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Subwoofer Design - Part 3

Building boxes.

by Julian Edgar

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This article was first published in 2003.
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Last week we covered the use of a PC software package for modelling enclosure designs, while the week before we looked at the different types of boxes that can be used - sealed, ported, passive radiator and bandpass. And 'boxes' is the right name for most of them - most speaker enclosures are just flat-sides boxes made from MDF. That makes them far more achievable for home construction than, say, multiple curved door pods (although we have covered an easy technique for door pods at "Making Door Pods"). In fact, even with simple hand power tools, you can expect a pretty high rate of success in making a subwoofer enclosure.

The Materials

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Almost all subwoofer boxes are made from high-density MDF. This is a composite material comprising small bits of compressed wood and glue (how small the bits of wood are depends on the type of board that is selected). The material is cheap, easy to work and is acoustically fairly dead. By the latter we mean that if you rap on a sheet of it with your knuckles, it doesn't ring like a bell. The 'deader' the material, the better - 'live' materials will tend to radiate sound, which isn't wanted. For example, if you use a sealed box, you want the sound waves coming out of the back of the cone to be absorbed and quelled, rather than excite the speaker box walls into becoming their own sound radiators.

The selected MDF should be thick - how thick depends more than anything else on how large the panels of the box are. If you are using a small box (say the largest panel is 300 x 400mm) then 15mm thick board can be used. But the thicker the panels, the better they'll be at resisting flex. The trade-off in a car is extra weight, which harms performance and worsens fuel economy. One way around the problem is to use material which is on the thinner side, but to brace the box internally, either by having ribs that stiffen unsupported panels or by using internal cross-braces. To emphasise the point, in any enclosure you want thick, solid and unresonant panels.

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It's common to place clear plastic 'windows' in enclosures, especially in bandpass enclosures where otherwise the driver would be hidden from view. That's fine, so long as the plastic is equally stiff and lacking in resonances as the rest of the materials that the box is made from. Oftentimes, it isn't.

Most speaker boxes use internal damping material. Fibreglass insulation batts, foam tiles or fluffy acrylic material can all be used. The latter is the most common - in addition to there being no health problems of glass fibres being pumped out through speaker ports, the acrylic stuff is cheap and readily available. Acrylic dampening material is widely available as 'quilt wadding' - the material used to fill quilts and available from craft and cloth supply shops.

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Box filling is used for two distinct purposes - to stop mid-bass reflections bouncing off the inside of the box and coming back out of a vent, and to change the behaviour of the sound waves being pushed out by the back of the cone, turning them into heat. The latter effect makes the box seem to the driver that it is bigger than it really is - the response is smoothed and often deepened. In all cases the amount of filling that is used should be designed in consultation with a PC software program - in some cases it can have a dramatic effect.

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Ports are most often made from PVC plastic pipe - it's cheap, easily worked, durable and rigid. Make sure that you measure the actual internal diameter of the plastic pipe that you are using, and input that data into the design program. If high port velocities are going to be a problem, plastic pipe can be teamed with commercially-available flared ports - like bellmouths on car intakes, they smooth the flow of air into and out of the ports. Use these flared bellmouths at both ends of the port, but make sure that the software modelling compensates for these changed ends - the port will need to be longer to have the same effect as a port with squared-off ends.

Construction

Furniture has been constructed for centuries, and one important aspect of those designs of the past has always been strength in joints. Dovetails, rebates, mitres - you name it and it's done to make furniture stiffer and stronger. Unfortunately, the same is almost never done in car subs - butt joints reign supreme... Typically, the two panels are placed up against each other, some glue is applied and a few self-tapping MDF screws are put in. To state the obvious, this isn't the best way of doing it - but it's easy, especially if you don't have good woodworking machinery available to you. At least when using butt joints make sure that the panels are flat and edges square, and use plenty of glue and screws, with each of the latter following a pilot hole drilled just for it!

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Sealing the enclosure so that no air leaks can occur is critical - in an enclosure that is supposed to be fully sealed, any air leaks will unload the cone, while in a ported or bandpass design, leaks can also cause squeals and whistles. To ensure that sealing is good, it makes sense to run a bead of sealant around the inside of all enclosure joins and around the back of ports. For the same reason, air should not be able to leak past the edge of the woofer's frame or around the connection terminal plate. Good quality components run gaskets in both of these locations; if there aren't any, you may need to make your own from neoprene rubber, or use sealant.

Pre-Built

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One really good way of getting a starting point that reduces your workload is to start off with a pre-built box. Often you'll find that they're already carpeted much better than you'll be able to do yourself (it's bloody hard to do a neat carpet job without getting glue and ragged edges everywhere), and if the rest of the enclosure is a bit flimsier than you'd like, well, it's pretty easy to add internal bracing. Modifying an enclosure that is already built is usually far quicker and cheaper than starting with one that you build from scratch. Note that in nearly all cases in these pre-built boxes, the diameters and lengths of the provided ports are too small - so they'll need to be changed.

Ports

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While we've said that ports can be made by combinations of plastic pipe and/or pre-formed flares, once you start punching box design numbers into a program you'll pretty soon find that there's a major limitation in your design process: the ports don't fit into the box. This is because if you're using a relatively small enclosure tuned to a low frequency, the ports will need to be long. And if you want to have a pretty major Xmax (cone displacement) without having large diameter ports, the air velocities inside the ports soon get too high. And then if you increase the diameter of the ports, you've gotta go longer again.... Aaaagh!

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There are a few ways around the problem. One way is to reduce the port diameter. Yes the port velocities will go up but if you're not intending to drive the sub a lot at max power, the trade-off will be worthwhile. (The effect of overly high port air speed is 'chuffing', a self-explanatory phenomenon.) Another way to do it is to have the ports external (or partially external) to the enclosure, allowing them to be longer. For example, in a bandpass enclosure (where all the sound comes through the ports), the ports can be used to connect the box to holes in the rear deck of a sedan. If 15 or 20cm length of the port is external to the box, then so what - it's channelling all of that sound into the cabin. Another approach is to have a long, thin box design, with the ports lengthwise within it. Ports up to 800mm long are easily incorporated within the box if it's of the sort that goes east/west across the car, up against the back seat for example.

Conclusion

There are three key points to getting a good DIY subwoofer happening:

  • Select a woofer that has a wide range of Thiele-Small parameters available for it.
  • Use a good software program to develop an enclosure design which is sensitive and broad in low frequency response.
  • Build a rigid, airtight enclosure that follows the design specs closely.

That all doesn't sound too hard, does it?

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