Performance of the Anechoic Room of the Parmly Sound Laboratory

The performance of the anechoic room of the Parmly Sound Laboratory has been measured to determine the characteristics of rooms of small volume with wedge-covered walls. The inverse square law holds to within ±1 db to 6.5 feet from 60 to 24,000 c.p.s., although the theoretical cut-off frequency of the room is 115 c.p.s. At 100 c.p.s., the radiation resistance of a loudspeaker placed near the walls was found to be 18 percent above the free-field value, and the radiation reactance was approximately 5 percent above the free-field value. The differences disappeared two feet from the wall. The wall transmission has been measured as a function of frequency, and the wedges themselves are found to act similarly to a thin plate in optics, having a maximum transmission at 110 c.p.s.