The first Subwoofer I built was for my brother while I was staying with him and his family in Western Australia. At that time (1996), Subwoofer speakers and passive crossovers were readily available from the local electronic stores. All that was needed was a suitable enclosure for the speaker and we settled for a box of approx 1.5 - 2 cuft with a port. The design realised a frequency response down to approx 30Hz. On my return to the UK I was keen to build my own but could not find a suitable drive unit, or at least one that did not require a box as big as the room it was going in! This year however I spotted two units in the Tandy catalogue. One an 8 inch driver and the other 10 inch. I chose the latter as it's resonant frequency is lower. There was also a passive crossover advertised, but this time I decided to go for an 'active' design so that I could alter the frequency response at will.

The box is made from good old MDF (medium density fibre board), It is cheap and very good to work with and once painted, looks good too. The main reason for it's use is that it does not resonate very much, similar in this respect to chipboard, but it is not so heavy! So, on to the design. My box is a sealed design (no port) with the speaker facing the floor and mounted on small feet. Inside is a pre-Amplifier/Mixer which takes a small amount of the signal present at the HI-FI amplifier's speaker outputs (both channels) and mixes them. The next stage is an active low pass filter followed by a volume control and finally a power amplifier. The choice of power amplifier is entirely yours up to 65W rms, which is the maximum that the driver can take. My HI-FI system is not very powerful so I used a PA of 30W rms.

My HI-FI speakers are an old pair of Wharfedale Laser 30s which have a frequency response down to 70Hz (-3dB) and before you real HI-FI buffs look away in disgust, let me tell you that this is not important because the Subwoofer is designed to take over from 70Hz to 30Hz anyway!

So here's how to build your own subwoofer.

1. Go to a Tandy store and buy a 10" subwoofer driver order No. 40-8055.
2. buy some MDF.
3. buy either an external ready made power amplifier or cheaper, buy an amplifer module and power supply. Electronic component companies sometimes sell these such as Maplin Electronic Supplies in the UK.
4. buy some black spray paint and grey primer!
5. buy some nails, screws and glue and finally put it all together.

OK it's not quite that easy. Firstly you need to know the frequency response of your existing stereo front speakers. I stress "front" because surround speakers (if fitted) don't count, as the frequency response of these is poor anyway, and because they don't have to reproduce bass frequencies. Then you have to design a Low Pass Filter so that the high frequency 'fall off' in the subwoofer matches the 'fall off' in low frequency response of your front speakers. If this sounds a bit complicated then have a look at the graph in the Low Pass Filter section and all will be clear (I hope). Once you have designed the filter and bought a power amplifer and power supply, you can connect them together as in the block diagram.

The low frequency 'fall off' of your front speakers can be found in the manufacturers specification (-3dB point) or if you cannot get this information, you can measure the response yourself by using a frequency generator connected to an auxiliary input of your stereo amplifier and use a sound level meter such as sold at Tandy (cat No. 33-2050 or 33-2055) but this of course adds to the cost unless you know someone who owns a meter. It is a nice piece of equipment to own if you want to design more speakers in the future.

Designing and Making the box

The above picture shows the box dimensions in inches. These are not too crytical to the nearest inch. Altering these measurements will affect the low frequency cut off. The MDF used is 0.5 in (12mm) thick, so the internal measurements of the box are 23in, 14.5in, 9in, giving a volume of 1.7cuft.

To find the low frequency cut off of this design, you need to know three specifications of the driver unit;

• 1) VAS (volume of air that offers the same degree of restoring force on the cone as that of the cone's suspension)=2.88cuft in this case
• 2) QTS (the total quality factor of the speaker at Fs)=0.42331
• 3) Fs (the resonant frequency in free space)=30Hz +- 20%

I don't want to get too involved with the mathematics but I will tell you how I arrived at the box size as simply as possible. I obtained my information from a book by David B. Weems called "Designing, Building and testing Your Own Speaker System", ISBN 0-8306-3374-X and those of you who want to know more should get hold of a copy. According to the book, a loud speaker and it's box have there own Q (quality) factor (Qcb=Q of closed box, Qts=Q of speaker)and together they form what is known as the system Q. A system Q of 0.7 gives the smoothest roll off in the low frequency response of the system. Above 0.7 causes a hump in the bass response and below this causes the bass to fall off at a higher frequency.

Now if the volume of the box is Vb, rearanging the following equation,

VAS/Vb=(Qcb/Qts)^2-1,
Vb=VAS(Qts/Qcb)^2+1
Vb=2.88(0.42331/0.7)^2+1
Vb=2.05 cuft (which is larger than the designed box)

The cut off frequency of this system is the same as the resonant frequency of the speaker ie 30Hz. But due to wood size limitations and to cut the cost a bit I chose a slightly smaller box. So rearanging the equation again this time to find Qcb.

Qcb= Qts.SQR(VAS/VB-1)
Qcb= 0.42331.SQR(2.88/1.7-1)
Qcb= 0.858 which is slightly higher than ideal but still fine.

Refering back to the book, a system Q of 0.85 gives a cut off frequency of Fs/0.865=34.5Hz which is nearly as good as the larger box and a lot better than the 70Hz cut off of the Wharfedales!
I hope that wasn't too involed. You don't need to understand any of it to be able to build the subwoofer if you stick to my dimensions.

So you need to cut some MDF or get your local DIY shop to do it on their circular saw. The sizes are as follows.

• Top: 24in by 15.5in by 0.5in (12mm MDF)
• Sides x2: 24in by 9.5in by 0.5in
• Ends x2: 14.5in by 9.5in by 0.5in
• Base: 23in by 14.5 in by 0.5in
• some 1 inch by 1 inch cross section wood, enough to go around the inside of the box for suporting the base with screws.

The box is put together with panel pins and woodglue and the inside edges sealed with rubber solution as used for sealing around baths etc.

Mounting the electronics

The low pass filter, power amplifier and power supply are all mounted on the inside of the base. Make sure that there are no boards, cables etc that can vibrate and keep all cables away from the speaker cone. There is so much bass in the box that it can rattle the electronics and make some horrible background noises. A hot glue gun was used in mine to seal things down to the base.

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