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How to Set Subsonic Filter on Car Amp

Whether you’re a serious basshead, audiophile, car audio fanatic, or simply someone who appreciates well-balanced sound, properly setting up your subsonic filter is of paramount importance.

A subsonic filter is only practical and functional as long as you set it up correctly. It won’t only make your music sound better, but it will also make your amp’s power supply and output devices, and the speakers, more efficient.

That said, what is subsonic filter? What does it do exactly? How do you set it up correctly and how does one use it effectively? Let’s find out…

What is Subsonic Filter?

Also known as an infrasonic filter, a subsonic filter is a feature that attenuates the intensity of low frequency notes. It cuts off (to a certain degree) extremely low bass notes (often below the range of human hearing) that many speakers and subwoofers cannot effectively reproduce, thereby making the amplifier and the speaker, more efficient. That said, it’s safe to say that a subsonic filter is nothing more than a high-pass filter with really low crossover points.

Subsonic filter is a very important feature for vented enclosures, as it can significantly reduce the loud booming notes around the box’s resonant frequency. I mean, think about it, if you have a custom built ported enclosure tuned to 31Hz, you don’t want to play frequencies that are way lower than that, otherwise you risk blowing your subs.

The human hearing ranges roughly from 20 Hz to 20000 Hz. So, if you send low frequencies below 20Hz to your subwoofers, you won’t be able to hear them anyway (you’ll mostly feel them). Most music doesn’t contain these frequencies for that very reason but sometimes even 30Hz can be hard on your subwoofer if it isn’t designed to dig down that low.

How Does a Subsonic Filter Work?

If you have an amplifier with a subsonic filter set at 40 Hz, this doesn’t mean, by any means, that everything below this is completely dead and gone.

In fact, it’s safe to say that when a subsonic filter is set to 40 Hz, a frequency like 25 Hz notes will still get through and be quite audible, albeit much quieter than it would be if the filter was not set at all. Think of the filter as a slope, not an on-off switch.

Speaking of “slope”, when talking about subsonic filters, you may hear that term thrown around quite often. It refers to the rate at which music notes will be attenuated to below the specified subsonic filter frequency.

Slope is expressed as decibels (dB) per octave, and the dB numbers (18dB, 24dB…) refer to the rolloff of the subsonic filter.

Taking into consideration that one octave corresponds to a doubling of frequency, if you set an 18 dB/octave subsonic filter to 40Hz, then at 20Hz it will sound 18dB quieter, and the 24 dB/oct would be 24dB quieter.

In other terms, if your subsonic filter has a slope of 18db/octave, then it will attenuate (make quieter) all the frequencies below the selected frequency at 18 decibels per octave. If it has a 12db/octave slope, then it will attenuate all the frequencies below the selected frequency at 12 decibels per octave.

That said, if you’re looking for extreme protection of your subwoofer, then a higher slope is better. However, if you want a little less rolloff, you can opt for a lower slope such as 18dB, or 12dB.

Is Subsonic Filter a Must?

A subsonic frequency is very low, so low the human ear is incapable of hearing it, however your body can feel it. Because of that and because these frequencies are prone to damaging woofers, filtering them out is a desirable characteristic in subwoofer amplifiers.

So, a subsonic filter isn’t mandatory, but it’s highly recommendable in some situations, specially when your sub is mounted in a vented box, or if you listen to heavy-bass music for extended periods of time.

How to Set Subsonic Filter on Car Amp

A subsonic filter is used differently in sealed enclosures than in ported. If you frequent any of the popular car audio forums, you’ll notice that there are many opinions about where to set a subsonic filter. 5Hz below what the box is tuned for… half (½) an octave below what the box is tuned for…etc.

In a Sealed Box:

Because lower frequencies are harder to re-produce, the lower you set them, the more excursion a woofer is forced to take in order to play them loudly and accurately.

Subsonic frequencies can potentially damage your subwoofer because they push it past its mechanical limit and make it play below the enclosure tuning. This is the reason why you want to cut those frequencies out.

As a general rule of thumb, if your sub is mounted in a sealed enclosure, adjust the subsonic filter to 25-35 Hz, to filter the extremely low bass frequencies your subwoofer is unable to play.

In a Ported Box:

Ported enclosures are tuned to a certain frequency range which is determined by a combination of port area, port length, and net volume of the subwoofer box.

When a vented box is tuned to a certain frequency, it will be capable of playing all frequencies above that tuning without an issue. It can also play below that frequency, but only half an octave, before the cone of the subwoofer is at risk of damage because of over-excursion.

So for safety reasons, we set the subsonic filter to half an octave below the frequency the box is tuned to.

To figure out how much half an octave is, let’s do some math.

Remember that One octave up is double the frequency, One octave down is half the frequency.

So, lets say you subwoofer box is tuned to 40Hz:

  • 40Hz / 2 = 20 Hz (one octave lower)
  • 20Hz /2 = 10 Hz (half an octave lower)

So we take 40 – 10 = 30 Hz

In other words, you set your subsonic filter to 30 Hz. So that way, if there’s music below the ports frequency, it gets filtered out protecting your woofer.

An easy way to this is to take your box tuning frequency and multiply it by .75

So it would be 40Hz x 0.75 = 30Hz

If the end result is a decimal number then you’ll need to round it up to the nearest whole number.

Keep in mind

Keep in mind that setting up your subsonic filter the way we described above isn’t by any means a bad thing, however, we do believe that this is one of those “rule of thumb” type things as every setup (box, subs, vehicle, …etc) is quite different.

So, it’s safe to say that no one can just tell you what the subsonic filter on your amp should be set to, therefore, the most important thing to do is to keep adjusting your system until you hear something you like.

Generally speaking, where you set the subsonic filter should be decided by how stressed the sub is. So, essentially, what you need to do is to start with the subsonic filter set a bit high and play a song that hits the lowest frequency, or find a test tone disc that has a variety of low frequency tracks, then gradually drop the subsonic filter until the tones cause the subwoofer to be stressed.

As you lower the subsonic filter, the woofer will move more and more until it gets to a point where the sound becomes less clear and/or just sounds off. You’ll notice the subwoofer is struggling (because its cone movement isn’t as linear and smooth) to retain its crisp clean bass, and starts to have over excursion. You want set the filter to right before that point where the subs play as low as possible without bottoming out.

Bottom Line

At the end, you should always keep in mind that the subsonic filter is NOT a cut-off filter. It isn’t a brick wall. It has a roll off “slope” where whatever frequency it’s set to will be attenuated, and the attenuation effect increases significantly as the frequencies get lower. Thus the power to the woofer decreases at filtered frequencies, which prevents over-excursion and risk of damage

Alex Brown

Hey There, my name is Alex Brown, I'm an LA-based sound engineer with over 10 years experience installing, troubleshooting, and repairing commercial, automotive, and household sound equipment. I've installed highly competitive car audio systems, and everything from navigation systems to full car stereo systems, remote starters, alarms and beyond. I enjoy creating solutions and simplifying everyday needs. I also love helping people get great sounding gear, thereby, saving the world from bad sound one customer at a time.

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