Build an Inexpensive Sub-woofer

by Marsh

17K14918

Intro: Build an Inexpensive Sub-woofer

With 5.1 Channel amplifiers in such abundance, there's a growing need for sub-woofers. Here I'll show you how to make your own for less than $30.

STEP 1: Tool & Materials

Tools

  • Tablesaw
  • Jigsaw
  • Chisel
  • Hammer
  • Nails
  • Wood glue
  • Soldering iron
  • Drill
  • Divider
  • Screwdriver
  • Tape Measure

Materials

  • Particle board or furniture grade plywood
  • Woofer
  • Terminal post
  • Wire
  • Screws
  • Nails

Cut list

  • 4 ea. 14" x 8" x 13/16" particle board
  • 2 ea. 14½" x 14½" x 13/16"" particle board
  • 1 ea. 13" x 13" x ¼" plywood
  • 8 ea. 13/16" x 13/16" x 7¾" particle board

A note about woofers in general. When it comes to speakers you generally want to look for three things.

  1. Big cone - The bigger the cone, the more air you're going to move. Especially in a woofer, you want to move a lot of air. I went with a 12" because this will be mounted between floor joists under my sofa. I got mine on clearance and they're out of stock now, but this is close.
  2. Heavy magnet - I'm not sure why a heavy magnet is good, I just know that it is.
  3. Rubber surround - The surround is the part that attaches the outside edge of the cone to the frame. If this part is made of foam, it will rot and tear in just a few years. Rubber is the best.

STEP 2: Cut the Sides, Top and Bottom

I usually refrain from giving precise measurements to allow you to use available materials and fit the project to your application. Since this project is designed to fit under the floor between the joists, I'm giving dimensions.

The raw material I'm using here is 13/16", particle board with Formica bonded onto it. People say particle board is cheap, and it is, but its also dense. And dense is what you want for a speaker cabinet. The size I'm shooting for is 14½" x 14½" x 8". This will fit between the floor joists under the floor and the woofer will blast up directly under the front of the sofa. The speakers in the room and the video monitor are in a fixed location so the sofa isn't going to move.

For ease of construction and durability, we'll use rabbet corner joints. Begin by cutting your particle board to 14" wide. Next cut four 8" wide pieces and taking several passes on the tablesaw, rabbet in a 5/16" deep by 13/16" wide notch along one 8" edge of each. Use a chisel to smooth off the surface. This notch will be the basis of a glued corner joint.

Also cut two 14½" x 14½" squares, one 13" x 13" square and eight 13/16" x 13/16" x 7¾" particle board.

STEP 3: Glue and Nail Corners & Bottom

Lay out the four sides and spread a good amount of glue into each rabbet. Insert the edges without a rabbet into the adjoining rabbet. Drive three nails through the rabbet edge and into the end of the adjoining side. Do this on all four corners. This will form an open ended box.

Apply a good amount of glue to one open end of the box and nail one of the 14½" x 14½" squares onto it. Now you have a box with one closed end.

Finally, glue and nail the 13/16" x 13/16" x 7¾" particle board pieces into all of the corners and lengthwise along all of the edges. This will reinforce the corners and provide a resting place for the plywood which will be the speaker mounting surface. The 13"x13" plywood should rest flush with the top of the box.

Now is as good a time as any to install the terminal strip.

STEP 4: Mark and Cut the Plywood

Draw an X diagonally from corner to corner across the 13"x13" and the remaining 14½" x 14½" squares. By aligning the speaker mounting holes with these diagonal lines, you can easily see that the speaker is centered. Trace a circle around the speaker frame onto both pieces of plywood.

Using a divider, measure from the outside of the speaker frame to where the frame cone begins. Transfer this measurement to the 13"x13" plywood. Cut this smaller circle out of the smaller piece of plywood. Also cut the full sized circle out of the larger piece of plywood.

STEP 5: Finish the Enclosure and Install the Speaker

Apply glue to the plywood mounting surfaces and set the smaller piece of plywood into its resting place. Apply glue to the entire back side of the larger piece, lay it in place and nail down the perimeter.

Once the glue dries, solder the wire from the terminal strip to the speaker. I use 16 gauge speaker wire. Now there is lots of science out there that says monster cable is better. AC (which is the kind of voltage traveling on a speaker wire) conducts over the surface of the wire, so the more surface area the better....right? Here's the thing. I've used that phenomenally expensive wire before and not once have I heard the difference or had the wire heat up. 16 gauge works fine.

From there, set the speaker down into its resting place and screw it to the enclosure. I've opted for no grill cloth because I'm going to apply weatherstripping and seal the front of the enclosure to the floor of my living room, running screws through the floor joists to secure it in place. Enjoy the surround sound!

14 Comments

BillD8 7 years ago

usually in sub-woofer cabinet design i would expect a "folder-horn" inside.
this in effect means, for example, recreating the dimensional theory behind a tuba!
which in the case of a tuba means the "horn" begins as a small piece where the vibration to be amplified begins, and a horn coming from the mouthpiece which builds deeper and deeper frequencies till it gets to the wide end peice where you here most of the sound from.

in the case of a folder horn bass speaker, we have the speaker itself (like the vibrating mouthpiece on a tuba) generally facing the front and on the front of the cabinet, but NOT always (back loaded).

directly behind the speaker cone (in a front loaded bass speaker) you'd have a hole cut that acts as the beginning of a wooden tunnel that folds it's way round in a spiral, and generally exits in a wide hole (the width of the cabinet) generally, but not always, below the speaker cone.
the effect here is that it offers the ideal bandwidth of waves needed for genuine sub-bass in wooden cabinet design.

also, generally, we would expect baltic plywood (no "voids" in the ply layers!)
and the glue used to make the ply to be heat/boil/steam/water resitant.

thye speaker above from it's appearnace and dimensions looks ideally more suited to perhaps a bass guitar speaker unit?
here is a very expensive exampl:

https://www.instructables.com/files/deriv/FJS/72NG/...

and here is a more typical DIY sub bass speaker (back loaded in this case, meaning the speaker driver is mounted inside and in the "back" of the cabinet) example:

http://es.virtualdj.com/image/62537/48114/folded%20horn%20sub.gif

planetp 8 years ago

I just wanted to comment on the thing with T/S parameters and building speakers. So, first, what TheAppleSauceMan says is certainly true and if one wants to build high quality speakers then traveling down that rabbit hole is certainly worth the effort.

Here's the thing though. The sub here is a sealed box design. It's known as an "acoustic suspension" design, which, isn't that important by itself, but the details might help people look things up. In any case, the thing with sealed box designs is that no matter the speaker, the design will roll off the bass smoothly largely as a function of box volume.

A brief discussion of speaker design history can be found at the following link.

http://ohmspeaker.com/news/sealed-box-speakers-or-...


What does that mean? It means that, to a point, the size doesn't matter in that you will simply get less bass than you expect if the box is too small for the speaker that you are using. So, in practical terms, just build a box that works for you and put your speaker in it. Yes, some speakers will require a larger box to produce the same bass response as others, so some speakers will work better in a box of a given size than others. But there's no magical box size such that if you get it wrong it will sound really bad. Note, this isn't true with more complex designs such as vented speakers or bandpass boxes. As long as you're building a sealed box, it will work fine. If it doesn't go low enough, you can build a bigger box, or, you can use different driver.

So, how do you choose a driver without crunching numbers? Just use a speaker designed for small boxes in the first place, get yourself a car subwoofer, just make sure that you have plenty of power to drive it with.

In conclusion, for reasonable results for your living room, build a box of a size that kind of works for you, as in this instructable, and stuff it with a car-subwoofer speaker. It will work.

TheAppleSauceMan 8 years ago

G'Day Marsh,

First off, I like it !

Nice build and good writeup. You must be pretty happy with yourself.

How does it play?

The reason i'm asking, (and this isn't a dig, just a suggestion)

is because as I was reading through the absence of TSP's bothered me.

What are TSP's you ask?

Thiele-Small Parameters.

They are a set of parameters, equations etc. etc. that govern how a sub works and how it will work best in a certain box (enclosure).

They just ensure that your sub is playing at maximum Sound Quality (SQ), most efficiently, and that it sounds great to you.

If you have already done all this stuff, then ignore me.

I suppose it will be for other people.

In the link you provided <https://www.parts-express.com/goldwood-gw-1244-12-...> under Product Details are that sub's TSP's.

They are as follows:

Thiele-Small Parameters

  • Resonant Frequency (Fs)29.7 Hz
  • DC Resistance (Re)3.6 ohms
  • Voice Coil Inductance (Le)0.6 mH
  • Mechanical Q (Qms)3.3
  • Electromagnetic Q (Qes)0.86
  • Total Q (Qts)0.68
  • Compliance Equivalent Volume (Vas)4.22 ft.³
  • Mechanical Compliance of Suspension (Cms)0.31 mm/N
  • BL Product (BL)8.48 Tm
  • Diaphragm Mass Inc. Airload (Mms)93.2g
  • Maximum Linear Excursion (Xmax)3 mm
  • Surface Area of Cone (Sd)525.9 cm²

Really the important ones are (in a rough order):

Re, Fs, Vas, Qms, Qts, Qes.

Now, you'll need these magical links, they thankfully do all the hard calculations for you, which is nice. :)

http://www.bcae1.com/spboxnew2.htm

http://www.rockfordfosgate.com/rftech/box_wizard.a...

http://www.diyaudioandvideo.com/Calculator/Speaker...

http://www.diyaudioandvideo.com/Calculator/Speaker...

So next step, we need to see if it will work better sealed or ported.

That is, in a completely sealed box, or one with a hole/tube a specific size.

So, we flick over to BCAE (second link), plug in our Fs & Qes, press calculate, and we magically get the EBP number (efficiency bandwidth product), which also tells us that this sub is best suited for a sealed enclosure.

Good, that means we don't have to calculate port sizes & area (which can not be fun).

So flicking back to the product page, they have kindly provided volume & tuning specs for that sub, which means we don't have to do that.

As follows:


  • Sealed Volume 5.99 ft.³
  • Sealed Tuning 39 Hz

However, 6 cubes sealed is a bit unrealistic. It's a bit big for the sub, but we'll stick to it, just for the sake of this exercise.

I'm going to argue with you over one point, please for the love of god don't put it under the floor. Not only are you restricted in size, which means you have to sacrifice SQ, but it makes the build much harder. Since you said the couch isn't moving, why not put it behind the couch? If it's against a wall, well bass is omnidirectional, it can be put in a corner (good) or anywhere really, just not under the floor, please.

For all the following i'm going to assume you are putting it behind the couch.

Let's say that you have the sub forward firing (towards you).

Let's just pull some magic numbers out of thin air for the box dimensions.

3ft wide x 2ft high x 1ft deep. These are based off my couch (not magic)

Plug that into the Rockford calc, we get, oh look, almost 5 cubes, not bad.

They said 6, so pretty close. This is the good thing about this, leniency.

First we will discuss tuning, then get on to designing a box.

Tuning is quite subjective, it depends on the type of music you listen to and what frequencies are involved. It also depends if you are going for SQ or SPL (sound pressure level). A good middle ground for most larger subs is 35Hz.

Some people who are going for optimum SQ build a bigger box and tune it lower, this means that the sub will be playing for optimum sound quality.

Some people who compete for SPL tune higher, so that when they play low notes, the sub unloads and moves a lot of air, this generates massive sound pressures.

But I doubt you are wanting to compete, so we will stick to what they suggest.

Now tuning:

using http://www.diyaudioandvideo.com/Calculator/Speake...

we plug in some figures and to get your sub to play @ 35Hz (a good middle ground) we need about 4.5 cubes, so our 5 cube box will play lower louder. Great!

So there you go, some tools and explanation for Thiele-Small Parameters.

For reference:

3ft x 2ft x 1ft box = ~5 cubes = about 35Hz.

Cheers,

TASM

studleylee 8 years ago

How would I quantify my chainsaw powered sub? I know for dang sure it thumps the hell out of anyone who asks...;-)

TheAppleSauceMan 8 years ago

Studleylee,

Well, you could try quantifying it it a Spacio-Temporal net, by analysing the ripple effect that the kickback generates due to spacial acceleration. Then you would need to cross-map the quality factor of total spectral energy produced in the ionic displacement field. That should hopefully land you somewhere close to the figures you need.

Hope this helps,

TASM

Marsh 8 years ago

Here's the thing about sub-woofers and sound quality. It only matters if the sub is in the room with you. This sub is mounted under the floor of my living room blasting up under my feet to provide the low end rumbling effect in surround sound movies. It is not there to bring out the dulcet tones of a baritone sax or Larry Gatlin. My front drivers (Infinity Reference R162s) and center channel (Infinity Primus PC351) are more than adequate for that.

TheAppleSauceMan 8 years ago

Sure thing Marsh,

I wasn't trying to take over your project, just trying to provide some helpful hints for others. Hope you didn't take my input the wrong way.

As you said, you wanted it for a specific purpose, and you created it with what you had. This project is in the true spirit of DIY, which is what Instructables is all about.

Regards,

TASM

Ripppr 8 years ago

Good 'ible for creating a home subwoofer enclosure. I would, as has been stated, like to point out that it is indeed only an enclosure, rather than a driver or speaker. I do have some other notes I'd like to add to help clarify some things:

1) Stranded wire will make little difference electrically when it comes to audio applications, except for at high frequencies, and, even then, the skin effect will be minimal, as well as the current through any given wire for most drivers for mids or highs. I would still recommend getting Oxygen Free Copper, OFC, stranded wire because it has low resistance and is easier to use than solid core wire.

2) As for the guage of the wire, check what how much power will be provided to or consumed by the woofer. RMS values should be fine. You can then calculate the current by dividing the power by the voltage of the amplifier (30 volts, ballpark). Compare that current to the current that can be handled by the wire on an AWG table. It's better to go a little bit bigger, just in case, but 16 gauge will likely be fine for low power and short leads.

3) You could probably get away will a thinner enclosure, but thicker enclosures do help absorb harmonics and other vibrations, especiallu with MDF. Good call.

4) Check the minimum nominal impedance of the surround sound before getting a subwoofer to go with it. A lower impedance may work, but it usually won't end well. The subwoofer's impedance is also related to its enclosure.

I'm sure I missed something, but those should help some. Just thought I'd clear the air about some of this audio stuff. Good luck.

Source: Electrical Engineer/Audio Enthusiast

HerbertD1 8 years ago

>>>"I would still recommend getting Oxygen Free Copper, OFC,"<<<

Out of curiosity, do you have a source for wire with non oxygen free copper? I allways thougt all copper wire for electric use is out of electrolytic-processed copper without oxygen

Slatu 8 years ago

Nice box, I find it very satisfying listening to my homemade speakers. From the ones I've made, here are a few suggestions: particle board doesn't take glue well, better to predrill and screw. With a sealed box, you could caulk all interior joints and stuff the box with the poly pillow stuffing material. A great supplier is Parts Express. They usually have project builds that guide you along well. Good box, and hope you don't stop there.

Creak 8 years ago

You did not make a speaker you made a speaker box and provided a list of prefabricated materials for a speaker. A heavy magnet reduces vibration. Typical magnets are made of an iron oxide and aren't very strong so they have to be large to produce the desired strength. Smaller rare earth magnets are available for thinner speakers.

Ripppr 8 years ago

A heavy magnet actually provides more motor force, which can increase the output of the driver in question, as long as the magnetic field produced by the coil is increased to compensate. If you look at the Theile/Small parameters for a driver, it would be labeled as BL.