Build Plans: Home Theater Subwoofer

In this post I am going to share with you plans for a home theater subwoofer with some interesting design elements.

 

 

 

 

 

 

Parts Used*

1. Dayton Audio Classic Series 4 Ohm 12" Subwoofer
2. Young 300 W Plate Amplifier
3. B52 3" x 5 " port flares
4. Birch Veneered Plywood
   
5. Wood Glue
6. Birch Edge Banding
7. Clear Poly
8. Textured Coating
   
9. Pocket hole screws
10. Cap Head Hex Wood Screws
    
11. Brad Nails
    
12. 5 minute epoxy
13. Polly-Fill Quilt Batting
    
14. Sound Deadener 
    

2 Design Goals 

 

The hardest part of building a speaker is making it look good, especially if it is a budget build.  Wood veneer is a simple solution that that looks great, but it can really run up the cost of a project.  It is no more expensive to use cabinet grade as compared to veneering MDF.  The problem with plywood is the exposed layers on the edges. Some people like the look, I don't.  So, this design is going to make use of overhangs to cover as many edges as possible, and edge banding to cover the rest.

As far as the sound goes, the goal is a a relatively flat response that reaches as low as possible, without exceeding X-MAX.  The design will make use of 2 3" ports with flares on both ends.  The port area is not big enough to prevent port noise, so flares are used to fix this problem. 

Cut List

• Baffle (2):  15" X 21.25"
  
• Back (1):  15" X 21.25"
• Sides (2):  21.75" X 21.25
  
• Top/Bottom (2):  22.25" X 17.5"
  
• Window Brace:  15" X 19"
  
• Port Support: 15" X 7" 
• Ridge Brace:  15" x 7
  

 

Dimensions 

 

 

You are free to cut up your plywood anyway you like. But, take some time to plan ahead.  Unless you have a large table saw with some big in-feed and out-feed supports you will want to have the plywood cut down to size when you purchase it.  This will make it easier to handle and transport.  Another alternative is to get a Kreg Rip Cut and some Styrofoam insulation.

I am very fortunate because the guys at my local big box stores are very accurate when they make cuts.  But don't count on that.  Ask them to oversize the cuts by at least 1/4", possibly more.  Plan this out before you go pick up the plywood.  In my area they will make two free cuts, but if they guy is in a good mood he will do more.  The best advice I can give is to make all of the cuts of a given dimension at the SAME TIME.  That way if you are off by some small fraction (1/16" or less), it will not make any difference in the finished product. 

Here is an example of how you might break down the material. This assumes that you can get two free cuts at  your big box store.

• Start with the largest dimension is the top and bottom piece, at 22.25" (22 1/4").  Have the store cut a piece that is 22.5"x 48".  Then when you get it home you can trim it down to size to 22.25" Then you can set the saw to 17.5" and cut out the top and bottom.  That will leave you with a 22.25" X 12.875" piece of scrap.  Save that piece, you can use it for the port support later. 
• The next largest dimension is 21.75.   So ask for a 22" cut.  Again, trim this down to 21.75" at home.  Then you can make the two 21.25" sides.  When finished you will have two side peices plus a 21.75" x 5.375" piece of scrap.  This one will not be very useful.  But it may work great for the port rings, so save it.
• Now with the saw still set on 21.75 take the last piece (this one will be large you may need some help if you are using a table saw, at the very least you will need some rollers if you are using a table saw) and rip that down to 21.75.  Then set the saw to 15" and make three cuts for the double baffle and the back piece.
• With the saw still set to 15" take the remaining plywood and rip it down to 15".  If you want to grab the scrap from before and make the port brace do that now.  Set the saw to 19" and cut the window brace, then to 7" and cut the port brace and ridge brace.  The ridge brace was added after the fact and even though it did raise the tuning frequency it stiffened up the box quite a bit. 
  

Port Design

The ports are made from inexpensive flared port tubes available from parts express.  The tubes themselves are not long enough to reach the desired tuning frequency.  So extensions were fabricated to lengthen and connect the tubes.  The internal tubes are connected to a brace inside the box and the tubes are mounted using 5 minute epoxy.  You will have plenty of 3/4" scrap from your plywood to connect and extend the ports.  When you take into account the fact that 3/4" plywood is undersized by 1/32" then four layers of plywood rings will get you very close.  Here is a video showing the fabrication process and the mock-up:

 

 

Assembly

 

You can use any method you like to assemble the enclosure.  Keep in mind that the wood glue does all of the real work, any mechanical fastener that you use in the build does the same thing that a clamp would do, it holds things in place until you can   Typically I build enclosures out of MDF, and I just use wood glue and brad nails.  One of the goals is to show off that nice expensive birch veneered plywood, so you will need to take care that you don't damage the surface veneer.  Therefore clamps and glue will work just fine.  But I am going to make use of some additional tools.  As described here.

You can start by laminating your two baffles pieces together, before you make the speaker and port cut out.  If you go this route you will need a rather large rabbiting bit in order to recess the speaker, or you can forgo the speaker recess.  If you don't have the proper rabbeting bit you can cut the circles before you laminate the pieces.  Here is a video with some tips to help make that process easy.

 

 

For the ports I made several test cuts in scrap material in order to get the best fit, and I used a round-over bit to match the curve of the ports.  Make sure to align  port holes on the brace support with the port holes on the baffle.  To do this I clamped them together, made sure that the pieces were square to each other and drilled the pilot holes for my circle jig at the same time.  My original plan was to recess the ports, but that did not go as planned so I just inserted them in the port holes and epoxied them into place with 5-minute epoxy.  I then  up the ports, the port support and the window brace an connect them all together.  Here is a video that will hopefully shed some light on the process.

 

 

That video also shows how I used pocket holes and pocket screws to connect the internal parts, as well as the process for the window brace.  The top, bottom, and sides overhang a bit.  This was an important part of the design as helped hide most of the plywood edges.

Finishing

The goal of the finished product was to hide as much of the plywood edges as possible.  This was done by paint the back, bottom, and the bottom edge with black duratex, and spray painting the subwoofer recess black.  Then iron-on edge banding was used on the top and the front of the sides.  The entire thing was finished with Polycrylic. 

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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.