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Pass filter explained: what is a low-pass filter and a high-pass filter?

Pass filter explained: what is a low-pass filter and a high-pass filter?

A pass filter is a really useful tool in audio mixing.

That’s because, unlike other filters which attenuate or boost frequencies by a certain amount, a pass filter allows you to cut sections of the frequency spectrum out entirely.

There are two main types of pass filter that are used in music mixing. Those are high-pass filters (HPF) and low-pass filters (LPF).

In this article, I’ll explain what pass filters do, how you can use them, and the benefits of doing so.

 

High-pass filters:

A ‘high-pass filter’, also referred to as a ‘low-cut filter’, does exactly what the name suggests. It allows only frequencies that are higher than a certain point to pass through. Simultaneously, it cuts out the frequencies that are lower than that point.

High Pass Filter (Low Cut Filter)

Low-pass filter:

A ‘low-pass filter’, also referred to as a ‘high-cut filter’, allows only frequencies that are lower than a certain point to pass through. Simultaneously, it cuts out the frequencies that are higher than that point.

Low Pass Filter (High Cut Filter)

Controlling a pass filter:

Pass filters have two main controls. They are the filter’s cutoff frequency and the filter’s slope.

Many filters allow you to control both, others have fixed settings, and some provide a selection of predefined options.

Let’s look at each of these controls in further detail…

Cutoff Frequency:

A pass filter’s cutoff frequency defines the point at which the transition is made between frequencies passing through and frequencies being cut. With a high-pass filter, frequencies higher than the cutoff frequency will pass through and frequencies lower than the cutoff frequency will be cut. With a low-pass filter, frequencies lower than the cutoff frequency will pass through and frequencies higher than the cutoff frequency will be cut.

Slope:

Some filters let you control the steepness of the filter’s slope. The steepness of the slope dictates how gradually gain loss is achieved. The slope is measured in decibels per octave (dB/oct). The fewer decibels that are cut per octave, the more gradual the slope will be. This can be seen here:

Low Pass Filter (gradual slope)

The higher the number of decibels that are cut per octave, the steeper the slope will be. This can be seen here:

Low Pass Filter (steep slope)

Why use a pass filter?

A filter which allows you to cut out certain parts of the frequency spectrum can be really useful. This is especially true in situations where cutting certain frequencies out completely is preferable to simply turning those frequencies down. For instance, you may want to use an HPF to cut the low end of a vocal track to eliminate plosives. Similarly, you may want to use an LPF to cut out the unwanted noise from a guitar amp in the high end.

Do you use pass filters in your mixes? If not, can you think of any occasions where a pass filter may come in handy? Leave your ideas in the comment section.

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