John Reekie
Technologicality, at work and play

The Convertible active loudspeaker – revisited

Some time ago. I wrote a proposal for an active loudspeaker called the Convertible, using an 8″ woofer and a tweeter with a waveguide. Since then, I came up with the Mini Convertible (additional articles still in preparation) project, which is a more compact speaker.

Having now ordered and received some waveguides from Dave Pellegrene, I wanted to revisit the design of the Convertible. Based on the measurements of the SB29RDC in the waveguide and some other considerations, I’ve switched to a 6.5″ woofer, the Seas U18RNX/P. Here’s the concept design for a “convertible” monitor using the U18 and the Pellegrene waveguide:

Earlier, I had always assumed that the ported base of the “convertible” speaker would only add air volume (and a port). However, I recently had one of those “Aha!” moments, and with a little further investigation realized that a much better approach would be to add an extra woofer as well, thus turning it into a “2.5 way” ported floorstanding speaker. Here’s the concept diagram of that configuration:

Why the switch from 8″ to 6.5″? First, take a look at the SB29 response in the waveguide (0 30, 60, 90 degrees):

And here is Seas’ published U18 response:

If you look at the dispersion shown by the 60-degree curves (orange in the top graph, bottom curve in the lower graph), the directivity of the woofer gradually increases above a few hundred Hz and then matches the waveguided tweeter around 2 kHz. It’s almost as if these two were designed for each other!

The other thing about the U18 is that it has a nice little bit of extra Xmax, at 6 mm (peak). In fact, it has almost the same volume displacement as the U22 that I wanted to use in the original proposal. The net result is that I think two U18’s, while a bit more expensive, are a more effective solution than a single U22, for this speaker. And why not use two 8’s? Because the ported box would be too large.

I’m quite excited by this design (at this concept stage anyway). Everything – size, output, plate amp power, etc – just all seems to fit together really well. But first, I’ll try to finish off the Mini Convertible series of articles, translating this same concept to the “mini” format.

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