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The High-Pass Filter: A Church Sound Board’s Most Underused Button

What they are, what they do, how to use them and more
This article is provided by Behind The Mixer.

One button that still looks pristine. No fingerprints. No worn finish. It doesn’t even have a name.

Working in audio, it’s easy finding my way into a foreign sound booth. It could be it’s my station for an event or it might be that I’ve been invited in by the sound tech on duty. 

Over the years, I’ve noticed one button that’s often not used. It’s the fixed-point high-pass filter button on each channel.

This single high-pass filter is often labeled as “/80” or “/100.” Honestly, not the best label choice. How many new sound techs would know what the number represented, let alone would know what happens when they pressed it?

What is a high-pass filter?

A high-pass filter (hpf) is an audio frequency filter that cuts (filters out) frequencies below a set level. 

For example, if you set a high-pass filter at 300 Hz, you’d lose a lot of the bass sound from instruments that primarily have fundamental frequencies in that lower range.

A high-pass filter also has a slope associated with it. Think of it as the rate of reduction of that filter.

In Figure 1 (below, right), you can see that the hpf filters out frequencies by attenuating (cutting) their decibel levels. 

Figure 1

In advanced filters, you can control the degree of the slope. In the case of the hpf button on the sound board channel, it’s a fixed slope.

What does “/80″ or “/100″ mean?

This label indicates a filter (/) and the frequency point for the filter. In the case of “/80,” it’s saying filter out all frequencies below 80 Hz. A resulting hpf then might look like the above image.

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