What Is a High Pass Filter?

Filters are a basic part of the audio experience

A high-pass filter allows high frequencies to pass but cuts, or attenuates, frequencies below a threshold point.

How Audio Speakers Use High-Pass Filters

A high-pass filter is used in an audio system to allow high frequencies to get through while filtering or cutting low frequencies. A high-pass filter is used with small speakers to remove bass, for example. In many cases, these filters are built into the speaker, but in some more elaborate do-it-yourself speaker setups, a high-pass filter unit can be wired to the system.

How High-Pass Filters Work With Microphones

Some brands of microphones include a selectable filter. These devices, usually intended for higher-end recording, automatically block sounds below a specific hertz rating. These settings are useful when recording in places like office buildings or warehouses where the HVAC system maintains a low rumble. These filters capture the signal while eliminating the nose.

It's better to capture good audio than to fix bad audio in post-processing. A microphone with an onboard filter yields better audio quality than a normal microphone that's been fixed algorithmically after the recording session concludes.

How to Use High-Pass Filters in Audio Editing

In tools like Audacity, a high-pass filter that's applied to a waveform simulates a hardware filter. It attenuates sound below a given threshold.

A screenshot of Audacity showing a high pass filter

Specify a frequency in hertz and the roll-off in decibels per octave. Audacity then attenuates the relative volume (the dB) of the signal with increasing intensity the further below the threshold frequency the signal lies.

Many audio-editing environments include similar functionality.

How High-Pass Filters Work

A high-pass filter (like its cousin, the low-pass filter) uses some technique to eliminate or reduce input below a specific threshold. There's no specific way that it works.

A graph of how the decibels fall off using a high frequency filter

Some filters offer a hard cap, effectively silencing noise below the threshold, while others taper off the signal below a certain point.

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