Table 3. Sound absorption coefficients and NRC value for varying the air gap depths

Thickness (mm)	Air gap depth (mm)	Sound absorption coefficients at frequency (Hz)				NRC
		250	500	1,000	2,000
10	10	0.09	0.42	0.51	0.32	0.34
	20	0.17	0.73	0.27	0.32	0.35
30	10	0.28	0.53	0.41	0.32	0.41
	20	0.32	0.51	0.25	0.31	0.35

NRC: noise reduction coefficient.