, e.g., if the test specimen is of uneven height or very thin.

For this paper, several reverberation room measurements have been carried out to investigate the importance of the height of the frame compared to the height of the test specimen and how critical it is that the heights match.

Two low absorbing and two high absorbing test specimens of different heights were used in the experiments, and the tests carried out in six series, one for each test specimen and two series where two test specimens of different heights were mixed.

For each series, an initial measurement was performed with a frame that was flush with the surface of the test specimen, and the results considered the true absorption coefficient of that specimen. Comparative measurements were then carried out with no frame and with frames with set height deviations compared to the test specimen.

The results will be presented in this paper.