Example of the Dependence of Sound Absorption on the Area of Sample

The sound absorption in a solid concrete room was measured by the reverberation method, first with the room entirely bare, then with a “border” of $114‐in.$ Acousti‐Celotex (46 ft² in area) around the ceiling, then with a “border” added around the top of the walls (total acoustical tile 112 ft²), and finally with the entire ceiling and one‐third of the side walls covered (265 ft² of acoustical tile). The test sound, generated by mechanical impact, was filtered on reception into third‐octave and half‐octave bands. The volume of the rectangular room was 1350 ft³ and its area 793 ft². With the border of 46 ft² the coefficients determined in the customary fashion by the Sabine reverberation equation were near those published for $114 in.$ Acousti‐Celotex. At 125 and 8000 cps the coefficients changed little with the area of sample. At 1000 cps, however, the measured coefficient dropped from 0.93 to 0.44, to 0.25 as the sample area was increased. This is possible evidence for a change in effective mean free path and for the need of an appropriate adjustment in the reverberation formula. The name Sabine absorption coefficient is suggested for the usual coefficient computed by the reverberation technique, to distinguish it from the energy absorption coefficient that does not exceed unity.