The effect of a deep niche on the laboratory measured sound transmission loss

Round robin measurements in acoustical laboratories have shown that laboratory design plays a significant role in the determination of the sound transmission loss (TL). An important factor affecting the TL is the occurrence of a so‐called “deep niche.” A niche occurs when the depth of the test opening is greater than the thickness of the object under test. Mounting a specimen at one end of the test opening creates a niche which is formed by the test opening surfaces and the panel surface itself. Mounting the panel in the center of the test opening creates equal niches on each side of the panel, etc. The effect of the panel location in the test opening on the transmission loss as well as the effect of overall test opening length was studied. Under some conditions, differences as great as 8 dB were observed. Effects occurred only at frequencies below the coincidence frequency. Variation in the transmission loss as a function of position in the test opening was less for deep niches than for short niches.