What is an Audio Weighting Curve?
Weighting curves are used as an accurate alternative to simply measuring a dB SPL level, which works out a sound pressure average, but does not account for different sensitivity to different frequencies.
"A" weighting curve
The "A" weighting curve, expressed as dBA, represents the way that humans realistically hear. It particularly gives a realistic representation of our frequency perception at low noise levels. It provides compensation for that low level sensitivity against the frequency curve of the ears perception. When measuring or predicting possible damage to the ear, the "A" weighting curve gives the most accurate representation of our hearing from which to assess it.
"A" weighting is also typically used when assessing public disturbance and excessive noise levels in an urban area.
"A" weighting is considered the same as a 40 Phon curve as is found in the Fletcher and Munson curves. These curves determine how physically loud a frequency must be in order to be perceived as being the same loudness as another frequency, relative to the absolute volume or reference level.
The difference between an A weighting curve and a 40 Phon curve is the following:
- A weighting cuts off frequencies so that it represents how we hear. It is the inverse of a Fletcher and Munson curve.
- Fletcher and Munson curves boost frequencies
"C" weighting curve
C weighting curves, expressed as dBC are different to A weighting curves in the respect that they work better at higher absolute sound pressure levels, and are therefore more sensitive to low frequencies than the A weighting curve. This results in a much flatter response curve than the A weighting curve, as at higher SPL's, (Sound Pressure Level) the frequency response of the ear flattens out considerably. C weighting is sometimes used when assessing industrial noise, and the threat it poses to workers who are exposed to a high SPL particularly over 100dB SPL for a long period of time. However, dBC is used rarely now because of alternatives like dBA being universally considered the standard for audio weighting measurements.
"B" weighting curve
B weighting curves, are an approximation of the 70 phon curve. It simulates the response according to medium level noise. Because of this, there is less filtering of low frequencies, but still more than dBC. It can be used when measuring a loud noise from a further distance away, but when it is still loud enough to disturb things such as normal conversation. dBB is very rarely used in modern days as it has also been universally replaced by dBA.
"Z" weighting curve
Z weighting curves, expressed as dBZ, range from 10Hz to 20 kHz are completely different from any other type of weighting curve, as the curve is completely flat. This means that it gives a completely neutral reading to any sound, and does not take the sensitivity of the human ear at different frequencies into account. Z weighting can therefore be used when trying to determine the frequency response of a room, or the effectiveness of acoustic treatment, by measuring the reverberation time and other things.
"D" weighting curve
D weighting curves, expressed as dBD, were specifically designed to measure aircraft noise. At 10Hz, the filter is at -40dB, and steadily increases until it reaches about 15 kHz, then rolls off dramatically as the highest frequencies a human can perceive are passed.
There are quite a number of different audio weighting curves, yet almost all have had their time and passed on, as the dBA weighting scale becomes normal. Whether the use of dBA for every situation is good or not is another story.
Author: Duran (Port Elizabeth, South Africa)
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Edited by: Rajesh Bihani ( Find me on Google+ )