Filters
Definition: An electronic circuit designed to emphasize or attenuate a specific range of frequencies.
(Creative Recording 1 Effects and Processors - Paul White 1999 )
The signal from the oscillator is sent through a filter which shapes the Timbre of the sound and makes it bright, dull, thin etc. Filters used within subtractive synthesis are used to the same extent within equalization.
Below are some examples of the types of filters which are commonly used in sound production today. The first clip is a sample of a piano with no equalization, filtering, etc to act as a reference so you can compare.
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Filters will typically have the following controls - Q, Frequency Pot and a Shelf or Bell Shape button.
Q - The gradient of a slope in db/octave or bandwidth of affected frequencies.
Frequency Pot - Alters the threshold at which the frequencies are cut off. In a LPF, the affected frequency range is typically from 20khz to 120hz and with a HPF, the range is from 20hz to 8khz. These frequencies will change per manufacturer but most modern filters will stay close to this range.
Shelf/Bell - A shelf option will normally have a range of 6dB/Oct to 24dB/Oct. Simply put: the higher the value,the sharper the filter slope. The Bell shape is set with either a set or specific bandwidth. The Shelf shape would be useful in filtering all frequencies above or below a certain range whereas the Bell can be used to isolate frequencies. For example, a ring in a snare drum can be removed by using a filter with a Bell shape.