Determining Enclosure Size for Small Speakers

Whereas the technical specifications of good quality larger loudspeakers are usually supplied by the manufacturer, small, cheap loudspeakers are sold with only the minimum specifications, usually only the impedance and power handling capacity, for instance 4 ohms and 5 watts. If you have comprehensive specifications like the Thiele parameters, it is easy to use freely available software to design enclosures with, but what to do with the small, cheap speakers? My solution was to build a box with an adjustable inside air volume, so that you can use your ears to determine the size of box the loudspeaker would sound the best with. The one I made can be use with loudspeakers up to 60 mm across, and the inside air volume ranges up to approximately 0.9 liters (or 900 cubic centimeters). I don't think one would like to go any larger than that for such small loudspeakers.

I built an open ended box with an internal volume of 9.4 cm high, 7 cm wide and 16 cm deep/long. A baffle with the speaker mounted on it is temporarily fitted to one of the open ends, and a piston (a block of wood 20 mm thick) is inserted from the other side to vary the enclosure volume with. By measuring the length of the piston's handle sticking out of the box, it is easy to calculate the volume at which the speaker sounds the best. The dimensions of the hight and width of the box differ from each other in order to prevent standing sound waves.

I used 6 mm masonite for constructing the sides of the box. It is important to keep the dimensions of the box as accurate as possible from one end to the other, in order for the piston to move freely while still forming quite a tight seal. To accomplish this, I sawed two pieces of wood the size of the piston area, and place them in the open ends of the box while glueing it up. By using one of them later to form the piston, I knew that it would be a good fit.

On the photo you can see the sides of the box clamped tightly around the end pieces (but not glued to them!) while waiting for the glue to dry. It is important not to use too much glue as dried glue on the inside will obstruct movement of the piston later on. Also you don't want to glue the end pieces to the sides.

On the first photo you can see the finished box. The second photo shows the piston fitting tightly inside. The screw in the middle of the piston holds the handle on the other side in place.

I wanted to test a 50 mm and a 40 mm speaker, and of course had to make openings of the appropriate sizes in the test baffles. On the first photo one opening is finished. I used the dangerous looking contraption in a drill press to cut the openings. On the second photo are the finished openings in the baffles, with the corresponding drivers sitting next to them. The baffles were sawn to size after that.

The first photo shows one of the drivers and its test baffle fitting nicely on to the test enclosure. On the second photo you can see where I filed a notch in the enclosure for the loudspeaker leads to pass through. In the third photo I've taped the baffle to the enclosure for a reasonable air tight seal.

The 50 mm loudspeaker being tested. The second photo shows the handle of the piston protruding from the back of the enclosure. Knowing the full length of the piston and subtracting the length protruding, it is easy to calculate the volume. Just for interest's sake, the optimum volume for this particular speaker I found to be approximately 395 cubic cm (front of piston was 6 cm from back of baffle, and with height = 9.4 cm and width = 7 cm, thus 6 x 9.4 x 7 = 394.8 cc, or a rounded 0.395 liter). Smaller than that the bass suffers, and any bigger it doesn't improve enough to justify the larger enclosure.