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This design of filter passes harmonics below its cutoff or corner frequency untouched, and reduces the level of lower harmonics depending on how far above the cutoff they are. In a 12dB/oct (decibel/octave) low pass filter, harmonics one octave above the cutoff frequency (in other words, double cutoff frequency) are reduced in level by 12 dB; harmonics two octaves above the cutoff (four times the frequency) are reduced by 24dB, and so forth. This is the most common type of filter used, as most natural sounds have stronger low harmonics and weaker high harmonics – especially as a note fades to silence.

low pass filter
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