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What are high pass and low pass filters and when should I use them?

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Topic What are high pass and low pass filters and when should I use them?
Stupid question but I always hear people talk about HPF and LPF and I'm still not sure what they do, when to use them, and if there are any 'rules' to follow?
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google is your friend
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I never appreciated that they call them High Pass filter and Low Pass Filters. Better names that would help you understand is Low Cut Filter and High Cut Filter (respectively). An HPF (or LCF) will allow all frequencies above your defined limit to pass through unchanged (ignoring all frequencies below this limit). Conversely, an LPF (or HCF) will allow all frequencies BELOW your defined limit to pass throughs.

The reason they aren't called low cut filters or high cut filters is because technically, the 'left out' frequencies aren't cut, but rather tapered. So for example, with a high pass filter set at 200Hz, every frequency below 200Hz will have a reduced amplitude. The farther the frequency is from the 200Hz limit, the more substantial the reduction (ie 20Hz will be inaudible, while 199Hz will be audible, with a much higher amplitude)

In general, HPFs offer a good way to clear out some unnecessary mud from your mix, but there are also countless other applications. Hope that helps.
4
For more info specifically about high-pass filtering, check out this Audiofanzine article: https://en.audiofanzine.com/recording-mixing/editorial/articles/cut-the-mud-from-your-mix.html