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:

 

 

Building your first Subwoofer: Before you start cutting

If you want to build a subwoofer, here is what you need to know before you start cutting wood.

You have the bug!  A friend with a great subwoofer (home or car) has shown you their setup.  You have been browsing various online retailers and you think you can build it better, or cheaper.  Perhaps you have been into audio for years and you are ready to up your game and DIY.  This is a multipart guide on how to get started!  Here is what you need to do before you start ordering parts and cutting wood.

1.  BEGIN WITH THE END IN MIND

This is an old quote from Steven Covey.  Focus on the result that you want, define the purpose of this project, and clearly understand why you are doing it yourself.  Here are some things to think about.

• Is this for music or home theater?  For home theater you need to reproduce bass frequencies that are below the normal human hearing range (20 HZ).  For most types of music you don't need to play as low.
  
• Is this for nullor home?  Car's have less space to work with, but benefit from cabin gain.  The design principles are the same, but the end goal is different.
• Do you want sound quality or do you just want to be loud?
• Are you going to build one subwoofer, or is this a lifelong hobby?  This will make a difference later when start to build your tool collection.
• How are you going to "finish" the enclosure?  Paint?  Carpet? Veneer?  Vinyl?  Make sure you plan ahead so that the end project will look good, or at least tolerable.  

 

2.  IDENTIFY YOUR CONSTRAINTS

Like any other DIY project you need to understand the factors that are going to prevent you from creating the perfect subwoofer.  The three most important constraints are:

• SPACE:  Big bass requires large drivers in large enclosures and it is very important to put the driver in an appropriate sized enclosure.  Putting a large driver in an undersized enclosure will give you horrible results.  So start by understanding your available space.  How much space do you have to work with?  Will your wife let you put a big wooden box in the living room?  How much room do you have under the seat in your four door pickup truck?  How much of your car trunk are you willing to sacrifice?   
  

• BUDGET: Most audio websites never talk about budgets, but they should.  This hobby ain't cheap.  If you are looking at entry level gear, you can buy it cheaper than you can build it.  You are only going to save money if you are trying to recreate a high end subwoofer, and only if you have access to the tools you need to do the job.   

• POWER:  If you are building a subwoofer for your car or truck there is one more thing to consider:  POWER.  Your alternator provides the juice while the car is running and your aftermarket subwoofer amplifier is going to compete with important things like headlights, spark plugs, and cooling fans.  Every car is different, so every car will have a different maximum amount of power available for your subwoofer.  Just remember that your car's electrical system is not an infinite source of instant power and modern car electrical systems are more complex than you can imagine.

 

3.  INVENTORY YOUR TOOLS

If you build things for a living (carpenter, electrician, etc) or if you are already an avid DIY'er you probably have the tools that you need.  Otherwise you will need to start collecting tools.  Don't be afraid to start small, and start with cheap tools.  Don't get stuck into the "buy once cry once" mentality, this is just a marketing slogan designed to get you to spend more money than you need to.  You don't need a dedicated workshop loaded with industrial grade woodworking tools to build a 2 cubic foot box with a single hole in it.  Here is a short list of basic tools that you will need to get started, along with links* to amazon so you can order the ones you don't have.

• A workbench, a pair of decent sawhorses and some scrap wood is a good place to start.  My first workbench was an old desk that somebody left on the side of the road.
  
• A jigsaw.  This is the cheapest tool for cutting speaker holes.  Don't buy an expensive corded jigsaw.  You will eventually want to upgraded to a cordless model. 
  
• A circular saw.  Again, don't spend a lot on a corded model.  You will eventually upgrade to a table saw and you will eventually want a cordless circular saw.
• A cordless drill.  You can get buy with just one, and it does not have to be expensive.  But life is easier if you have two.  That way you don't have to waste time swapping out a drill bit for a Phillips head bit while you are assembling the enclosure.  Some speaker builders will glue and screw the box together.  But there are plenty of other ways.  You will, however, need to drill holes and drive screws to mount the driver in the enclosure. 
  
• Clamps. Some speaker builders clamp and glue their entire enclosure, this works great.  I use clamps as extra hands. 
  
• A sanding block or a handheld electric sander.  Again, you will upgrade to a cordless one eventually so don't spend a lot of money on this item.
• An edge guide, so you can make straight cuts.  Even if you just use a straight piece of wood you need an edge guide.  I like the kreg rip cut.
• Wire crimpers, strippers, and cutters.
  

If you want to kick it up a notch here are some more advanced tools that will make life a lot easier:

• A brad nailer, either battery or pneumatic.  Your enclosure will be held together with wood glue.  Clamps, nails, and screws are just used to hold things together while the glue sets. My preferred method is to apply glue, clamp the pieces in place temporarily, drive in a few 18 gauge brad nails, remove the clamps and move on to the next piece.   
• A router with a circle jig.  Cutting perfect circles with a jigsaw is hard.  A circle jig on a router is almost idiot proof.  You can also use the router to flush trim and round-over the edges.  This will really improve your end product.
• A table saw.
• A torch for solder and shrink wrap

The DIY Audio Guy has a video showing how to build a box with basic tools:

 

4.  INVENTORY YOUR SKILLS

What do you know how to do?  What are you good at and what is beyond your ability? Are you good at drawing?  How about basic arithmetic?  Do you know how to read a tape measure?  Are you a professional cabinet maker?  If you don't have some basic DIY skills, don't let that stop you.  Instead, practice a bit.  If you have never used a circular saw, then go get one, get some wood and start cutting.  Don't know how to solder?  Go get a soldering iron and spend a few hours a night practicing.  You will get better.  Your first box will look like crap.  So what?  Go get some more wood and try again.  Learn from your mistakes.  You will get better.  Jump on YouTube and watch some woodworking videos.  I recommend Steve Ramsey's Wood Working for Mere Mortals.  He teaches basic woodworking techniques and he uses affordable tools.  Here is his video on how to make a simple box:

Another great resources is Toid's DIY Audio, Toid covers topics and builds from beginner level to advanced.  Here is one of my favorite videos where he helps us understand and fix a very common mistake that is often made when building a ported subwoofer enclosure.

 5.  START SIMPLE, NOT SMALL

Large speakers are no harder to build than small speakers.  In my experience small projects (boomboxes, small bookshelf speakers) are much harder to work with because of the tight space inside the enclosure.  So don't start with a 6.5" woofer, start with a 12.  But, keep it simple.  Start with a sealed enclosure.  If you start with a ported enclosure then build an existing design, or a kit, or stick to the manufacturer's recommended specifications.  Don't try something complex until you have mastered something simple. Avoid exotic speaker coverings at first, then expand as your skill increases.  Don't start with plexiglass windows, exotic shapes, or advanced techniques. Sure, that stack-fab, fiberglass, passive radiator, eighth order bandpass you saw online looks nice.  But you need start with a simple rectangular box with a single hole in it.  After you can make a simple box look and sound nice then you can advance to the next level.

*Most of the links on this page are affiliate/associate links, if you click on the links and make a purchase I may earn a small commission.  As an Amazon associate I may earn a small commission from qualified sales.