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What Is a Subsonic Filter & Why Should You Care?

Car stereo controls

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’ve got 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 that 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 named the main mechanic these filters use: attenuation. It’s not super important to understand exactly how they’re wired – it’s a lot of electrical engineering without much effect on what you need to know as the end-user.

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 as which it starts to reduce the intensity of the sound.

If your subsonic filter is set to 40 Hz, 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

OctaveRangeCenterDescriptionInstrumentsProblem

1

20-40 Hz32 HzSub-bass, punchKick drum, bass, organRumbling

2

40-80 Hz64 HzLow bass, depthKick drum, bass, pianoThud

3

80-160 Hz125 HzBody, fat, boomingDrums, bass, keyboardUnclear

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 love the feeling of low notes thudding through our whole bodies. But there’s something else we love just as much: not having to replace our subwoofers once a month.

Subsonic notes sound awesome, but they also 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 you visual learners, the video below is a great illustration of how low frequencies can distort a subwoofer’s cone alignment without a filter.

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

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.

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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 down until the low notes start to reverberate. 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 just want a hint on where to start. If either of those are 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 one thing: whether your subwoofer is set in a sealed or ported enclosure.

If you aren’t sure which one you have, you can tell by just looking at the box. Is the hole for the speaker the only hole? If yes, you’ve got 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...
  • Ix Series 8-Inch Dual 2-ohm loaded Subwoofer Enclosure
  • Peak Power: 300 watts | RMS Power: 150 watts

What if I have a ported enclosure?

Then it gets a little more complicated. On ported speaker enclosures, you have to tune the port to its optimal frequency. Explaining that is beyond the scope of this Q&A, though if you’re just now learning you should tune your subs, 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.

Rockford Fosgate Punch P2-2X12 P2 Dual 12" Loaded Enclosure...
  • The P2-2X12 is a dual P2 12” DVC subwoofer in a vented enclosure that...
  • The P2-2X12 vented loaded enclosure is Internally wired to create a 1-Ohm...

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.

Conclusion

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 last a lot 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!

Header image credit: slightly_different, Pixabay

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