The frequency response is the decisive characteristic value for the frequency-dependent transmission behavior of active or passive quadripoles(amplifiers, cables, microphones, loudspeakers, filters, etc.).
The frequency response describes the level deviations of all frequencies lying in a frequency range to each other. These are given in decibels( dB) and determine the transmission characteristics of the quadripole. It is a measure of the ratio of the level deviations to each other. In the audio range, frequency response deviations would be noticeable by a greater or lesser attenuation of individual frequencies.
Typical frequency response specifications refer to the frequency range. For example, an audio amplifier may state that the maximum level deviation in the frequency range between 20 Hz and 20 kHz is +0.1 dB and -0.3 dB. Such level deviations are not perceptible.