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 EQs which turn frequencies up or down (e.g. parametric EQs and shelving filters), a pass filter allows you to filter out portions of the frequency spectrum. There are two main types of pass filters 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.

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Click here for a video tutorial on pass filters:

 

High-pass filter:

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 filters out the frequencies that are lower than that point.

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 filters out the frequencies that are higher than that point.

Pass filters have two controls. They are the filter’s cut-off frequency and the filter’s slope.

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

Cut-off Frequency:

You use a pass filter’s cut-off frequency as a reference for the point at which frequencies to one side are filtered and to the other side are allowed to pass. It is worth noting that the cut-off frequency is not the point at which attenuation begins. It is actually the point at which 3dB of attenuation is achieved. As such, it should be noted that some attenuation does occur even on the side of the cut-off frequency to which frequencies are allowed to pass.

Slope:

Some filters let you control the steepness of the filter’s ‘slope’. The steepness of the slope dictates how gradually attenuation occurs. 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:

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

Controlling a pass filter:

The designs of different EQs vary greatly. Some equalizers allow you to control both the cut-off frequency and the slope of a pass filter. Others may have fixed settings for both the cut-off frequency and the slope, allowing you simply to switch the filter between active and inactive. Some offer only control over the cut-off frequency and have a fixed slope setting. Others may offer only a HPF but no LPF.

Why use a pass filter?

A filter that allows you to cut out certain parts of the frequency spectrum can be really useful, especially in situations where filtering frequencies out is preferable to just turning them down in the way that you would with something like a shelving filter.

High pass filters are often applied to things like acoustic guitars, electric guitars, pianos, and any other instruments other than kick and bass guitar that contribute low end to your mix and could cause your track to sound muddy. Using a high pass filter allows you to filter out some of this low frequency content. This cleans up the mix and allows the kick drum and bass guitar to take precedence in the low end.

Another common use for a high pass filter is to cut the low end of a vocal track to eliminate plosives. Other noises or unwanted parts of a signal like mains hum, the rumble of traffic or an accidental knock against a mic stand can all usually be filtered out of a recording using an HPF. Similarly, you could use a low pass filter to cut out the unwanted noise from a guitar amp’s high end.

Do you use high pass filters or low 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.