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What Should I Set My Low Pass Filter To

There’s no denying that getting the exact bass sound you want from your subs can be a daunting task. However, the rewards of a well-tuned subwoofer system are gratifying and satisfying.

Getting that energetic bass isn’t actually so hard if you know the basics. The right low pass frequency can make a night and day difference!

In this article… I’ll show you what to set your low pass filter to in order to get your subwoofers to sound just the way you want them to.

What is a Subwoofer’s Low-Pass Filter?

To put it simply, a low-pass filter is an electronic circuit that allows signals with a frequency lower than the cutoff frequency to pass through it while blocking those any higher (because they are not produced efficiently). For example, if you set a low-pass filter to 100 Hz, it will block the notes above 100 Hz to a certain degree (depending on slope) and allow those under that to pass.

Why Use a Low-Pass Filter?

The reason we use low pass filter is because when higher frequencies get to the subwoofer, they will not be reproduced faithfully. This results in distortion and poor sound quality.good low pass filter setting

In other words, we use the low-pass filter to allow the subwoofer to run more efficiently. Speaking of efficiency, the less broad a spectrum of frequencies a subwoofer has to play, the more efficient, clear and loud the frequencies will sound.

Generally speaking, by asking your subwoofer to play tones all the way down to 20Hz and up to 200Hz or possibly more, you’re forcing it to operate in a sonic range that it was not designed for. This doesn’t only put a ton of strain on the subwoofer, but it also makes it sound terrible the entire time.

I mean, the higher the frequency a subwoofer is asked to reproduce, the greater the potential risk of mechanical failure, as well as its inability to blend smoothly with the main speakers (this holds true for both a home and car audio system).

As a general rule of thumb, always keep in mind that a low crossover frequency minimizes the sub’s potential to do more harm than good.

We always recommend starting with the cutoff frequency point set to 80Hz. That’s because most often your highs won’t go much lower than that, so the sub will “take over” from that point down.

Should you Turn your Subwoofer Low Pass Filter On or Off?

Every musical note, has a precise frequency. Higher-pitched notes have higher frequencies, and vice versa.

Regular speakers aren’t designed to reproduce low frequency sounds. This makes a lot of music sound flat.

A subwoofer on the other hand is built to focus on producing the lower frequencies within music, known as the bass.

If you turn the low pass filter off, your midrange speakers and subwoofer would duplicate too many of the same frequencies and your sub would waste time trying to reproduce high notes it wasn’t meant to handle.

So, It doesn’t make any sense to turn your subwoofer low pass filter off. However, if both the receiver and your amplifier have low-pass filters without an option to turn it off on neither of them. What should you do? Use one or the other, or set it on both of them?

Well, in such case, you must use one or the other, but not both. This is because something called phase distortion generates around each filter crossover frequency, muddying up the sound.

What Should You Set Your Low Pass Filter To?

Well, this is not an easy question to answer. In fact, there are many different answers depending on your system and your preferences. However, in most cases it’s safe to set your low-pass filter to 80 Hz or so.

I a perfect world, everything below that cutoff frequency point would be unaffected, and everything above it would be completely filtered out. But that’s not the case in the real world.

I mean, no filter is a true brick wall, and some frequencies above the desired passband gets through.

If, for example, you set your low pass filter to 80Hz, a frequency like 90-95Hz notes will still get through and be slightly audible, albeit much quieter than they would should the low pass filter not be set. Think of the filter as a slope, not an on-off switch.

  • In the line graphs above, the passband refers to the range of frequencies for which a low-pass filter does not cause significant attenuation, while the stopband refers to the range of frequencies for which the filter does cause significant attenuation.

 

  • As you can notice in the graphs above, the transition from passband to stopband in a low-pass filter happens gradually. This means it’s impossible to precisely identify one frequency at which the filter stops passing signals and starts blocking signals. This is where the concept of cutoff frequency comes into play.

 

  • In the line graphs above, the ƒc refers to the cutoff frequency point which is the frequency at which the amplitude response of the input signal is reduced by 3 dB relative to the level of the main passband output of the filter. This value (-3dB) was chosen because it represents the frequency where the output power is reduced by one-half. This is the reason why the cutoff frequency is most often referred to as the (–3 dB) frequency, and in fact this name is much more accurate and more informative.

The (b) line graph above shows how the low-pass filter should work if it were a true brick wall. However, that’s not the case.

The (a) line graph above shows a more realistic view into what really happens when a low-pass filter is set to a certain value.

As already mentioned, when you look at the (a) line graph above, you’ll notice that the term “cutoff frequency” is not that accurate. The image of a signal’s spectrum being divided into two parts, one of which is retained and one of which is discarded, does not really apply, because attenuation increases gradually as frequencies move from below the cutoff to above the cutoff.

I mean, notice how the attenuation begins even earlier than the cutoff frequency point (ƒc) in the pass-band and frequencies higher than the cutoff frequency point in the stop-band are still getting through, even though they are attenuated.

How Do You Set the Low-Pass Filter on a Subwoofer?

Theoretically speaking, you could just set the low-pass filter on the subwoofer to the lowest frequency that your main speakers can handle faithfully without their own attenuation. However, most often than not, this might not be a good way. For good low pass filter setting, follow the step below:

Step #1

As mentioned and as shown in the graph above, the low-pass filter isn’t perfect — the attenuation begins below the frequency you set in the filter. So you want some overlap to allow for smooth transition.

A commonly-stated rule of thumb is you want to dial-in a bit of overlap (roughly 10 Hz or so), so for example if your main speakers are reasonably flat down to 90 Hz, you would set the low-pass filter on the subwoofer to (90 Hz + 10 Hz) 100 Hz. Of course, this is just a general rule of thumb, not set in stone.

Step #2

What makes things worse is that it’s not always obvious what frequencies your main speakers can handle faithfully. I mean, any specifications that are in the form of “50 Hz to 22 KHz” provided by the manufacturer are meaningless for the most part. However, some manufacturers do provide actual frequency response graphs for their speakers.

So, let’s say you’ve managed to find a frequency response graph for your speakers. It should look something like this:

If you take a closer look at the line graph curve above, you could arguably say that the corner or cuttoff frequency should be set exactly at 90Hz (below which it is on a clear downward trend), or you could say that it hits -3 dB at 85 Hz. Either is defensible, but I’d personally start with the former, and say 90 Hz.

In this case here, add roughly 10 Hz of overlap, and set the low-pass filter frequency on your subwoofer at 100 Hz.

Now listen carefully and make sure your sub sounds the way you want it to. Play a well-balanced soundtrack with full-range music, or alternatively play a frequency sweep (use e.g. this video on Youtube), where a tone (typically a sine-wave tone) goes from 20 Hz to 20 KHz.

As the tone passes through the frequencies around the transition between the subwoofer and the main speakers, listen carefully. How does it sound? Is it louder or quieter at the transition section? Is there a build-up or a drop-off around that point? If there is, fix it. You want the transition to be smooth and seamless.

Understanding Crossover Slope Options

Before we get into the crossover slope options, it’s important to understand what crossovers are and how they operate.

A crossover is a term that comes from the idea of crossing over from one frequency range to the next.

A crossover serves as a network of filters that block out unwanted frequencies to a speaker or group of speakers. You’ll find crossovers in some form almost any time speakers are present.

Basically, what a crossover does is that it divides an input signal into two or more outputs of different ranges of frequencies (high-, mid-, and low-range frequencies), so that tweeters, speakers, and subwoofers will each get only the range of frequencies they were designed to play. Any other frequencies outside each designated range are attenuated or blocked.

Think of a crossover network as an audio traffic cop, directing highs to your tweeters, midrange to your woofers, and lows to your sub.

Crossovers are generally defined by three main parameters; a cutoff frequency, a slope, and an implementation.

The cutoff frequency or corner frequency refers to the frequency at which the output signal of the system starts to take a nosedive. More precisely, at the cutoff frequency, the ratio (Output power)/(Input power) attenuates by a factor of 1/2. This is generally “-3dB” and “-6dB” respectively. This defines the shape of the filter around the cutoff frequency.

The crossover slope defines the rate at which the filter response falls beyond the cutoff frequency. In other words, it’s how effective the filter is at attenuating sound frequencies beyond the cutoff frequency point. This is defined in dB/Octave and the most common slopes are 6, 12, 18, and 24 dB/octave. The higher the dB/octave rating, the steeper the slope on the crossover.

  • 6dB/octave is referred to as first order
  • 12dB/octave is referred to as second order
  • 18dB/octave is referred to as third order
  • 24dB/octave is referred to as fourth order

Crossover slope is a crucial element you need take into account when you’re setting the low pass filter for your subwoofer.

Changing your crossover slope will significantly affect your sub’s power handing performance and overall frequency response.

A 6dB per octave crossover for instance reduces signal level by 6dB in every octave starting at the crossover point.

So, since an octave is a doubling or halving of a frequency number (i.e: 50Hz –> 100Hz –> 200Hz –> 400Hz, etc.), every time the frequency of the audio signal is increased by one octave, the level of the audio signal will change by 6dB.

When a crossover is said to have a cutoff of -6dB per octave, that means it will keep decreasing the output by an additional 6dB for every doubling of the previous frequency.

Example

  • 6dB @ 1KHz
  • 12dB @ 2KHz
  • 18dB @ 4KHz
  • 24dB @ 8KHz

What Crossover Slope Should You Use for a Subwoofer?

When it comes to settings the crossover slope for a subwoofer, most people usually put the sub on the steepest slope that the receiver allows. We’re not going to say that this is wrong, because each subwoofer is built differently. So, it really depends on the natural roll-off of the driver.

12dB/octave or 24dB/octave are some of the most commonly used slope options in car audio. However, experimentation is a must.

So, try switching the slope and listen for any improvements in sound quality. If sound quality is below par, simply switch it back.

12 dB per octave slope (green) vs 24 dB per octave slope (orange) on a LPF set at 80 Hz.
  • A 12dB/octave slope is a more gradual cut off and is best suited for coupes or sedans that have the subwoofer(s) mounted in the trunk. The reason the 12dB/octave slope works well for these types of cars is because the rear seat material soak up some frequencies and acts as a complementary filter which can relatively reduce upper bass range amplitude. The more shallow slope of 12dB/octave will allow a bit more bleed through of frequencies above the filter frequency to help counter this.

 

  • A 24dB/octave slope on the other hand is twice as steep as a 12dB/octave slope. It’s a more abrupt and dramatic cut off and works well in open vehicles such as hatchbacks, wagons and SUVs, since the bass notes aren’t soaked up by seat material. Furthermore, since a 24dB slope is more abrupt, it can sometimes allow for a slightly lower crossover point between the tweeter and midrange while still offering more protection specially for tweeters.

12 dB/octave Vs 24 dB/octave – Which is Better?

When you listen to your music, whether in a car or in a room, you’re not supposed to “hear the speakers”. In other words, you’re not supposed to locate them. The music should just sort of appear in the car and reach your ears coming from the correct direction, and sound right.

A properly mounted speaker, or set of speakers with the correct audio settings will simply disappear.

Same thing holds true for a subwoofer. When you add a sub to your system, there should be more bass (or, more correctly, the right amount of bass), but you shouldn’t be able to locate the subwoofer when it’s hitting hard, and more importantly, you shouldn’t be able to hear when the sound “shifts” from the speakers to the subwoofer.

The best crossover slope should be the one that allows a smooth “handoff” from one driver to the other, so all you can hear is well-balanced music.

So should you choose a sharp slope or a more gradual slope?

Well, as with everything in life, it’s all about tradeoffs. In the real world, each has its own benefits and drawbacks.

Gradual Slope

Unlike sharp slopes, gradual slopes don’t have as sharp a “handoff” from one driver to the other. In other words, they have more overlap which allows for a relatively smoother transition. In fact, the more you overlap the area where both the subwoofer and the speakers meet, the more gradual the transition between them, and possibly the smoother the transition.

Most importantly, these gradual filters cause less “sonic issues” than sharper filters. This is the reason why gradual slopes are said to have more musical quality than sharp slopes.

Nevertheless, keep in mind that the wider the overlap area, the more likely the drivers won’t blend smoothly, or that you will get some interference between them.

Steeper Slope

A steeper slope, with a faster and narrower transition means less overlap. This means sharper slopes are less likely to have interference between the subwoofer and the speakers.

I mean, think about it, since most cancellations happen in the transition area where the overlap occurs, shrinking this area should significantly lessen the likelihood or severity of those problems, but make it more likely to hear the “dividing line” between the drivers.

Additionally, steep slopes are highly likely to cause some unwanted artifacts such as resonance or ringing (the steeper the slope, the more ringing).

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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