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This design of filter passes harmonics above its cutoff or corner frequency untouched, and reduces the level of lower harmonics depending on how far below the cutoff they are. In a 12dB/oct (decibel/octave) high pass filter, harmonics one octave below the cutoff frequency (in other words, one half the cutoff frequency) are reduced in level by 12 dB; harmonics two octaves below the cutoff (one quarter the frequency) are reduced by 24dB, and so forth.

High pass filters are typically used to create bright sounds where the higher harmonics are much stronger than the fundamental and lower harmonics – for example, the sound of a harpsichord. (What – you haven’t played a harpsichord? Sorry; it was the example taught to me when I learned this stuff, and I haven’t been able to forget it. Not that I was around a few hundred years ago when they were popular – I’m not that old. But I digress…)

high pass filter
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