Frequency Dependence of Different Ways to Obtain Diffuse Sound Fields in Reverberation Chambers

Abstract

In the reverberation chamber of the Physikalisch-Technisehe Bundesanstalt (PTB), the absorption coefficient of two absorbing arrangements has been measured for testing different states of equipment of the room with diffusing elements. The room has fixed convex cylindrical segments on two walls and the ceiling. In addition, movable spherical segments can be fastened to the walls and/or diffusing elements can be suspended in the space. With uncovered mineral-wool slabs as the testing material, additional space diffusors change the resulting absorption coefficient more or less in the frequency range above 200 cps. Decrease or increase of the absorption coefficient depends on the test-area size and on the number of diffusors. In the second arrangement, a cover of linoleum was fixed on the same slabs, thus performing an efficient low-frequency absorber with a resonance at 140 cps. With diffusors of 2 m2 each present in the space (their total area amounting to at least 20 m2), reasonable results were gained. The available cylindrical and spherical segments on the boundaries could not produce sufficient randomness in the low-frequency range.