• Audio DIY Projects
• Subwoofer DIY Page

Introduction
A subwoofer made out of cardboard?  I'm kidding, right? Well, see for yourself...

Construction
Constructing this subwoofer is very simple, and can be done in a few minutes.  All you need are the following:

1. A cardboard box
2. An inductor
3. An 8-ohm resistor (25W)

To start, draw the subwoofer's circular diaphragm out on a piece of paper.  The best way to do this would be to use a graphics program like CorelDraw to create the circular diaphragm.  Once you've completed it, stick it to the box as shown in the picture below.:

On the rear of the diaphragm, glue the inductor to the box as indicated in the picture below:

Once you've done that, connect the resistor in series with the inductor, then connect both to your amplifier.  That's it! Done properly, your subwoofer should exhibit a smooth but deep response, and be a perfect match for your mains speakers, whatever size they are.

After experimenting with this configuration a bit, I noticed the following:

1. You get a better response if one side of the box is left open.  The driver has a very high-Q response, and converting the box into an open baffle compensates for this.
2. Larger boxes result in better production of lower frequencies, but the upper frequencies are attenuated. You can negate this effect by sticking additional diaphragms to the front of the box.
3. Larger inductors = more low frequencies, but they tend to fall off the box if driven too hard.
4. Drawing a larger surround on the diaphragm increases the apparent excursion, but total output drops. There is a "sweet spot" for the diaphragm/surround diameter ratio of about 7.2637:1

I took the opportunity to measure the T/S parameters for my cardboard subwoofer driver, and they turned out as follows:

Below is the measured response of one of my cardboard subwoofer test systems, this with the ideal diaphragm/surround ratio:

I believe the 400 Hz peak is caused by the cheap glue I used to glue the inductor to the back of the diaphragm. The use of a better-quality glue should reduce this peak a bit.

Overall, I'm very happy with the output of this subwoofer, given its cost.  If you do decide to try building your own cardboard subwoofer, please let me know how it turns out.

Comments:
Dr. Lirpa - This is excellent! I used the box my new refrigerator came in, and the output far surpasses my two Dayton 15" subs! I used 4 diaphragms to increase the mid-bass output - it worked!

Brian Steele
06 June 2018