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2.4.2 Physiological and subjective acoustics

The sound pressure level, in decibels, of a sound or noise is 20 log10 (p1/p0) where p1 is the root-mean-square (r.m.s.) sound pressure and p0 is the reference sound pressure of 20 μPa (see ISO 131:1979).
   The A-weighted sound pressure level, in decibels (dB(A)), is defined in the same way except that p1 is the r.m.s. sound pressure of the sound or noise as modified by a filter with a standardised frequency characteristic known as the A-weighting. This provides a measure of the noise more closely related to the frequency response of the human ear. Values in the following table are taken from IEC 651:1979.



 

Filter characteristics for A-weighting

 

Frequency/Hz

A-weighting/dB

Frequency/Hz

A-weighting/dB

   31.5

− 39.4

1 000

   0.0

63

− 26.2

2 000

+ 1.2

125  

− 16.1

4 000

+ 1.0

250  

  − 8.6

8 000

− 1.1

500  

  − 3.2

16 000  

− 6.6




The subjective loudness of a sound as a function of frequency, as perceived by the human ear/brain system, varies with the level of the sound. The figure overleaf is taken from ISO 226:1987 and shows equal-loudness level contours measured for young listeners with normal hearing for sinusoidal plane sound waves coming from directly in front of the listener; the sound pressure level is the level in the undisturbed wave.
   Each contour on the graph overleaf is labelled with the loudness level, in phons, which refers each contour to the sound pressure level at 1000 Hz. The A-weighting characteristic roughly mirrors the 40 phon contour. The minimum audible field (MAF), which is illustrated by a broken curve, represents the threshold of hearing and is tabulated overleaf.


(Click the Image to view Larger Image)
Normal equal-loudness level contours for pure tones (binaural fee-fielding listening








 

Minimum audible field

 

Frequency/Hz

MAF/dB
(re 20 μPa)

Frequency/Hz

MAF/dB
(re 20 μPa)

   31.5

56.3

1000

4.2

63

35.5

2000

1.0

125  

20.7

4000

− 3.9   

250  

11.2

8000

15.3  

500  

  6.0

 

 


Sound at high levels is capable of causing temporary and permanent damage to hearing. Susceptibility to damage differs between individuals, but it can be noted that the European Community Directive (1986) on workplace noise requires that preventative action be taken if a worker is exposed to an A-weighted sound pressure level (averaged over an eight-hour day) of more than 85 dB(A).




References

European Community (1986) Directive 86/188/EEC, OJ No L 137/28-34 of 24-5-1986.
IEC 651:1979 Sound level meters.
ISO 131:1979 Acoustics–Expression of physical and subjective magnitudes of sound or noise in air.
ISO 226:1987 Acoustics–Normal equal-loudness level contours.

G.R. Torr

 

 

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