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What Is a Subsonic Filter & Why Should You Care?

Car stereo controls, subsonic filter

Even among seasoned audiophiles, subsonic filters are a topic guaranteed to sow confusion. Some people own amps with subsonic filter buttons that they never turn off for fear of breaking something. Others have adjusted theirs at multiple volumes and haven’t noticed any difference.

Yet it’s not a topic amp owners can blow off. If you have an amplifier, preamplifier, subwoofer, or any other unit with a subsonic filter, and you want to use it in a way that won’t destroy it prematurely, you need to know what your subsonic filter is and how to adjust it. We’re here to help. Our Q&A covers all the big questions you might have about subsonic filters. If you have one we missed, let us know in the comments!

What Is a Subsonic Filter?

A subsonic filter is a component on your subwoofer that reduces the intensity of notes which come through at lower frequencies. It decreases the amplitude of those low notes you feel more than you hear. (To learn more about frequencies and how they work, check out our Beginner’s Guide to Frequency Ranges.)

In other words, it’s what we know as a high-pass filter: a filter that lets through signals above a certain Hz threshold and attenuates any signal below that threshold. It’s the opposite of a low-pass filter, which only attenuates signals above a certain line.

How Does a Subsonic Filter Work?

We’ve already mentioned that the main mechanic these subsonic filters use is attenuation. It’s not important to understand exactly how they’re wired. The only thing you do need to know before we move on is that attenuation doesn’t completely dampen sounds of a certain frequency. Instead, the number you set the filter to represents the frequency at which it starts to reduce the intensity of the sound.

If your subsonic filter is set to 40 Hz or 30 Hz, notes will still get through but will be much quieter. Think of the filter as a slope, not an on-off switch.

Our Quick Guide to Bass Frequencies

Octave Range Center Description Instruments Problem


20-40 Hz 32 Hz Sub-bass, punch Kick drum, bass, organ Rumbling


40-80 Hz 64 Hz Low bass, depth Kick drum, bass, piano Thud


80-160 Hz 125 Hz Body, fat, booming Drums, bass, keyboard Unclear

Why Would I Want a Subsonic Filter?

If you’re a true bass-head, you might be incensed at the idea that you’d want to cut out subsonic frequencies. That’s where all the bone-shaking, window-rattling fun is! We’re right there with you. We absolutely love the feeling of low notes thudding through our whole bodies, but we love something else just as much: not having to replace our subwoofers once a month.

Subsonic notes sound awesome, but they damage your hardware when they get out of control. Too many deep bass thuds can tax the limits of your woofer and cause it to wear out much faster than it otherwise would. For visual learners, the video below illustrates how low frequencies can distort a subwoofer’s cone alignment without a filter.

The trick is to find the right balance; it should be loud enough that you can feel the bass but attenuated enough that it’s not destroying your amp.

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What About Speakers Without Subsonic Filters?

Some bass units on the market don’t have subsonic filters built in. While filtering is important, this isn’t necessarily a dealbreaker; it all depends on what kind of music you listen to.

Plenty of genres never get anywhere near the 50 Hz range. But if you’re a fan of the ones that do (and you know if you are), you need a subsonic filter to keep your subwoofers from rattling themselves to bits before their time. Check out the MTX Audio THUNDER1000.1 Thunder Series Car Amplifier for a great amp with a subsonic filter.

MTX Audio THUNDER1000.1 Thunder Series Car Amplifier
  • Class D Topology
  • Variable LPF
  • Selectable Bass Boost

How Do I Use a Subsonic Filter?

We’ve now covered the purpose of a subsonic filter and how it works. The next step is knowing how to make your high-pass filter work for you. The best thing to do is listen. Play some of your favorite bass-heavy tracks and adjust the filter threshold until the low notes start reverberating. Then adjust it back up until the bass notes tighten up again. That’s a great way to find that balance we talked about above.

However, you might be the kind of audiophile who’s more into hard numbers, or you may want a hint on where to start. If either of those is true, read on, and we’ll get just a little bit into the weeds. Your approach to adjusting your subsonic filter depends most heavily on whether your subwoofer is set in a sealed or ported enclosure.

If you aren’t sure which one you have, you can tell by looking at the box. Is the hole for the speaker the only hole? If yes, you have a sealed enclosure. If not, it’s ported.

Pyle Car Vehicle Subwoofer Audio Speaker

What’s the Best Way to Adjust a Subsonic Filter in a Sealed Enclosure?

Sealed enclosures work just like we’ve described. Low subsonic frequencies, especially 20 Hz and below, damage the structure, so we want to attenuate them as much as we can. Since the Hz setting on the subsonic filter is the start of a slope, not a cutoff, we want to set it higher than 20 Hz.

The sweet spot for getting some pounding bass without destroying your subwoofer is between 25 Hz and 35 Hz. Within that band, it’ll vary depending on your chosen music and the rest of your setup.

Skar Audio Single 8" 300W Dual 2 Ohm Loaded Sealed Subwoofer Enclosure | IX-1X8D2
  • Ix Series 8-Inch Dual 2-ohm loaded Subwoofer Enclosure
  • Peak Power: 300 watts | RMS Power: 150 watts
  • Subwoofer comes pre-wired and loaded in enclosure

What if I Have a Ported Enclosure?

Now, it gets a little more complicated. On ported speaker enclosures, you must tune the port to its optimal frequency. Explaining that is beyond the scope of this Q&A, but if you’re just now learning, you should tune your subs, and this video is a great explainer. The tuning frequency acts as a second high-pass barrier, but it does not replace the function of a subsonic filter. Too many subsonic notes can still damage your subwoofer.

A ported sub can safely play about half an octave below its tuned frequency before there’s a risk of damage. That means the safest setting for its subsonic filter is half an octave below whatever frequency the port is tuned to.

The hertz scale is logarithmic. Each octave up is twice as many Hz as the previous one, and dividing any frequency by 2 takes it one octave lower. Therefore, to get your subsonic filter setting, take your subwoofer’s tuned frequency and multiply it by 0.75. So, if it’s tuned to 40 Hz, set the filter knob to 30 Hz.

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With all the detail we’ve gone into, it’s easy to forget that most of these changes aren’t audible to the human ear. If you don’t like to crank your volume up that far, you might never notice any difference in subsonic sounds. But if you understand how to adjust your subsonic filter and practice doing it right, you might reap some big rewards. Not only will your low-frequency sound be less distorted, but your expensive subs will also last much longer.

Thanks for reading, and as always, feel free to sound off in the comments below. Enjoy your next ride with your newly balanced bass track!

Featured Image Credit: slightly_different, Pixabay


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