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We know that shelving filters boost or cut energy at the target frequency and all audio frequencies either above or below it. A pass filter also affects all energy above or below the target frequency, often referred to as the cutoff frequency in a pass filter. However, instead of boosting or cutting that content by a specific amount of gain, it instead removes that frequency content completely. Any frequency content outside the cutoff frequency is attenuated or cut gradually in a downward slope, heading towards negative infinity.
Let's listen to some audio examples. Listen to the following drum loop with a high-pass filter engaged. (music playing) Next, let's listen to the loop with a low-pass filter engaged.
(music playing) Like the name suggests, a high-pass filter allows the highs to pass through the filter unaffected, while the low frequencies are attenuated, or removed, starting at the cutoff frequency and lower.
Likewise, a low-pass filter allows the lows to pass through the filter unaffected while the high frequencies are attenuated or removed, starting from the cutoff frequency and higher. Sometimes these filters will be referred to as high-cut and low-cut filters. High cut is simply another name for a low-pass filter and low cut another name for a high-pass filter. Don't let the names confuse you. Fortunately, the names are actually quite descriptive. Low-cut filters cut the lows; high-pass filters let the highs pass through, et cetera.
When both filters are used simultaneously, one filtering out the lows and another filtering out the highs, it is often referred to as a band-pass filter, since in that case only a specific band of frequencies are allowed to pass through. Pass filters generally only have two controls: frequency and Q. The frequency sets the cutoff point of the filter, while the Q control, sometimes called slope in a pass filter, determines the steepness of the cutoff slope.
Q in this case is usually measured in dB per octave and determines how aggressive the EQ will begin removing frequency energy above or below the cutoff point. Q settings on high- and low-pass filters generally start at 6 dB per octave, which is a rather gentle slope, and work their way up to more aggressive settings--sometimes as much as 48 dB per octave, which looks basically like a vertical line here on the graph.
Let's look at an example of a high-pass filter. A setting of 12 dB per octave with a cutoff at 100 Hz means that after passing through the filter, a signal of 50 Hz or one octave lower will be reduced by 12 dB, while a signal of 25 Hz, two octaves lower, is reduced by 24 dB. High- and low-pass filters are especially useful in restricting or bracketing frequency content of signals in a mix. Many times rumble, or frequencies lower than the fundamental of the instrument, can be completely removed using a high- pass filter set just below a signal's fundamental frequency.
For example, a male vocal isn't going to have much below 80 Hz, so cutting off everything below 80 with a high-pass filter can help ensure that no additional low-frequency content unrelated to the vocal itself makes its way into the mix. Likewise, low-pass filters can be used to restrict and reduce high-frequency content. In the case of DAWs, unlike analog mixing consoles and tape, high-frequency retention is 100%, meaning no high frequencies are soaked up by the DAW's mixer.
This can lead to mixes that have too much high-frequency extension in too many of the instruments. This can result in a subtle building up of undesirable high-end frequency content that is hard to describe. I like to use low-pass filters on instruments that aren't contributing significantly to the high frequencies of my mix. For example, a 12-inch guitar speaker cabinet doesn't produce much useful audio information above 8K, so why keep it in the mix? Reducing or eliminating those high-end frequencies helps to remove any noise or hiss that may be present in the recording, but not part of the actual instrument.
And reducing these frequencies on multiple tracks keeps them from adding up in a mix. Using high- and low-pass filters to help tighten up frequency content and improve focus has been a secret of the pros for years, and I would find myself hard-pressed to do a mix without them.
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