Measurement In Decibels: What Is the Difference Between dB and dB(A)?
First of all, a dB (decibel) is a ratio between two quantities that has been reported on a logarithmic scale. Moreover, dB allows a realistic modelling of human auditory perception, since the ear reacts to relative changes in noise level. In many cases, dBs are followed by a suffix to define a reference variable. For example, dBm means that the reference variable is the milliwatt; dBV, the volt.
In acoustics, sound is a change in pressure relative to atmospheric pressure. 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. However, the human ear does not perceive all frequencies in the same way. Indeed, the ear of a healthy person is more sensitive to frequencies between 2 and 5 kHz.
This is why dB(A) is used. 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. The blue curve shows the gain for a type A weighting. There are also B, C, D and Z weights. However, the B and C weights are only valid for pure signals (signals with a single frequency).