(Fr. filtre; Ger. Filter; It. filtro).
An electrical device that enables a sound to be shaped by amplifying or removing one or more areas of its total sound spectrum. The principal types of filter are: band-pass, band-reject (or notch), high pass, low pass and parametric. A band-pass filter retains or boosts the region or regions in which sound is to be passed, while the complementary band-reject filter eliminates only those that are to be rejected; these are defined in terms of independent bands, normally one octave, 2/3, ˝ or 1/3 of an octave in width, which can be individually controlled in loudness by means of parallel slide controls (a form of band-pass filter, in which the removal of pitch bands is less complete, is known as a graphic equalizer, and is found in all types of mixing desks and in some domestic hi-fi equipment). High and low pass filters remove areas of the sound spectrum that are respectively below and above a selected cut-off frequency, with a single overall loudness control; the sharpness or roll-off at which this is applied can be adjusted by a response, bandwidth or ‘Q’ control, and in certain synthesizers this can be set so sharply on a low pass filter that it oscillates as a sine-wave at the cut-off frequency. The ‘wah-wah’ pedal (often associated with the Electric guitar) is a type of limited low pass filter. High and low pass filters are sometimes combined in a single unit; two such combination filters are prominent in Stockhausen's live electronic Mikrophonie I (1964). Parametric filters feature elements of all the types of filters described above; several subdivisions of the spectrum (typically three or four) can be independently filtered in terms of central frequency, loudness and response. Further details are given in T. Cary: Illustrated Compendium of Musical Technology (London, 1992), 187–90, 195–203.
HUGH DAVIES