A device used to affect certain parts of the spectrum of a sound, by causing the attenuation of certain frequency bands, while allowing other bands to pass unattenuated. Some common types of filters are:
high-pass filters (which attenuate low frequencies below the cut-off frequency);
low-pass filters (which attenuate high frequencies above the cut-off frequency);
band-pass filters (which combine both high-pass and low-pass functions);
band-reject filters (which perform the opposite function of the band-pass type);
octave, half-octave, third-octave, tenth-octave filters (which pass a controllable amount of the spectrum in each band);
shelving filters (which boost or attenuate all frequencies above or below the shelf point);
resonant or formant filters (with variable centre frequency and Q).
The Q of a filter is a measure of its resonance and is defined as the ratio of centre frequency to the bandwidth. The narrower the bandwidth, the higher the Q, and the more the filter will ’ring’ or go into oscillation when stimulated by a signal with energy near the centre frequency. A constant Q filter varies its bandwidth as a function of the centre frequency, always keeping the ratio between them the same.
A group of such filters may be interconnected to form a filter bank. (Reduced from Barry Truax - Handbook for Acoustic Ecology CD-ROM Edition. Cambridge Street Publishing, 1999 - CSR-CDR 9901)
With Comb-Filtering, a sound is passed through a group of filters, generally distributed equidistantly. The mode of sound processing changes the spectral content of sounds around the selected frequencies, possibly boosting them to the extent of generating resonance, when the width of the filtered areas become very thin. Sinusoidal frequencies then start to oscillate in response to the feedback created in those frequencies. (Source - Rodolfo Caesar (1992). The Composition of Electroacoustic Music. PhD Thesis, University of East Anglia.)
Filters have also been used as musical ’instruments’ in live performance, most significantly in a number of works by Karlheinz Stockhausen, including Mikrophonie I (1964), Prozession (1967), and Kurzwellen (1968). The filters were used to transform the sounds of a large tam-tam picked up by microphones moved by performers over the surface. In the latter two works the filters were also applied to the sound from a contact microphone attached to a viola. (Source - Richard Dobson (1992). A Dictionary of Electronic and Computer Music Technology. Oxford University Press.)
UNESCO DigiArts- Digital Music online course: Editing/Filtering ( English )