What is a Bandpass Subwoofer?

What exactly is a Bandpass Subwoofer?  Let’s take a look and see!  Along the way we will dispel some popular myths about bandpass subwoofers.

 

Forth Order Bandpass Subwoofer Enclosure

The image above shows a speaker enclosure with the subwoofer mounted on an internal baffle that divides the box into two separate airspaces.  One of those chambers is sealed, one is ported.  At first glance this seems like an odd configuration.  How does it make any sound if the speaker is inside of the box?  The answer is simple once you understand that a port is just a speaker made out of air.  As the air in the ported side of the enclosure is compressed and decompressed it causes the air in the port to move back and forth and the port produces the sound.  A common myth is that the subwoofer driver makes the sound and the port “let’s the sound out”.  That is not accurate, what you are actually hearing is the sound waves produced by the port itself.

DIY 4th Order

It turns out that there are many different ways to configure a bandpass enclosure.  The simplest, the one shown here is known as a 4^th order.  This article will only focus on 4^th order enclosures.  The image at the left shows the internals of a 4^th order bandpass that I built a few years ago. 

Why would you want to use a bandpass subwoofer?  The answer is simple “efficiency” which is code for “loud.”  A bandpass enclosure has the potential to be much louder than the same subwoofer in a sealed or a ported enclosure.  This is, of course, not an absolute statement.  From my experience modeling, designing, and building speakers a ported subwoofer can often play just as loud as bandpass subwoofer.  But in general, if you want to get the “most” out of a subwoofer then bandpass is the way to go.  They pop up quite often in car audio SPL competitions.  

 

Mackie Professional Bandpass Subwoofer (click image to view on Amazon)

 

This is where one of my favorite myths about bandpass enclosures pops up. 

If you do some searching online you will learn that there is a “magic” ratio that you need to use to build the “best” bandpass enclosure.  This ratio is 3:1, meaning that the ported chamber has three times the volume as the sealed chamber.  This is bunk.  One only needs to crack open Vance Dickson's Loudspeaker Design Cookbook to discover a lot of very complicated formulas, none of which have anything to do with ratios. Next, what do we mean by “best”?  Best in this context means loudest.  Check out the following screen captures from WINisd showing the response for three different bandpass designs. 

 

Three Bandpass Response Functions

Dayton Audio High Output Reference 10"

These graphs are for the Dayton Audio High Output Reference Subwoofer.  It is an awesome driver with an cool aluminum cone.  What you will notice is that the 3:1 (blue) is louder than the 2:1 (Black), which is still louder than the 1:1 (Red).  You will also notice that as the ported chamber gets larger, relative to the sealed chamber, that the bandwidth (i.e. the passband) gets smaller.  The 3:1 box is a “one note wonder”, also known as a “burp box.”  The idea is to play a single frequency, the one where the box peaks, and throw two or three times the rated power at the subwoofer (cooking the voice coil in the process).  You then pop the driver out, throw in your replacement driver and re-cone the original driver. 

 

This leads us to the second myth.

Bandpass subwoofers sound bad.  Not at all.  In fact Bose, a name that many** associate with sound quality, patented a very complex bandpass design back in the 1980’s.   A 3:1 bandpass sounds like utter crap.  But, a bandpass enclosure can be tuned to sound absolutely amazing.  The other contributing factor to this myth are the cheap pre-fabricated bandpass subwoofers designed for car audio.  These often have flimsy plexiglass viewing windows and fancy lighting.  They look flashy, and they can play loud, but they use cheap drivers and sound terrible.

 

A flashy, but otherwise terrible enclosure.

 So how do you design a bandpass enclosure so that it sounds good?  The answer is tricky.  There is a reason why most subwoofers use a ported cabinet.  Designing a bandpass enclosure that can outperform a ported enclosure is a real challenge, building the enclosure is also a challenge.  The stars have to align.  You need the right driver in the right box for the right application, and no amount of cleverness can ever overcome Hoffman’s Iron Law. 

 

 

But this is a place where we learn new things and challenge ourselves, so let’s go over some basics.  The best common sense explanation of bandpass tuning that I have been able to find is on JL Audio’s website.  I am going to break it down here into it’s three most important components.

 

The sealed portion determines how low the subwoofer will play.

This is of course very dependent on the driver, some will play lower than others.  But, for any given driver if you want to hit the lows then the sealed side needs to be large.  There is, of course, a tradeoff.  If the box is tuned low you will have to worry about power handling.  Always remember that the air in the enclosure is part of the subwoofer’s suspension and smaller boxes have a stiffer suspension.  Stiffer suspensions equate to higher power handling.  This is one of the reasons behind the magic 3:1 ratio.  That small rear chamber means you don’t have to worry about mechanical failure when playing loud.  You will melt the voice coil long before you hear the subwoofer make any mechanical noise from over-excursion or hitting the back plate.  The car audio SPL crowd will typically place the magnet in the ported side of the enclosure so that they can smell the voice coil heating up and pull back on the power before cooking the coils. 

The ported side determines how loud the subwoofer will play.   

Again, this is the justification for the 3:1 ratio.  A big ported side will be very loud, but there is a tradeoff.  That tradeoff is called the “passband,” the passband is how the bandpass enclosure got its’ name.  Consider the following diagram:

The Passband of a Bandpass Subwoofer

You can clearly see that the bandpass enclosure plays a range, with a high-end cut-off and a low-end cut-off.  It is not unlike the crossover that you apply to your midrange speakers, it does not play highs, it does not play lows.  The crossover allows a narrow bandwidth to pass through to the speaker. A passband.  As the ported side gets bigger the passband range gets narrower, giving us a one-note wonder.  As we make the ported section smaller, we get more music out of the box, but the music gets quieter.  The other problem is a thing known as the passbrand ripple.  Consider the strange “dog ears” in the diagram.  There is a peak at the upper and lower frequencies, and there is a dip in the middle. That is going to sound bad. The difference between the peaks and the valley tend to increase as the passband widens.

Port tuning is counter intuitive.   

The port tunes the ported chamber.  But the port tuning is based on the sealed chamber.  The port should be tuned to the center of the passband and the center of the passband is determined by the resonate frequency of the sealed side.  The resonate frequency of the sealed side is determined by the subwoofer’s free air resonance (Fs), the stiffness of its’ suspension (Vas) and the volume of air in the sealed side (Vb).  Based on this simple formula:

 

Adjusting the volume of the sealed side will adjust the center of the passband, as the sealed portion volume (Vb) gets smaller Fc and thus the center frequency of the passband gets higher, a larger box and the center of the passband gets lower.  This formula also gives us some insight into selecting the driver.  If a low tuning is desired then look for a woofer with a lower Fs and a lower Vas.  If you do not tune the port to the center of the pass band you will exaggerate one of the dog ears and flatten out the other.  This will sound like garbage.  Here is what that looks like:

A Poorly Tuned Bandpass Response Function

There is a whole lot more to it, but this is supposed to be a brief, non-technical article.  So, I will stop here!  If you have enjoyed this article consider becoming a patron on Patreon, or you can always drop me a $5 or $10 tip via paypal:  paypal.me/diyaudioguy

*Some of the links on this website are affiliate links.  If you order something through one of these links, I may earn a small commission.  As an Amazon associate, I may earn a small commission on qualifying purchases.

**I don’t associate Bose with sound quality.  There stuff sounds OK, but it is overpriced.  You can get something just as good for less money, or something much better for the same money.

Subwoofer Ports

Subwoofer ports (a.k.a vents) are interesting things.  In a previous post I provided some background into how ports worked, in this post I am going to dig a little deeper into port design, a.k.a. port tuning.

Flared Ports with Wooden Connector Rings

The Port Formula

There is a formula that you can use to design a port, since formulas tend to scare people away and there plenty of tools available to do the calculations for you this blog post will focus on understanding the four main parameters that you need to think about when designing a port. 

• Tuning Frequency
• Enclosure Volume
• Port Cross Section
• Port Length

We can manipulate any three of these four items and the formula dictates the forth parameter.  For this post we are going to examine how the tuning frequency, enclosure volume, and port cross section influence the length of the port.  It is very important to understand the impact of these four parameters as they will determine the size, shape and sound of our subwoofer.  We need to get the sound we want, while keeping a reasonably sized box, a reasonably sized port, and a reasonably simple port.  We also need to design the system so we don't have any port noise (a.k.a. "chuffing").

  

Tuning Frequency

This is the resonate frequency, expressed in hertz (HZ) of the subwoofer enclosure.  Sometimes we call this Fb, or the box frequency.  We could have an entire blog post dedicated to understanding this concept.  But for now all you need to know is that if you want your subwoofer to play lower then you need to to tune the box lower.  For a given enclosure volume and port cross section you can adjust the tuning frequency by adjusting the length of the port.  As the port gets longer the tuning frequency gets lower, As the port gets shorter the tuning frequency gets higher.  This can be clearly seen in in this plot, which is based on a 4" diameter port in a 2 cubic ft box.

Port Length VS. Tuning Frequency, 2 CU FT Enclosure, 4" Diameter Port

For those that have a hard time visualizing a plot consider these images, each shows an identical sized enclosure (2 Cu FT), each with a 1 x 12 slot port.  The first image shows a 25 3/16" long port, which corresponds to a 24 HZ tuning frequency.  The second image shows a 17 3/4" long port, this enclosure is tuned to 28 HZ.  The third image shows a 12 59/64" long port, and a 32 HZ tuning frequency.  The key take away here is that a lower tuning frequency requires a longer port, holding everything else constant.  The ports are shown outside of the enclosure so that the relative size of the ports are easier to visualize. 

24 HZ Fb, 2 CU FT, 1X12 Slot Port

28 HZ Fb, 2 CU FT, 1X12 Slot Port

 

32 HZ Fb, 2 CU FT, 1X12 Slot Port

 

The ports in the above three images are relatively small and can be quite easily folded into their respective enclosures (obviously the enclosures will need a slight redesign in order to maintain the same airspace).  But, port design is not quite that simple.  There are several other factors that must be considered.  

Enclosure Volume

Now consider a slightly different scenario, let's hold the tuning frequency constant, along with the port cross section and look at what happens when we adjust the box volume.  As the enclosure volume gets larger the port gets smaller, and vice versa.  This plot shows the relationship for a 4" diameter port tuned to 40 Hz.

Port Length Vs. Box Volume, 4" Diameter, 40 HZ Fb

 

 

Again, for those that find it difficult to visualize this consider these two images which both show two absurd enclosures.  Neither of these is very good design, they were just selected for.  The first is a 1 CU FT enclosure tuned 28 HZ with a 1X12 slot port, and the second is a 4 CU ft enclosure tuned to 28 HZ with a 1X12 slot port.

1 CU FT, 28 HZ Fb, 1X12 Slot Port

4 CU FT, 28 HZ Fb, 1X12 Slot Port

The 1 CU FT enclosure has two main problems.  First the port is comically long.  It would be very difficult to build that port (it is over 3' long) into the enclosure.  This is the result of using an undersized box.  As the box gets smaller, holding everything else constant, the port must get longer.  Second, the small size of the box will have adverse effects on low frequency response.  The 4 CU FT box, on the other hand is not a reasonable solution.  For starters it is a comically oversized enclosure.  With a box this large the port length becomes trivial.   

Port Cross Section

The port cross section, which rather the cross sectional area is another important factor.  The port needs to be large enough to prevent port noise, also called chuffing.  This happens when the air in the port moves fast enough to create turbulence.  As a general rule of thumb the air speed velocity needs to be less than 5% of the speed of sound.  There are two solutions to this problem, the first is to flare one or both ends of the port.  The second is a larger port opening.  But, that presents a problem.  As the cross sectional area of the port increases, holding everything else constant, the port length increases.  Here is yet another chart showing the relationship.  This example uses a 2 CU FT enclosure tuned to 30 HZ.

Port Length vs Port Cross Sectional Area, 2 CU FT, 30 HZ Fb

Here is a example of a comically long port.  This was designed to show the absurd result you wold get if you were to take things to the extreme in order to get rid of port noise.  This design uses a 2 CU FT enclosure, tuned to 28 HZ with a 3X12 slot port.  The end result is a port that is almost 5'.

3X12 Slot Port, 2 CU FT, 28 HZ Fb

So there you go, now you know how box tuning, port area, and enclosure size impacts the length of you port. Hopefully you found this post interesting and informative.  I am planning a few more posts that dig deeper into the port tuning formula, so make sure you check back in frequently.  If you want to learn more you can also check out this video on the DIY Audio Guy YouTube channel: