I've had a bit of experience with building subs recently, as well as a grounding in the theory behind their operation. My findings are quite different to what you find on most web pages.

Building a sub involves trade-offs between:

- Reducing driver excursion.
- Increasing efficiency.
- Lowering cutoff frequency.
- Reducing the size of the enclosure.

And like anything derived from good physics, you can't have all of them at the same time :) Well, you can, but you end up with VERY large enclosures. A friend of mine suggested that the ratios between size, efficiency and bass cutoff is very approximately:

- Half bass cutoff = 8 times the required volume(size)
- Half bass cutoff = 1/4 the efficiency (-6dB)
- Twice the volume = twice the efficiency (3dB)

And a very approximate "reference" sub: 100dB SPL : 100 litres : 100Hz

Given that, I start with an approximate maximum size of sub - in my case a 90 litre enclosure. Then sort through a list of drivers applied to an enclosure of that size until you find one which gives the required Hz and SPL. You cannot get a driver which has the same Hz but higher SPL. If you find one, the manufacturer has some bad figures. It is not possible to have "higher efficiency" drivers at bass frequencies.

Enclosures are all about dealing with the rear wave. I've got a ton of stuff about enclosure choices to write, but I'll save that till tomorrow (beer beckons). Basically, 6th order bandpass boxes are amazing things:

- Transient response is as good as a sealed enclosure. Don't believe me? Create a good enclosure and see that group delay follows closely the delay of a sealed enclosure, up to where a sealed enclosure would have stopped producing bass. From there on, the group delay becomes bad, but hey the sealed box wouldn't be producing that bass would it? And it's usually less than 20ms, so you probably won't even notice it.

- Bass response is excellent. You can shift the parameters that a driver determines by quite a lot (cutoff, sensitivity, bandwidth), leaving a lot of design freedom.

- Excursion control is excellent. It's a shame people quote frequency response as the determining factor. It's not. The true maximum SPL a sub can generate is the minimum of max excursion and max wattage at a frequency. This makes sealed boxes have much much lower maximum SPL at low frequencies. And because excursion is lower, so is distortion - another thing people usually attribute the other way around!

- Lots of freedom in enclosure design. You have two ports, two partitions and a driver to play around with. You can generate pretty much any frequency/excursion graphs you want provided you can find the drivers.

Food for thought :)



- John (from empeg)

(The above may not represent the views of empeg :)