2.4.4 A-weighting (dBA) and C-weighting (dBC) | Owners Corporation Network

2.4.4 A-weighting (dBA) and C-weighting (dBC)

Loudness is the human impression of the strength of a sound. The loudness of a noise does not necessarily correlate with its sound level.

Decibels (dB) are a measurement of sound intensity over the standard threshold of hearing. However, you will often see noise levels given in dBA (A-weighted sound levels) instead of dB.  dBA is sound intensity with an "A" contour filter. The filter adjusts the measurement to account for the way in which the ear responds to different frequencies of sound. Measurements in dBA, or dB(A) as it is sometimes written, are decibel scale readings that have been adjusted to attempt to take into account the varying sensitivity of the human ear to different frequencies of sound. The sensitivity of the human ear to sound depends on the frequency or pitch of the sound. People hear some frequencies better than others. The main effect of the adjustment is that low and very high frequencies are given less weight than on the standard decibel scale.

Many regulatory noise limits are specified in terms of dBA, based on the belief that dBA is better correlated with the relative risk of noise-induced hearing loss.

Another system of adjustment is C-weighting, the dBC scale. dBC is sometimes used for specifying peak or impact noise levels, such as gunfire. Unweighted dB readings are also used for this purpose; there is usually not much difference between the two.

Various noises and their sound level measurements

Noise Source

Sound Pressure,Pa

Weighted, dB(A)

pneumatic chipper at 1 metre

10

115

textile room

5

103

newspaper press

1

95

power lawn mower at 1 metre

0.8

92

diesel truck 50 km per hour at 20 metres

0.5

85

passenger car 60 km per hour at 20 metres

0.05

65

conversation at 1 metre

0.02

55

quiet room

0.002

40

baseline

0.00002

55

Sound pressure levels in decibels (dB) or A-weighted decibels [dB(A)] cannot be added or subtracted in the usual arithmetical way.

If there are two sound sources in a room - for example a lively discussion producing an average sound level of 62.0 dB, and a television producing a sound level of 73.0 dB - then the total combined sound level is a logarithmic sum = 10 x lg ( 10^(62/10) + 10^(73/10) ) = 73.3 dB.