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. 

*This site uses affiliate links, if you click on a link then we may earn a small commission on some purchases.  As an Amazon Associate we may earn a commission on qualifying sales. 

 

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: