As mentioned in our "7 design principles", we think the midrange driver should be as close as possible to a full range unit.
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...

A Uni friend of mine was having a party in Poitiers - a mere 300kms away...- and asked me if I could produce the hardware for the sound sytem. He would organise the software, and provide us with travel money, accomodation and cost to build/rent the hardware. It was going to be a huge affair in a large reverberant hall at the Uni in Poitiers.
I can't remember what amplification I got my hands on, but I can remember visiting the factory of Pierre Clement, and imploring them for a "loan" of their new tangential arm turntable, and guess what, they agreed!
Now that I think about it, I believe they also loaned me a Shlumberger Broadcast amp destined to the ORTF. For more info on this product check this excellent website (time to sharpen your French...)http://www.audiofolia.com/clement.php
Most readers would know that Sclumberger gobbled up Pierre Clement soon after
I suppose it is a case of "follow your passion" and it will take you anywhere...
The next step (and time wise, it was the first one) was to build 20 identical speakers. I had in mind that if I could make these speakers in raw MDF, I could them finish them to any finish and sell them afterwards. Some ended up lacquered in white, or black or red. (remember, we were in 1972/73...)
They were designed to be a cube of 30cm when fitted with a grille (always black) on top to hide the tweeter.
Equipment was a full range 17cm from SIARE, and a TW8 from Audax of Elipson's fame- aluminium dome, apparently good up to 40khz. The filter was a simple 2.2uF capacitor for an 8KHz cut-off frequency. The tweeter was time aligned to the main driver.
So, here I am with a very expensive turntable and amp (probably six months of my salary at the time...) 20 speakers (sans tweeter to save space and risk of destruction) , the GF of the time, all to be fitted into a Peugeot 205 Convertible, and torrential rain in a 6 hour drive (no freeway  past the outskirts of Paris then)
We arrive in a fairly angry/exhausted mood to discover that accomodation was going to be a very bare student room, and too late for dinner.
We managed to get a good breakfast the next morning and by mid afternoon, we set up the whole system, and it worked fine all night and the fact this was all in mono did not bother any of the dancers.
We dismantled after the party, packed everything in the car and managed a good night sleep before driving back to Paris the next day.
I ended up selling all these speakers, including a few pairs to a vinyl pressing factory that used them as monitors in their QA department, because of their amazing image and detailing.
I am afraid I don't have any photo of this early design. If you happen to have one, or think you have, get in touch, please!

When I was at Uni - and still living at home, as most students did at that time - my passion developed further, and I was keen to use my new found knowledge - I was studying for a BSc in Electronics - to build my first hifi system.
Coincidentally, my sister and I were gifted a DUAL 1219 turntable for Christmas, so I decided to get down to work.
My budget being quite limited, and having settled for a full range high efficiency speaker and valve amplifier, I went shopping...
I was confident I could design and build a solid state preamp, so I did, but for a strange reason, I didn't feel confortable designing the amp from scratch, so I went and bought a kit from Cibot Radio and assembled it myself, crossing fingers that I would not put the house on fire on first power up. As I am still here to tell the story, you have your answer...an added bonus though was that my room was
always a couple of degrees Celsius warmer than the rest of the house, as you wouldn't switch that amp off, would you?
It was quite an ugly thing in military green lacquered perforated metal for the cover, but the sound...ah that glow in the valves transcribed itself in velvet music! Even my crude preamp could not ruin the sound...
For the technically inclined, it seems that this was the valve configuration: 2xel84;2x12ax7;ez80, but it is a while back, so I can't be sure.
If anybody has more information let me know via our contact form or the comments function. Thanks in advance.
Now, for the speakers, I used a full range 17cm drive from SIARE in a closed box made of MDF and filled up with wool  bed covers kindly given by Mum...
At the time, Michel Visan was the technical director of SIARE. He ended up starting Davis Acoustics, who continues to make some full range speakers inspired by the ones he designed for SIARE - see photo above. We ended up quite good friends and I would use some of his paper cone woofers in future designs, but more on that later.
I have never been a fan of Kevlar which started to be the rage in the 80s, when both Jacques Mahul at Focal (ex technical director of Audax and arch rival of SIARE, and Michel Visan at Davis Acoustics introduced drivers using this "space age" material.
I still believe paper cones have the ability to sound better than any thing else for the midrange, as they respect the timbre of instruments better.
The whole system was housed in a white laminate "structure" with the amp on one side, the preamp and turntable on the other side to avoid picking up rumble from the amp, a desk in the middle and the "legs" containing the speakers at the top and some space at the bottom for records stored vertically (bien sur...). I will try to find a photo from my parents archives or else, I will make a drawing of it
(but do you really care...?).
This system kept me happy for quite a while, until I came across the Elipsons and Supravox. But that will be my next story.