Air Absorbtion Excess Attenuation

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Aa, the attenuation due to air absorption is dependent upon temperature and relative humidity.  The values used in the PEN noise model are based on Sutherlan, LC JF Piercy, H.E. Bass & L.B. Evans 1974. Method for calculating the absorption of sound by the atmosphere.  Journal of the Acoustical Society of America 56, Supplement 1 (abstract).

 

       Aa  = m r

 

where

       m is the absorption per m; and

       r is the actual distance from source to receiver

 

Attenuation due to Atmospheric Absorption

Relative Humidity %

Temperature (c )

dB per 1000m for Frequency (Hz)

 

 

63

125

250

500

1000

2000

4000

8000

25

15

0.2

0.6

1.3

2.4

5.9

19.3

66.9

198.0

 

20

0.2

0.6

1.5

2.6

5.4

15.5

53.7

180.5

 

25

0.2

0.6

1.6

3.1

5.6

13.5

43.6

153.4

 

30

0.1

0.5

1.7

3.7

6.5

13.0

37.0

128.2

 

 

 

 

 

 

 

 

 

 

50

15

0.1

0.4

1.2

2.4

4.3

10.3

33.2

118.4

 

20

0.1

0.4

1.2

2.8

5.0

10.0

28.1

97.4

 

25

0.1

0.3

1.2

3.2

6.2

10.8

25.6

82.2

 

30

0.1

0.3

1.1

3.4

7.4

12.8

25.4

72.4

 

 

 

 

 

 

 

 

 

 

75

15

0.1

0.3

1.0

2.4

4.5

8.7

23.7

81.6

 

20

0.1

0.3

0.9

2.7

5.5

9.6

22.0

69.1

 

25

0.1

0.2

0.9

2.8

6.5

11.5

22.4

61.5

 

30

0.1

0.2

0.8

2.7

7.4

14.2

24.0

58.4

 

PEN Implementation

 

The distance is the calculated distance from the source to the receptor.  It ignores any intervening terrain (or barriers) but places the source and receptor at the appropriate location in three dimensional space.  The model then applies the appropriate air absorption attenuation for the prevailing meteorology.  The default temperature is 20 C and 50% relative humidity. 

 

Note since this is an attenuation, the negative sign in the original equation 1 becomes positive.