Table 2. Average sound absorption coefficient at each frequency range and NRCs

Average Sound absorption coefficient@ frequency range	Group	Filling height (cm)
		6	8	10	12
α250–500	L	0.22	0.32	0.51	0.68
	M	0.27	0.40	0.58	0.80
	S	0.20	0.29	0.51	0.71
α500–1,000	L	0.53	0.74	0.65	0.53
	M	0.64	0.82	0.72	0.55
	S	0.51	0.77	0.67	0.53
α1,000–2,000	L	0.33	0.38	0.39	0.57
	M	0.44	0.38	0.51	0.62
	S	0.34	0.36	0.44	0.61
α2,000–6,400	L	0.50	0.66	0.76	0.75
	M	0.71	0.73	0.73	0.81
	S	0.58	0.72	0.68	0.76
NRC	L	0.49	0.43	0.43	0.58
	M	0.59	0.48	0.49	0.58
	S	0.57	0.44	0.41	0.53

NRC: noise reduction coefficient.