Mr. Ray Purchase wonders: Speaker Design – Is there much to it? (Part 3)


I used the program WinISD to calculate the ideal size of sealed enclosure for the bass drivers. Ideally it would be 120 litres. Quite big! I managed to find a pair of 1969 Goodmans Magisters going locally for £20, which have about 90 litres internally. Luckily they weren't in great condition, so when I butchered them, jig sawing chunks out of the baffles, my conscience didn't trouble me too greatly. These enclosures contain large lumps of open cell foam rubber which were still in good condition.

I figured that standing the woofer enclosures vertically with the mid & tweeter enclosures on top would place the tweeters at almost the right height. I cut out new baffles from 18mm MDF to screw onto the old ones, and made circular apertures for the 12” drivers with a router and crude circle jig. I put the woofers as close to the top of the enclosure as I could manage. I braced the potentially flappy lower portion of the baffle by screwing a length of batten onto the inside.

I was sure I would want to fiddle with this arrangement, so I decided not to glue the baffle on, but to make a neoprene gasket instead. In the event, having decided to do all the woodwork in one day (which I managed to do – just), I didn't get round to making the gasket, and merely snaked self-adhesive foam draught excluder around a perimeter defined by the fastening screws and left it at that.

I also used WinISD to calculate the correct volume for the SKO100 mids as if they were woofers, and found that I had some old Acoustic Research enclosures with rotted foam-surround drivers that were just right, and very solidly made. Again, I butchered them and made new 18mm MDF baffles, this time radiusing the edges to reduce diffraction (that's what they advise, these days, although many older speakers with square edges are still highly rated). I also radiused the inner edges of the mid drivers' apertures. I mounted the mids and tweeters very close together and did the same draught-excluder trick to quickly seal the enclosures without gluing. I re-used the original wadding material that was stuffed in the cabinets.

Continues tomorrow

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