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What Is a Car Audio Amp or Processor Infrasonic/Subsonic Filter?

August 18, 2021 By 1sixty8 media

Subsonic Filter

If you’ve looked at any top-quality car audio amplifiers or signal processors, you may have seen an adjustment labeled as a subsonic filter. If you have subwoofers or speakers that may not play the lowest of audio frequencies at high levels, your installer can adjust this control to provide protection. First, let’s look at how it all works.

Proper Filter Terminology

First and foremost, the proper term for this feature is an infrasonic filter. The term subsonic refers specifically to aircraft or projectiles traveling at less than the speed of sound, or 767.27 miles per hour. The word infrasonic refers to sound waves that are at or below the lower threshold of human hearing. We typically associate this limit as being around 20 Hz.

If you have seen an amplifier or processor with the label subsonic, it’s incorrect.

What Is an Infrasonic Filter?

The subwoofer amplifier you choose to have installed in your vehicle will almost always include a low-pass filter. This type of filter passes audio frequencies below the crossover point. In the case of a subwoofer, we’d typically set this at 60 or 65 Hz so that the output blends nicely with the midbass drivers in the car.

Subsonic Filter
The ARC Audio PS8-Pro software showing the predicted response of a 60 Hz low-pass filter.

An infrasonic filter is nothing more than a high-pass filter set at a very low frequency. Many subwoofer amplifiers have an analog control set at frequencies between 10 and 40 Hz or a switch that enables the filter at a specific frequency.

Subsonic Filter
We have applied a 25 Hz infrasonic filter in the PS8 Pro software.
Subsonic Filter
The Rockford Fosgate P1000X1bd amplifier has a selectable infrasonic filter fixed at 28 Hz with a slope of -12 dB/octave.
Subsonic Filter
The 1,100-watt ARC Audio X2 1100.1 amplifier has an infrasonic filter that’s adjustable from 10 to 80 Hz.

Why Would a Subwoofer Need Less Bass?

The question remains, why would we need to apply a high-pass filter to our subwoofer system? Isn’t the point of a subwoofer to reproduce as much low-frequency information as possible? The answer lies in the power handling capabilities of the subwoofer system. For all speakers, cone excursion increases by a factor of four for every octave lower a speaker plays. The graph below shows the cone excursion of a high-performance subwoofer with an Xmax specification of 17.6 mm in a sealed enclosure and driven with 1,000 watts of power.

Subsonic Filter
The predicted cone excursion of a 12-inch subwoofer in a sealed enclosure powered with a 1,000-watt amplifier.

As you can see, the driver never exceeds the maximum excursion rating, so the system doesn’t need an infrasonic filter. What if we want to increase the efficiency of the subwoofer system and decide that a vented enclosure is better suited to our application? Here’s the cone excursion graph of the same woofer in a 2-cubic-foot enclosure with a vent tuned to 31 Hz.

Subsonic Filter
The predicted cone excursion (in violet) of our 12-inch subwoofer in a 2-cubic-foot vented enclosure.

Now we have a problem. The woofer doesn’t move significantly around the tuning frequency of 31 Hz (which reduces distortion) but goes crazy below that. So at frequencies below 25 Hz, when driven with 1,000 watts of power, the subwoofer may be damaged.

If we implement a high-pass filter at 25 Hz, we reduce the output at that frequency by 50% (-3 dB) and attenuate frequencies below that point even more. Unfortunately, the BassBox Pro software doesn’t easily model cone excursion with a high-pass filter applied.

Subsonic Filter
The Audison Voce AV uno is a 1,700-watt amplifier with an infrasonic filter that’s adjustable from 20 to 50 Hz and has a slope of -24 dB/octave.
Subsonic Filter
The 1,000-watt ML Power 1 from Hertz includes an -24 db/octave infrasonic filter that can be adjusted between 18 and 40 Hz.

Pick the Right Subwoofer Amplifier for Your Car Audio System

When you’re working with the local specialty mobile enhancement retailer designing the next upgrade for your car audio system, be sure they model the subwoofers you have chosen in the enclosure they will be constructing. If the amp you’ve picked can drive the subwoofers beyond their excursion limits at low frequencies, make sure it has an infrasonic filter built in.

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Filed Under: ARTICLES, Car Audio, Resource Library Tagged With: Amplifiers, Digital Signal Processors, Infrasonic Filter, Subwoofers

About 1sixty8 media

1sixty8 media is a content marketing, website development, and business solutions provider specializing in the mobile enhancement industry. The 1sixty8 media team includes many household names in the industry, from store owners and industry writers to award-winning installers and fabricators. To learn more about 1sixty8 media, visit www.1sixty8.com.

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