What is a Low Pass Filter?

R. Kayne

A low pass filter, a feature found in many of today’s car audio systems, allows the user to exclusively designate lower frequencies to a subwoofer. In the absence of a subwoofer, the filter can do the same for rear, full range speakers. Once it is set by the user at a specific cutoff point, all frequencies lower than the cutoff will be passed along, while higher frequencies will be muted or filtered out. Used in conjunction with a high pass filter, one can “place frequencies” to the proper speakers, creating a total sound environment.

Man playing a guitar
Man playing a guitar

Generally, low frequencies, like bass guitar and bass drum, are poorly reproduced through small-coned speakers. While high-end smaller speakers might do a satisfactory job for some, for heart-pounding, chest-thumping bass, many audiophiles will want a speaker made for the job: a subwoofer. A subwoofer is designed to handle lower frequencies, delivering an astounding fullness and depth to any music that leans heavily on low-end frequencies. This is true of rock but even more so for hip-hop and rap.

A low pass filter will enhance the lower end of the sound spectrum by keeping the bass channel cleaned of higher frequencies so that the subwoofer gets just the frequencies it was designed to reproduce. By playing with the cutoff point, a listener can find that sweet spot that lets just enough midrange frequencies through to spread the sound envelope out. With front speakers handling higher frequencies, he can tweak to taste for optimum enjoyment.

If there is no subwoofer, the low pass filter can direct lower frequencies to the rear speakers. Rear speakers tend to be larger and better equipped than front ones to handle bass. Front speakers are commonly smaller and better suited to reproducing high-end frequencies. A high pass filter can block bass to the front speakers. By using low pass and high pass filters together, a listener can improve the sound of the entire speaker system without changing the speakers out.

This type of filter helps create an impressive sound environment by channeling the proper frequencies to the appropriate speaker(s). Once it has helped establish a nice bassy rear, other audio functions can help round out the sound environment. Equalization settings and in some cases compression settings can all help to fine-tune the music experience.

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Discussion Comments


I can imagine more and more people being interested in this with the recent explosion of UK bass music like dubstep, a genre which is almost entirely focused on bass and hard hitting percussion. Tune your low pass filter and turn it up as you head out for a night in the city, your passengers will thank you.


Great explanation. i finally know how to use this feature properly on my sound card application. After messing around with EQ settings for ages, i simply set the low-cut filter to 70hz and my sub woofer is much more punchy now, thanks a lot.

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