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The audio filter is a frequency dependent circuit, operating in the audio frequency range, from 0 Hz to 20 kHz. Audio filters can amplify (gain), transmit, or attenuate (cut) certain frequency bands. There are a variety of filters for different audio applications, including hi-fi stereo systems, synthesizers, effects sets, PA systems, instrument amplifiers, and virtual reality systems.
What is low-pass filter?
A lowpass filter passes frequencies below the cutoff frequency and gradually attenuates frequencies above the cutoff frequency. Lowpass filters are used in audio crossovers to remove high frequency content from the signal sent to the low frequency subwoofer system.
What is high-pass filter?
A high-pass filter does the opposite, passing high frequencies above the cutoff frequency and gradually attenuating frequencies below the cutoff frequency. A high-pass filter can be used in an audio crossover to remove low frequency content from the signal sent to the tweeter.
What is band-pass filter?
A bandpass filter transmits frequencies between its two cutoff frequencies, while attenuating frequencies out of range. The band-reject filter attenuates the frequencies between its two threshold frequencies, while also passing frequencies outside the "rejection" range.
What is all-pass filter?
An all-pass filter passes all frequencies, but affects the phase of any given sinusoidal component according to its frequency.
Applications
In some applications, such as in the design of graphic equalizers or CD players, the filters are designed according to a set of objective criteria such as passband, passband attenuation, stopband, and stopband attenuation, where the passbands are the frequency ranges for which audio is attenuated less than a specified maximum, and the stopbands are the frequency ranges for which the audio must be attenuated by a specified minimum. In more complex cases, an audio filter can provide a feedback loop, which introduces resonance (ringing) alongside attenuation.
Audio filters can also be designed to provide gain (boost) as well as attenuation. In other applications, such as with synthesizers or sound effects, the aesthetic of the filter must be evaluated subjectively. Audio filters can be implemented in analog circuitry as analog filters or in DSP code or computer software as digital filters. Generically, the term 'audio filter' can be applied to mean anything which changes the timbre, or harmonic content of an audio signal.