Frequency response is used to describe how an audio system responds to various frequencies of sound. As noted above, at low volume the ear favors middle-frequency sounds, and at high volume its frequency response is more even or flat. A good audio recorder is capable of providing a fairly flat frequency response throughout the frequency range of human hearing.
Because all sounds incorporate a spread of frequencies, if you change the frequency response of a recording by increasing or decreasing the low, middle, or high frequencies, you can change the character of the sounds. The bass and treble controls on a radio do this to some extent; most people like to turn the bass up in dance music to make the rhythm, carried by low-frequency instruments such as the bass guitar and bass drum, seem more powerful. Equalizers (see Figs. 11-12 and 15-17) are often used to alter the frequencies of sounds during recording or after. With an equalizer, you could boost low frequencies to make, say, a truck engine sound deep, rumbly, or menacing, or you could boost the high frequencies of a piano to make its sound “brighter.” If we diminish high frequencies without changing the bass, the effect is like putting cotton in your ears: the sound is muddy and dull.
Telephones have a fairly limited frequency response, which is centered on the middle frequencies needed to understand speech. In movies, the sound of someone talking through a phone can be simulated with an equalizer by cutting the low and high frequencies and boosting the midrange.