Audio Filters Circuits

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An audio filter is a frequency dependent amplifier circuit, working in the audio frequency range, 0 Hz to beyond 20 kHz. Many types of filters exist for applications including graphic equalizers,synthesizers, sound effects, CD players and virtual reality systems. Filters are defined by their slope, which determines the attenuation of signals outside the ‘pass’ band. Most audio filters on mixing desks (and DAWs) will have a slope of 12dB or 18dB per octave, and in synthesizer filters the slope may be as steep as 24dB per octave. If an 18dB/octave high-pass filter is set to 80Hz, any audio an octave below that (at 40Hz) will be attenuated by 18dB, and an octave lower still, at 20Hz, it will be attenuated by 36dB... and so on.

For example, high-pass filters are often used in studio recording and sound reinforcement to attenuate extraneous low-frequency content like mechanical rumble or vocal plosives. By choosing a filter with a cutoff frequency below the fundamental frequency range of the program, a HPF can be used to differentiate between program signal and low-frequency noise. Low-pass filters can also be used to eliminate unwanted, counter-productive bandwidth. One common example is using a LPF to establish the limited bandwidth of a low frequency transducer, like a ‘sub’ mic on a kick drum. Other examples include the entire universe of subtractive synthesis.

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