|Low pass filter||reduce high frequencies|
|High pass filter||reduce low frequencies|
|Band pass filter||pass frequencies within a band|
|Band stop filter||reduce frequencies within a band|
|Vibrato||regular and repetitive change in pitch|
|Equalisation||increase or reduce the amplitude of bands of frequencies|
The filter table lists four types of filter together with two more techniques for processing frequency and briefly summarises what each does.
Filtering is a process of reducing the frequencies present in a sound.
Subtractive synthesis is another name for filtering. It is the opposite of additive synthesis.
The most important features of a filter are its cutoff frequency and the shape of its rolloff curve
The cutoff frequency is the frequency at which a filter becomes active. In practical terms, the cutoff frequency is the frequency that a filter begins filtering. In technical terms, filtering actually starts at a point such that the cutoff frequency becomes the half-power point, the point at which power has dropped by -3 dB.
Rolloff describes the shape of the amplitude of the frequencies being filtered. In an ideal world, the shape of a rolloff should be a brick wall. A filter should completely eliminate all the frequencies to be filtered and the amplitude of filtered frequencies should thus be zero. In practice, there are technical limitations to the design of a filter, and the amplitude of filtered frequencies reduces gradually not abruptly. This reduction in amplitude is shaped like a curve, called the rolloff curve. The steepness of the rolloff curve is determined by the order of the filter. A first-order filter reduces the amplitude of filtered frequencies by 6dB per octave, a second-order filter by 12dB per octave, and so on. The higher the order, the steeper the rolloff curve, and the more evident the filtering effect.
Filters are widely used to clean up a sound by removing unwanted frequencies. They are less often used to create sound effects although they are well suited for this purpose. Filtering frequency-rich sounds such as white noise and pulse trains produces all sorts of weird and wonderful effects.