Absence filter

An absence filter is a filter arrangement that is often used in recording studio technology for sound shaping . This lowers the levels in narrow frequency bands . The counterpart of an absence filter for raising the level of frequency bands is called a presence filter .

An absence filter generates a bell-shaped frequency response that is lowered downwards , similar to an inverse resonance curve , which suppresses a very specific frequency  f ₀ . Particularly narrow and steep-sided lowering curves are called notch or notch filters.

To set an absence filter, you need:

• the center frequency  f ₀
• the gain ( amplification factor ) in  dB for setting the maximum reduction
• the bandwidth  B per octave or the quality factor  Q = f ₀ / B for setting the narrow bandwidth or edge steepness; the lower the bandwidth and the higher the quality factor (with constant center frequency and constant gain factor), the narrower the filter or the steeper its edges.

The individual starting points for lowering the desired frequency can usually be selected in small dB steps.

Narrow-band absence filters are often used (large Q) to eliminate conspicuous interference in the audible frequency range. To do this, the fault is first searched for in the "presence" filter setting and then switched to absence.

See also

• List of audio terms

literature

• Roland Enders: The home recording manual . 3rd edition, GC Carstensen, Munich 2003, ISBN 3-910098-25-8
• Karl Hermann Huber: Filtering and sifting of audio frequencies. 1st edition, Frech Verlag, Stuttgart 1974, ISBN 3-7724-0162-7
• Hubert Henle: The recording studio manual. 5th edition, GC Carstensen, Munich 2001, ISBN 3-910098-19-3

Web links

• Absence filter and presence filter - a collection of formulas (pdf; 44 kB)
• Presence filter according to Jens Blauert (pdf; 744 kB)
• Conversion of bandwidth in octaves into the quality factor and back