What is a low pass filter? Filter types explained

Filters are a staple tool in electronic music production and can be found on drum machines, samplers, mixers, synthesizers and in every DAW (typically in VST and/or AU plugin formats). But what exactly are filters and what’s the difference between a low pass, high pass, notch and band pass filter?

Long before the arrival of DAWs and plugins, filters existed in the hardware realm only. They were/are electronic devices designed to reduce a signal’s energy at a specific frequency.

Filters act on a signal in a similar way to an EQ (filters of different frequencies can be combined to create equalizers) but while EQs can be used to boost and cut at specific frequencies, ‘true’ filters were subtractive and cut (or ‘attenuated’) the signal. In many hardware filters, an amplifier was/is incorporated into the circuit, allowing the frequency to be boosted as well as cut and now in the software realm most filters follow this model.

Learn what 4 of the most common filter types do:

Low Pass Filters: A low pass filter cuts high frequencies in a signal. Low frequencies – such as the lower end of a bass drum – can pass through the filter.

High Pass Filters: As you might expect, a high pass filter acts in the opposite way of a low pass filter. High pass filters cut the low frequencies in a signal and allow the higher frequencies to pass through.

Band Pass Filters: With a band pass filter both high and low frequencies either side of a range of frequencies are cut. Band pass filters are good for filtering out unwanted lower and higher frequencies – in a drum loop, for example – while allowing the body of the signal to pass through.

Notch Filters: Notch filters create a’u’ shaped cut within a specific range of frequencies and the signal either side of the notch passes through. Notch filters are a great tool for finding and cutting unwanted chunks of audio that might be taking up unnecessary space in your mixes.

Filters typically boost and attenuate – cut – a certain range of frequencies as determined by the ‘Cutoff’ and ‘Resonance’ settings on the filter.

What is the ‘Cutoff’ on a filter?

On filters you’ll see a ‘Cutoff’ knob, dial or slider of some kind. You use the ‘Cutoff’ to sweep through the frequency spectrum and listen to determine where exactly you want the filter to take effect.

What is ‘Resonance’ on a filter?

Alongside the Cutoff knob you’ll find a Resonance knob of some kind. On some filters Resonance is sometimes labeled ‘Q’. Resonance typically – not with notch filters – creates a boost around the Cutoff point and is used to add a sonically pleasing brightness, clarity or aggressive ‘bite’ (depending on the nature of the filter).

Multimode Filters:

As the name suggests, a multimode filter can operate in different ways depending on which filter type the user wishes to switch to.

Morphable Filters:

Some filters can morph between different filter types for example you might want your filter to smoothly change from a low pass filter to a high pass filter during different parts of your track.

Three free filter plugins to try out:

If you’re using a modern DAW then chances are you already have access to at least one stock filter plugin but it’s still worth spending some time exploring some of the other filters out there – filters are a fun tool and different filters can offer creative possibilities and sonic characteristics that go beyond your stock plugins.

Stagecraft Auto Filter: With this free filter you can sweep the filter frequency with a range of LFOs, vary the range of the frequency sweep, duty cycle warp the LFO, and control the wet/dry mix. (Mac/PC 64 Bit, Vst, AAX and AU.)

Download Stagecraft Auto Filter

TAL-filter 2: TAL-Filter-2 is a host synced filter module with multiple filter types, pan and volume modulation possibilities. Filter options include: LP 24 dB, LP 18dB, LP 12dB, LP 6dB, HP 12 dB, BP 12dB, N 12 dB.

With TAL-filter 2 you can easily modulate the pan or volume of any incoming audio signal to create tremolo or trance gate type effects. (Mac/PC 64 Bit, 32 Bit, Vst and AU.)

Download TAL-filter 2

Filterjam: Filterjam from respected plugin developer AudioThing is a multi-band resonant filter delivering weird ringmod-like filtered sounds. The input signal is divided into 4 bands that are then summed or multiplied together according to the selected mode. Filterjam can be very harsh or gentle (Mac/PC VST2, VST3, AU, and AAX 64-bit.)

Download Filterjam

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