The main effect of this adjustment is that low and very high frequencies are given less weight than on the standard decibel scale.Ĭompared with dB, A-weighted measurements underestimate the perceived loudness, annoyance factor, and stress-inducing capability of noises with low frequency components, especially at moderate and high volumes of noise. Measurements in dBA, or dB(A) as it is sometimes written, are decibel scale readings that have been adjusted in an attempt to take into account the varying sensitivity of the human ear to different frequencies of sound. Because the human ear is not equally sensitive to all frequencies of sound, variations of the decibel sound-power scale. A 100 dB sound would be 10 10 (ten billion) times greater than the threshold of hearing. You will often see noise levels given in dBA (A-weighted sound levels) instead of dB. A sound measuring 40 dB on the decibel sound scale would be 10 4 times greater than the threshold of hearing. For this reason, sound levels in the low frequency end of the spectrum are reduced as the human ear is less sensitive at low audio frequencies than at high audio frequencies. Although dB is commonly used when referring to measuring sound, humans do not hear all frequencies equally. However, the B and C weights are only valid for pure signals (signals with a single frequency).Ī dBA is a weighted scale for judging loudness that corresponds to the hearing threshold of the human ear. The blue curve shows the gain for a type A weighting. The reference quantity remains the same 20 µPa and the units are still dB SPL, but each value has a different gain depending on the frequency in order to better represent human auditory perception. Indeed, the ear of a healthy person is more sensitive to frequencies between 2 and 5 kHz. However, the human ear does not perceive all frequencies in the same way. Thus, the reference quantity is the smallest pressure change detectable by the ear (hearing threshold), 20 µPa in air, which corresponds to 0 dB SPL.

In acoustics, sound is a change in pressure relative to atmospheric pressure. For those looking to gain a deeper understanding of how audio works, whether to make better-informed decisions or simply for the sake of curiosity, it's useful to lay down some ground rules that govern how audio systems behave relating to loudness and the decibel. For example, dBm means that the reference variable is the milliwatt dBV, the volt. The Decibel (dB) Scale & Audio Rules 101. In many cases, dBs are followed by a suffix to define a reference variable. Moreover, dB allows a realistic modelling of human auditory perception, since the ear reacts to relative changes in noise level.

First of all, a dB (decibel) is a ratio between two quantities that has been reported on a logarithmic scale. Substituting in the known values the expression is: dB 10 log (3 10-5/10-12) negative exponent in the denominator of a fraction is a positive value when.

Decibel reference scale