However, if you are trying to keep the cabinet small and, consequently, the WAF high, then you will need a dedicated driver for the lower octaves of the spectrum.
Hence, our SWS subwoofer design...
The idea was to have a modular offer: one could buy a pair of satellites as bookshelf speakers, and then later, when money and/or space become available you could add the subwoofer(s) to your existing speakers.
Interestingly enough, very few customers took advantage of this feature.
Most bought either the satellites on their own, or the full system.
So, to come back to the design of the subwoofer, it was to be like a pedestal to the SATs and we decided early on that the driver would be invisible, as we wanted the SAT to be the main design feature.
So, we were constrained to a 200x200mm envelope and between 700 and 900mm height to bring the SATs to ear level.
The SATs themselves would be used horizontally in this configuration.
This was made possible by the fact that the tweeter was offset from the midrange and once horizontal, the dispersion pattern would still be good.
This is confirmed by listening tests: these speakers have an excellent image way beyond their own position, and providing quite a wide range of listening positions.
The SATs are 150mm high, then it leaves us between 550 and 750 mm for the subs.
Once the driver was chosen and we could plug its parameters into our box calculation, we ended up with a 600mm high cabinet.
The driver firing down would then be at the mercy of the floor material, and we didn't like that, hence the special socle with a pattern to match the angled design of the satellite, adding some extra stability to the whole unit.
That was going to be tested in years to come by the toddlers in our household...
The driver was originally an SEAS 17cm with an extra rubber treatment on the membrane. Further down the track, we used a SIARE unit.
The filter was again a Linkwitz-Riley with an upper frequency of 100hz
L=25mh (air core), C=100uF (made out of 4x25uF in parallel for better quality.
The final response being 30-100Hz within 1dB, thanks to the mechanical feedback of the base, close enough to "laminate" the air, hence linearising the response.
This is quite an amazing result from a 17cm driver.
The integration with the satellite is seamless and also prevent the satellite to generate too much distortion at the low end.
The only drawback is obviously a little loss of efficiency as a complete unit, but still in the 90dB range.
Later on, we designed a central subwoofer, starting the triphonic fashion, way before Bose entered the market...but this is another story for a later post...