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How to Tune a Subwoofer Box

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

You can use the greatest, most powerful subwoofer and amp in the market but if your subwoofer box isn’t properly tuned, you will not get that energetic bass you want from your subs.

An improperly tuned subwoofer box can leave a lot to be desired. It’ll hold your audio system back from reaching its full potential and sounding good, which can turn it into an expensive disappointment.

In this article… We’ll show you how to properly tune your subwoofer box in order to get your subwoofers to sound just the way you want them to.

What Is Port Tuning Frequency?

To put it simply, port tuning frequency (AKA: Fb or box tuning frequency) is the frequency where the subwoofer port is doing the majority of its work to reinforce the low end.

Different drivers in different box sizes will all end up with different tunings depending on their parameters.

Port tuning frequency can also be described as the frequency at which air naturally resonates through the port, and it’s determined by a combination of port’s cross-sectional area, port length, and net volume of the subwoofer box.

It goes without saying that your tuning choice will be determined by your goal. That is to say, if you’re more into SPL (getting as loud as possible), you’ll need to tune your sub box fairly high.

High port tuning frequency for SPL is usually somewhere around 40-45Hz or slightly higher. At this range, the sound quality will be nothing to write home about.

40-45Hz is a horrible tuning frequency range for listening purposes, but it allows your subwoofer box to sound much louder than if it’s tuned to hit lower frequencies.

For this reason, if SQ (sound quality) is high on your list, you’ll want to tune your sub box fairly low – even down around 25Hz. This will allow your subwoofer to dig down into the depths of 20Hz. But by tuning low, you are sacrificing overall SPL.

A lower-tuned subwoofer box will produce better sound quality that’s more comparable to a sealed box. The main downside to lower tuning is that it flattens the frequency response, which causes the box to lack the boost around the tuning frequency.

All in all, tuning a subwoofer box is all about tradeoffs. Therefore, you have to find a balance of all factors that suit your needs.

Metaphorically speaking, you can think about this like the tachometer (RPM gauge) redline in your car. As you approach the redline, performance just gets better and better. The redline is your tuning frequency and the closer you drive to that frequency the better your driver performs. However, when you reach the redline, you have entered a danger territory where the mechanical and thermal limits are being pushed to the limit and your performance begins to take a sharp noose dive.

What is Resonant Frequency?

To put it simply, resonance refers to a frequency point where the resistance provided by the inertia of the moving mass (port) and that of the driver’s suspension are equal.

So, with that in mind, and since resonance is the point where these two suspensions are equal, it’s safe to say that at other frequencies the two suspensions will not be equal.

I mean, when you are above resonant frequency, the mass (port) dominates, whereas when you’re below it the suspension (spring) of the sub is dominant. The resonant frequency is where the two suspensions complement each other and become friendly.

P.S: In theory, you’ll get the highest efficiency if your sub’s Fs (the driver’s natural resonance frequency) and Fb (tuning frequency of the box) are identical, but that’s only if all the other parameters work with that scenario as well.

 

Technically speaking, when you’re above resonant frequency, the resistance of the air mass (port) is significantly high due to the fact that the port isn’t quick enough to react to the quick changes in direction. This results in an inactive port, making the box behave as if it were a large sealed box. This doesn’t happen instantly. It’s more of a gradual progression. Still, you’ll get a minor boost in output for a range of frequencies.

To put it another way, when you’re above this frequency, the movement will be controlled mainly by the mass. And since force acting on a mass is proportional to the acceleration of the mass (F=ma), we will have an acceleration proportional to the driving current. Furthermore, since sound pressure was proportional to acceleration, you get a flat response for frequencies above the resonant frequency.

When everything is tuned perfectly, the port becomes active since it matches the resistance of the frequency played (inertia). It uses the speaker cone as a fulcrum for its own output, and sucks energy through the internal air pressure and loads the driver in the process. This allows for higher output, lower excursion for the subwoofer since it’s controlled by the port’s activity.

Below resonance, the movement is controlled mainly by the spring. This means the excursion of the cone will be proportional to the driving current running through the coil.

In other words, when you’re below resonance, the port is no longer contributing or adding any significant amount of resistance. At this point, the suspension of the subwoofer is higher. While the inertia remains more or less the same as the excursion requirements become substantially higher at any given output level, the influence of the driver’s suspension increases significantly, becoming more of a factor than the port. The latter becomes like a hole in a baffle and doesn’t load the driver anymore, causing cancellation and pushing the subwoofer past its mechanical limit (outrageous excursions) due to the lack of control.

How to Calculate Port Tuning Frequency

The tuning frequency (Fb) is a function of the port’s cross-sectional area and its length, as well as the net volume of the subwoofer box as given by this formula:

  • Av is the cross-sectional area of the port (in square inches) : when the vent cross section increases, the tuning goes up. In other words, you get a higher tuned box with a bigger opening. However, you need to keep in mind that the larger the area of your port (bigger Av), the longer it needs to be to maintain the tuning frequency.
  • Lv is the length of the port (in inches): as the port gets longer, the box gets lower.
  • Vb is the enclosure’s net volume (in cubic inches): the bigger the box the lower the tuning frequency.

Why Tune a Subwoofer Box?

The main reason why you should tune your ported subwoofer box to a certain frequency is because the port makes the enclosure more efficient at the tuned frequency.

The port increases efficiency by allowing the back-wave generated by the driver to reinforce the front-wave.

If the enclosure, for example, is tuned to 30Hz, there will be an increased output at this frequency. The driver, however, will appear not to move as the output will be coming from the port.

Keep in mind that when you run your subwoofer below the tuning frequency, the port will allow the driver to unload. That’s to say, it’ll behave as though it’s not mounted in a box. So, it’ll move much more in each direction with less power thus making it very easy to push the driver past its mechanical or thermal limits.

P.S: The sub’s suspension system plays a huge role on how far below tuning it can play before unloading.

 

Generally speaking, people have different preferences when it comes to tuning their subwoofer boxes.

Some prefer to tune their box fairly high, while others prefer to tune it slightly low.

The thing is that for deep musical bass, you’ll need to tune the enclosure just above the free-air resonant frequency (Fs) of the subwoofer. That’s the sweet spot where the output of the box will spike and hit hardest.

On the other hand, the higher you tune the box, the boomier it’ll sound, which isn’t audibly pleasing, but tuning high does increase SPL.

It goes without saying that people who are more into sound quality tune their subwoofer box to a low frequency like 28Hz or 30Hz, and rarely run their subs below that frequency, so the driver hardly ever unloads.

SPL people on the other hand tune their enclosures to a higher frequency like 40Hz and run their sub below the tuning frequency a lot, moving more air and thus sounding louder, yet worse.

The reason a higher tuning sounds worse is that a box tuned to ~40+ is going to roll off below tuning, thereby you loose a lot of low end output.

With that in mind, if you are into SQ, you’ll need to set an infrasonic filter to prevent the sub from unloading below tuning frequency. So, anything below the port frequency is cut off sharply, meaning if you tune to 35Hz, you get no bass below 35Hz, or not much anyway with a steep Q.

 

P.S: To set the record straight, if an enclosure is tuned to 30Hz, the driver does not fully unload at 29Hz. The “unloading” is an incremental process so the lower you go below 30Hz, the more it starts to unload. In other words, the driver may not be completely unloaded until around 15Hz or so.

 

Why Does Every One Tune Their Box to ~30 Hz?

The reason why ~30 Hz is the favorite tuning frequency of most guys out there, is because it’s a sweet spot that offers a perfect mix of SQ and SPL.

Keep in mind that when a subwoofer box is tuned to 32Hz for example, it can still play well below and above that frequency. The lower you play under your box’s tuned frequency the more easily the subwoofer is going to unload. But, that’s where the subsonic filter comes into play.

When the subsonic filter is set, it’ll prevent your driver from bottoming out and getting damaged from playing beyond its mechanical limits.

What Should I Tune My Sub Box To?

First and foremost, it’s worth mentioning that sub box tuning is entirely a personal preference. Furthermore, it’s also vehicle dependant. The same sub/box/amp combo in your car will sound quite different in another car, truck, or van.

Usually we tune boxes at or a bit above (Fs) of the driver in case of ported enclosures. The Fs of a subwoofer is simply where the impedance curve of the driver peaks in a free-air environment. As soon as you mount the driver in an enclosure its impedance curve changes substantially.

It used to be that you had to have a subwoofer with a really low Fs (resonant frequency) in order to play true sub bass, but with the abundance of wattage and tweaking equipment (DSP, EQ ..etc), nowadays you can add enough boost below (Fs) to compensate. This is not to mention the fact that once you factor in cabin gain, this might not be necessary anyway.

So, if you’re wondering what to tune your subwoofer box to, bear in mind that there is no magic port tuning frequency to use. It all depends – to a great extent – on the subwoofer and the vehicle it’s mounted in in.

Every vehicle will sound different and require a different type of tuning. Some will play one genre of music just fine when tuned to 30hz. Others will require 40hz tuning, so it’s best to test out the cabin gain of your vehicle to see what frequency you peak at.

Additionally, it must be noted that each car has multiple resonant frequencies. This is because every pair of parallel surfaces has one frequency. Additionally, there’s the Helmholtz resonance, which occurs when a car window is open and makes a loud sound, also called side window buffeting or wind throb – it’s the sound that an empty bottle makes when you blow across the opening). Furthermore, there’s panel resonance, and the list goes on…

 

That being said, the most important resonant frequency is the box’s your subwoofer is mounted in.

There’s no denying a bad box can ruin the best subwoofer, and even a well-built box that’s not the right size or design will be nothing to write home about.

So, instead of speculating on what to tune your subwoofer box to, you can simply run a sine sweep to identify your frequency range. The key here is to figure out your vehicle’s resonant frequency. Then, coincide that with your subwoofer box for a nearly perfect port tuning.

Best Tuning Frequency for Ported Box

Generally speaking, the best tuning frequency for ported box would be anywhere from 27-35Hz (depending on your subs and what you want out of them). This is especially true for everyday tuning. However, you’ll need to tune to 35Hz or higher if you are into SPL competition.

For a daily driver, we would recommend you to tune your box to sound great playing the types of music you listen to the most. For example, if you’re listening to classic rock and country music most of the time, it makes sense to tune your sub box fairly high to make the most of it in that range. On the contrary, there is no reason to have a flat response down to 22Hz if the lowest you’ll ever play is 46Hz.

If you listen to a multitude of genres, then simply pick a happy medium and you won’t be disappointed.

How to Find What Your Subwoofer Box Is Tuned To?

The best and the easiest way to find what your subwoofer box is tuned to is by playing a series of test tones around where you think it is tuned and find the point where the subwoofer barely moves or where it moves the least, yet you feel plenty of air movement in the vent. That frequency will be the tuning frequency.

Another easy, yet expensive way to figure out what your subwoofer box is tuned to is to use Dayton Audio DATS, which is a computer based audio component test system. The system will run a quick sweep and plot the impedance magnitude on screen (as shown in the screenshot below) making it easy to verify the actual tuning of your system.

You can also find what a subwoofer box is tuned to by plugging in the dimensions of the box, and the dimensions of the port(s) into a subwoofer box tuning calculator or a modeling software such as WinISD or BassBox Pro.

How to Tune a Subwoofer Box To Hit Lower

There are many different ways to make the bass in your car sound its best. Tuning your subwoofer box to hit lower frequencies is one way. But wait! Is there a way to tune a sub box to hit lower after it was already built? Well, the short answer is yes.

It must be noted that when it comes to tuning a subwoofer box: longer port equals lower tuning.

So, if your subwoofer box has a slot port, you can simply add another section to the inner port by going through the subwoofer cutout, and attaching the extra section with screws on the sides or with L brackets, and then use silicon glue on the sides to make it air tight. You will have to calculate the total loss of net volume to get your new tuning though.

Another easy method to lower the sub box’s tuning is to add a piece of MDF to the port wall(s) to reduce the port area, which in turn will lower the tuning.

It must be noted that if you can’t afford to lose any net volume, reducing the port area will not affect your volume, but extending inner port length will.

Also, adding polyfill will lower your box’s tuning frequency but only by a few Hz.

As mentioned above, longer port = lower tuning. So, another method to lower the tuning frequency of your subwoofer box is to build an external port and attach it to the existing port opening. But, we wouldn’t recommend this, unless you find a creative way to wrap it around the box, and make it look neat. Because otherwise, it will not look good.

Although it’s possible to tune a pre-built sub box to hit lower, it’s not something we recommend at all.

This is because changing the physical parameters of the enclosure after it was originally designed/built for a specific driver, will change the way the driver performs all together. This is especially true if your enclosure can’t afford losing any net volume.

Do Sealed Boxes Have a Resonant Frequency?

Sealed enclosures don’t have a resonant frequency in the sense that ported enclosures have a resonant frequency, near the cutoff point of a typical bass driver.

As mentioned above, in a typical ported subwoofer box, the resonant frequency is set up by a combination of port’s cross-sectional area, port length, and the net volume of the air in the box. Without a port or passive radiator, you have no resonance.

Still, it makes sense to think that a sealed volume of air is a damped mass-spring system, and therefore might have some resonances. Furthermore, typical subwoofer box panels can also generate standing waves, which also count.

In other words, a driver that’s mounted in a sealed enclosure has both a mass and a stiffness (think of a spring), and this combination does resonate at a certain frequency. This resonance is second order because there are two energy storage devices. The mass and the spring.

In a sealed enclosure, the backwaves are isolated in order to prevent cancellation, and in order to act as a stiffness against which the driver must rely on in order to move. The more stiffness there is, the higher the driver’s resonance.

Speaking of stiffness, the bigger the cone, the stiffer an equal volume of air will appear because more air is displaced for the same distance moved – that’s to say the air is compressed more.

Do Sealed Boxes Have a Tuning Frequency?

Sealed enclosures don’t have a tuning frequency in the sense that ported enclosures have a tuning frequency.

What most people mean by tuning a subwoofer box, is adjusting the port for the frequency at which air naturally resonates through the port. Thereby making use of the rear waves. However, in the case of sealed enclosures, there’s no port to tune.

Nevertheless, you shouldn’t forget that Qtc (the total Q of the speaker in an enclosure) determines how the speaker performs in the enclosure. Every enclosure has a Q (quality factor), and the Q of a sealed enclosure can either make it a peaky boomy box, a low frequency monster, or anything in between. It all depends on how you design it.

Generally speaking, a Qtc of .707 will give the flattest frequency response (most musically accurate), while a Qtc of .9 to 1.1 (which most people prefer) produces emphasized bass. Anything above 1.2 will sound unnatural and boomy.

It goes without saying that typical sealed enclosures have a peak with a high Q which can be undesirable, but some people tend to like it.

So, although you can adjust the peak of a sealed subwoofer box, that doesn’t really equal tuning.

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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