For example, the non-uniform frequency response curve of AC-coupled cascade amplifier is an example of frequency distortion.
From the en.wikipedia.org
Typically this is achieved by altering the frequency response of the capsule and electronics to result in lower noise within the A-weighting curve while broadband noise may be increased.
From the en.wikipedia.org
These settings, the frequency level of the peak or trough, are two of the parameters of a frequency response curve, and in a two-control equaliser they completely describe the curve.