Wind turbine acoustics – Thai Wind Energy Association

dB(A) Sound Levels in decibels and Sound Power in W/m2

Level dB(A)	Power W/m 2	Level dB(A)	Power W/m 2	Level dB(A)	Power W/m 2
0	-121,000*10	55	-73,162*10	83	-41,995*10
10	-111,000*10	56	-73,981*10	84	-42,512*10
20	-101,000*10	57	-75,012*10	85	-43,162*10
30	-91,000*10	58	-76,310*10	86	-43,981*10
31	-91,256*10	59	-77,943*10	87	-45,012*10
32	-91,585*10	60	-61,000*10	88	-46,310*10
33	-91,995*10	61	-61,259*10	89	-47,943*10
34	-92,512*10	62	-61,585*10	90	-31,000*10
35	-93,162*10	63	-61,995*10	91	-31,259*10
36	-93,981*10	64	-62,512*10	92	-31,585*10
37	-95,012*10	65	-63,162*10	93	-31,995*10
38	-96,310*10	66	-63,981*10	94	-32,512*10
39	-97,943*10	67	-65,012*10	95	-33,162*10
40	-81,000*10	68	-66,310*10	96	-33,981*10
41	-81,259*10	69	-67,943*10	97	-35,012*10
42	-81,585*10	70	-51,000*10	98	-36,310*10
43	-81,995*10	71	-51,259*10	99	-37,943*10
44	-82,512*10	72	-51,585*10	100	-21,000*10
45	-83,162*10	73	-51,995*10	101	-21,259*10
46	-83,981*10	74	-52,512*10	102	-21,585*10
47	-85,012*10	75	-53,162*10	103	-21,995*10
48	-86,310*10	76	-53,981*10	104	-22,512*10
49	-87,943*10	77	-55,012*10	105	-23,162*10
50	-71,000*10	78	-56,310*10	106	-23,981*10
51	-71,259*10	79	-54,943*10	107	-25,012*10
52	-71,585*10	80	-41,000*10	108	-26,310*10
53	-71,995*10	81	-41,259*10	109	-27,943*10
54	-72,512*10	82	-41,585*10	110	-11,000*10

Sound Level by Distance from Source

Distance m	Sound Level Change dB(A)	Distance m	Sound Level Change dB(A)	Distance m	Sound Level Change dB(A)
9	-30	100	-52	317	-62
16	-35	112	-53	355	-63
28	-40	126	-54	398	-64
40	-43	141	-55	447	-65
50	-45	159	-56	502	-66
56	-46	178	-57	563	-67
63	-47	200	-58	632	-68
71	-49	224	-59	709	-69
80	-50	251	-60	795	-70
89	-51	282	-61	892	-71

How to use the table above:
If a wind turbine has a source noise level of 100 dB(A), it will have a noise level of 45 dB(A) 141 m away. [100 – 55 dB(A) = 45 dB(A)].
The sound level decreases by approximately 6 dB(A) [ = 10*log 10 (2) ] every time you double the distance to the source of the sound. The table assumes that sound reflection and absorption (if any) cancel one another out.
How to derive the table above:
The surface of a sphere = 4 pi r2 , where pi = 3.14159265, and r is the radius of the sphere. If we have a sound emission with a power of x W/m 2 hitting a sphere with a certain radius, then we’ll have the same power hitting four times as large an area, if we double the radius.

Adding Sound Levels from Two Sources

dB	41	42	43	44	45	46	47	48	49	50
41	44.0	44.5	45.1	45.8	46.5	47.2	48.0	48.8	49.6	50.5
42	44.5	45.0	45.5	46.1	46.8	47.5	48.2	49.0	49.8	50.6
43	45.1	45.5	46.0	46.5	47.1	47.8	48.5	49.2	50.0	50.8
44	45.8	46.1	46.5	47.0	47.5	48.1	48.8	49.5	50.2	51.0
45	46.5	46.8	47.1	47.5	48.0	48.5	49.1	49.8	50.5	51.2
46	47.2	47.5	47.8	48.1	48.5	49.0	49.5	50.1	50.8	51.5
47	48.0	48.2	48.5	48.8	49.1	49.5	50.0	50.5	51.1	51.8
48	48.8	49.0	49.2	49.5	49.8	50.1	50.5	51.0	51.5	52.1
49	49.6	49.8	50.0	50.2	50.5	50.8	51.1	51.5	52.0	52.5
50	50.5	50.6	50.8	51.0	51.2	51.5	51.8	52.1	52.5	53.0

Example: A turbine located at 200 m distance with a source level of 100 dB(A) will give a listener a sound level of 42 dB(A), as we learned in the table before this one. Another turbine 160 m away with the same source level will give a sound level of 44 dB(A) on the same spot. The total sound level experienced from the two turbines will be 46.1 dB(A), according to the table above.
Two identical sound levels added up will give a sound level +3 dB(A) higher. Four turbines will give a sound level 6 dB(A) higher. 10 turbines will give a level 10 dB(A) higher.

How to add sound levels in general

For each one of the sound levels at the spot where the listener is located, you look up the sound power in W/m 2 in the first of the three sound tables. Then you add the power of the sounds, to get the total no. of W/m 2 . Then use the formula dB = 10 * log 10 (power in W/m 2 ) + 120, to get the dB(A) sound level.

Source : http://www.windpowerwiki.dk/