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.

The Best (and Worst) Subwoofer Enclosure on Amazon.

 

This website* is primarily dedicated to teaching you how to build your own subwoofer.  If you want to get into box building check out this blog post, and this podcast where some top DIYers talk about how to get started:

  

But, not everybody can do that.  You need tools, time, and skill.  There is no shame in ordering a pre-fabricated (a.k.a pre-fab) subwoofer enclosure from Amazon and installing your own speakers.  Here is some examples of good and bad enclosures along with a breakdown of what makes a good enclosure.  We will cover unloaded (no subwoofer included) enclosures in this post and save pre-loaded (subwoofer included) enclosures for another day.

Unloaded:  BYOS (Buy Your Own Subwoofer)

Before we get into reviewing boxes keep in mind that if you go with an unloaded enclosure you will need to order a subwoofer to go with it.  Your goal is to match the subwoofer to the enclosure.  Typically this is just a matter of looking up the manufacturer specifications and looking for a prefab box that will work.  But, before you do that you need to know what makes a "good" box good and what maxes a "bad" box bad.

A Bad Box (click on the image to view on Amazon)

So, what's so bad about this box?  Everything, this box is why pre-fab boxes have such a terrible reputation in car audio.

It is made out of 5/8" MDF.  That's bad.  MDF is a great material, but it needs to be thick.  The walls on this enclosure is just a tad thicker than 1/2".  The standard size is 3/4"  PLUS a double baffle (1.5").  The walls on this box are going to flex.  In medium to high power applications your subwoofers will tear this box apart.  If you plan on running cheap subwoofers and a small amp it might work.  This box could be fixed with some simple braces and reinforcement.  But that will take up air space in the box, which brings us to the second problem with this enclosure.

The net internal airspace is unknown.   The net external dimensions are listed, and the entire enclosure is just a hair over 4 cubic feet.  Some quick tells me that this enclosure has no more than 1.7 cubic feet per side BEFORE you back out the port displacement.  The actual number is probably closer to 1.4.  This is a problem because that is not enough space for a 12" subwoofer in a ported enclsoure.  Speaking of the port, what about the port?

Based on the external dimensions, the material thickness, and the internal pictures showing the port it looks to be about 10" long.  It cannot psychically be any longer than 12.  My best estimate of the port tuning frequency is going to be between 40 and 45 HZ.  This is tuned absurdly to high.  Check out this video for more information on port tuning:

How is that going to sound?  It is going to loud, but it is not going to be low.  In fact, you will probably rip your subwoofers apart if you try to play low frequencies without an infrasonic filter, here is a video explaining how this works:

It is important to remember that there is a market for gear at all price points.  So some of you will still want to buy this box.  There is no shame in owning entry-level gear, and there is nothing wrong with trying to stay in a reasonable budget.  If you are trying to bass on a budget go ahead and grab this box.  Just match it with an appropriate subwoofer, like the GRS 12:

I am a huge fan of this driver and I recommend it for anybody that is on a budget.  Check out this video of this sub in action:

A Better Box (click on the image to view on Amazon)

This box is the BBox from Atrend, they have been in the game quite a while and they make decent pre-fabs, a lot of brick and mortar stores stock these enclosures.  I use one of their under seat boxes in my truck.  Currently, this enclosure is only about $30 more than the previous box and it is $30 well spent.  What makes this box better?

Thicker MDF.  Some Atrend boxes still use 5/8" MDF, but this one steps up to 3/4" plus a 1" baffle.  Thicker material flexes less, and will sound better.

Better airspace (but not quite enough), the specs on Amazon are a little confusing.  One of the images claims 1.32 cubic feet per side.  But the specs indicate 1.8 cubic feet per side.  You need a big box to make big bass, so any increase in box size is an improvement.  For a pair of 12 inch subwoofers I would rather see 2 or 2.5 cubic feet per side.  But that makes for a HUGE box, and it is going to scare away some customers.

Better construction.  Mitered joints, and dado grooves, probably all CNC cut.  This makes for stronger joints, which makes a stronger box and a better sound.  

Lower tuning, means better bass.  This enclosure is tuned to 36 Hz which is typical for car audio designs.  Judging from the images we can see that the port wraps around the interior of the enclosure, giving adequate length to reach that tuning frequency.

The Best Box (click on the image to view on Amazon)

This box is manufactured by SKAR Audio.  I mention  them in my post on 10 subwoofer brands that you have never heard off.  This company has developed a reputation for delivering quality bass at reasonable prices.  Even though this is one of the more expensive enclosures on Amazon it is still a great bargain.  

Finally a box with a reasonable amount of air space!  This enclosure claims to have 2 cubic feet per side.  

Finally a box with a reasonable port tuning!  A nice low 32 HZ.  This is spot on for most cars.  

Although not specifically stated in the specs on Amazon it appears to use 3/4" MDF along with internal bracing.  Plus it has some extra touches that make it worth the extra money:

1. Quality speaker terminals
2. Pre-wired internally
3. Acoustic stuffing attached to the inside of the enclosure.
4. A kerf-cut flared port that will help reduce port noise.  
   

 Conclusion

There are a lot of other enclosures on Amazon, with varying degrees of quality at many different price points.  One key thing to remember is that you don't always get what you pay for, read the details and don't pay $90 for a junk box when a decent box can be purchased for the same price.  Hopefully this article has helped you not only pick out a quality enclosure, but it has also shown you what makes for a quality enclosure.

If you have enjoyed this post and would like to support DIY audio content you can become a Become a Patron!

*This website uses affiliate links, to learn more click here.

How to Design a Dual Subwoofer Enclosure

There seems to be a lot of confusion about how to design and build a subwoofer enclosure with multiple subwoofers in the same airspace.  This blog post is going to clear it up.

A Single Subwoofer in one enclosure

Let's start off with something simple, a single subwoofer in a 2 cubic foot enclosure tuned to 28 hertz.  In this example the port has been drawn outside of the enclosure for easier comparison.  The port is 1"X12" and is 17 3/4" in length.  Before you go out an build this enclosure keep in mind that a 1"X12" port is probably not large enough to prevent port noise.  This size was chosen just as an illustration.  

 

 

 

 

If you wish to add a second subwoofer then you will need to increase the size of the enclosure.  If you increase the size of the enclosure you will need to make a shorter port.  This is because you tune the port to the airspace in the box, not the number of subwoofers in the enclosure.  So here is a 4 cubic foot enclosure with a cut out for a single driver.  We can see that the port is a lot shorter.

Two Subwoofers in a Single Enclosure

 Now Let's add a second woofer cut out and we see that the port is exactly the same.

However, now we have created a new problem.  Now we have two subwoofers, so we are moving twice the air.  That means the port needs to do twice the work.  So the port cross section needs to be larger or else the air speed velocity in the port will create turbulence in the port.  This will lead to audible port noise, also known as "chuffing."  We need to make the port opening larger.  As described in this blog post, when the port opening becomes larger the port needs to be longer.  So let's double the port area. This image shows the same enclosure with a 2"X12" slot port, and now the port is longer.

This final design is perfectly acceptable, it may be a good idea to make the port area a bit larger or  flare the port to minimize chuffing, beyond that all that is needed is to fold the port into the enclosure and adjust the enclosure size in order to account for the volume that the port takes up.

 

Dual Ports

 

Some people might prefer the look of an enclosure with two ports instead of one.  How do we design that?  Nothing like you might expect.  When you add a second port each port will tune half of the enclosure.  In this example, where we have doubled the enclosure volume, and double the number of subwoofers, an enclosure with two ports will look exactly like two enclosures with just one subwoofer.  You could divide this enclose into two equal sized chambers and get the same frequency response (but 6 db louder since you will have double the cone area and double the power).

In fact that is how most pre-fab subwoofer enclosures are designed.  After browsing Amazon for some examples I found it difficult to locate an enclosure that had one chamber with two drivers, and even more difficult to find one that had one chamber with two ports.   The typical dual subwoofer enclosure available on Amazon has two chambers with a single port down the middle such as this one (click on the pic for details):

Dual Chamber enclosure with a single port.

For this type of enclosure the port is typically half-sized on each chamber and then merges into a single, larger chamber.  I imagine this is done so that the ports serve as a brace. 

If you are looking for a good subwoofer then you should check out my list of 10 little-known but totally awesome subwoofers.  To learn more about port tuning here is a great video over on the DIY Audio Guy YouTube Channel:

If you have enjoyed this post and would like to support DIY audio content you can become a Become a Patron!

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: