The Metronome Mass Loaded Quadratic Quarter Wave Resonator Loudspeaker
After the amp fest we had on March 25th at Steve Shiels’ place I began to think about all we had talked about regarding Steve’s Lowthers and the way they just seemed to suit my amp to a tee.
Listening to my own system at home with my sub sats, there was a definite lack of life compared to Steve’s giant horns. Of course in our tiny 12 x 12 foot living room there was no way I could accommodate speakers like the Lowthers. Nonetheless I found myself hankering after that sense of immediacy horns have compared with conventional speakers. I had felt just the same on the two occasions I had visited Simon Chambers and heard his big Fostex horns. Returning to my own speaker set-up had been a bit disappointing but the spouse acceptance factor and the sheer impracticality of such a speaker in my own circumstances had caused me to have to learn to accept that in life we can’t have everything we want.
However, this time the feeling that something was missing from my own set-up kept nagging so I began investigating further. I came upon a review of a commercial speaker by Terry Cain called the Abby which used quarter wave loading and a single Fostex full-range driver. The reviews were generally very good indeed but the speaker was over 6 feet tall , therefore spouse acceptance factor was off the radar.
The concept however was intriguing. Visiting the single driver website revealed lots of designs that used the QWR folded to more acceptable dimensions but this time they were far too deep front to back to be acceptable in our living room.
So if I wanted a shallow front to back measurement the speaker would be far too tall but if the height was made acceptable then the speaker would be too deep. It seemed to be another dead end.
It was one of those “eureka” moments that are rare, but when they come you’d better take notice. All the speakers I had looked at were ostensibly trying to cover a wide frequency range, hence the monstrously long line lengths being used. But I had a sub. I did not need to get low with my design. All I needed was a cutoff at around 80 Hz; the sub could take over from there.
Looking at the Fostex website it didn’t take long to turn up a suitable driver. The FE108 Sigma had an Fs of 77Hz. Out with the calculator and it turned out that the wavelength of this frequency was around 14 and a half feet.
Dividing by 4 gave a line length of 44 inches for a quarter wave tube. That was more like it.
Sd for the speaker was 50 sq cm so going for 4 x Sd gave a mouth area of 200 sq cm which gave an internal base footprint of 20 cm across by 10 cm deep. YES!
The drivers were ordered from Wilmslow Audio and a sheet of 18mm birch ply was procured from the local B&Q cut into four 4 x 2 lengths by the nice man at the cutting station.
I decided very early on in the design to taper the enclosure in both directions. Not only would this ensure that there were no parallel sides to set up standing waves but also the top of the speaker would be very small, reducing the visual impact within the small space I had. This was very important if I wanted the finished design to be acceptable to my wife and not dominate the room. The woodwork was a bit more involved but would be worth it from both a sound quality and aesthetic point of view.
Internal walls of the enclosure were lined with a heavy rubber carpet underlay the fitters had left when they had put in our front room carpet. I knew it would come in handy some day!
The arches cut into the cabinet bases on all four sides serve as the cabinet feet. Drivers are ready to be installed.
Purists may blench at the fact that I’ve used crimp on spade connectors to connect to the drivers. I was not however prepared to risk damage by attempting to solder directly to the driver lugs. I’ve built speakers before and know only too well how easy it is to b**ger things up!
It’s not worth the aggro believe me.
Here’s a close up of one of the Fostex FE108 Sigma full-range drivers.
They are very well put together with hand-made cones fashioned from banana plant fibre of all things.
Mass Loading port is made from a 2 inch diameter 3 inch long plastic pipe set into a piece of three quarter inch veneered chipboard.
The base plate in place at the bottom of the cabinet. The joins are sealed with PVA glue and silicone caulking material.
And here they are in the system. I must admit they look very handsome in their beeswaxed finish. My other half is happy with them too, which is a definite plus.
They cross over beautifully to the sub which fills in the lower bass nicely.
If the speakers sound closed in and nasal then remove all the fibre stuffing. The sound will open out and become clear and detailed. Truth be told it is probably best just to use the carpet felt lining without the fibre wool.
If you build these speakers drop me an email and let me know how you get on.
Thanks must go to:
Dave Dlugos of Planet-10 Hi-Fi and Scott Lindgren without whom this project would never
have sounded as good as it does.
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