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Copenhagen Transmission-line Loudspeakers

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Project by bunkie posted 07-23-2012 01:18 AM 1343 views 2 times favorited 2 comments Add to Favorites Watch

This is the pair of speakers I built for the recent Midwest Audio Fest which is sponsored by Parts Express. I had never entered any of my designs in a competition before . It was a very interesting experience. More about that later.

I named this pair “Copenhagen” because, 1) I needed a name for the competition, 2) the woofer and tweeter are made in Denmark and 3) I went to school in Copenhagen when I was a lad.

This is essentially the same design that I’ve been building for the last decade. What makes these different is that I’m going for aesthetics to match the acoustics. The cabinet is made from 3/4” MDF. It looks like a box, but internal, there’s a pathway from the space just behind the woofer to the opening on the back. There’a an internal baffle that creates this pathway whose length is tuned to the resonant frequency of the woofer. The pathway is tapered and the area of the opening on the back exactly matches that of the woofer cone. This is known as a quarter-wave tapered transmission-line design. It has the effect of extending the low frequency limit of the woofer beyond what it can normally produce.

I used paper-backed maple veneer and I rounded over the front edges of the cabinet so that I could wrap the cabinet with a single piece of veneer. I used contact cement to attach it. The feet are made of cherry and incorporate cone-shaped metal spikes to tie the cabinet firmly to the floor.

As for the competition, I didn’t do too badly. There were over 50 different speakers entered. I was entered in the “over $200” category. I just missed the cut as it was based on the cost of the drivers and mine cost $203.50 for both speakers. The winner in this category spent quite a bit more than I did. Here’s a link to a write-up and gallery of the entrants by one of the participants.

I highly recommend looking at what the speaker-building community is doing. There are some exceptionally talented woodworkers doing top-notch work.

What was amazing was how good all these speakers sounded. There was very little sonic variation. I had a great time and I can definitely tell you that I came away determined to up my game. I now know what I’m up against.

BTW, anyone who wants to make a copy of my design is welcome to do so. Send me a PM and I’ll be glad to help.

-- Altruism is, ultimately, self-serving





2 comments so far

View bunkie's profile

bunkie

411 posts in 1898 days


#1 posted 07-23-2012 01:29 AM

A couple more things…

You might notice that there’s no visible means of holding the grill covers in place. That’s because it’s done with magnets. I embedded some screws in the face of the cabinets and veneered over them. The grill frames are made of a single piece of 1/4 birch plywood with the opening cut on my router table. In an incredible bit of luck, I had a forester bit that was the exact same diameter as a set of neodymium magnets I had. The magnets are pressed into the holes.

The finish on the speakers is rattle-can lacquer. On one cabinet, I brushed on two coats of shellac before spraying the lacquer. On the other, it’s just lacquer. It’s hard to tell, but the one with the shellac has a slightly warmer color. I had never tried this before and I will definitely try it again in the future.

One thing I struggled with was hiding the veneer seams at the top. I’m not entirely happy with the result and next time, I’m going to use resawn thick veneer to cap the cabinet. One more issue I had was that the contact cement got into the bearing on my flush-trimming bit which seized up and caused some burning. After that, I used a thin razor saw to remove most of the overhanging veneer and finished up with sandpaper.

-- Altruism is, ultimately, self-serving

View TheGman's profile

TheGman

5 posts in 1391 days


#2 posted 07-23-2012 04:31 PM

I really like your simple design, especially with the grill covers off. I usually make it to the midwest audio festival but I couldn’t make it this year. Its a great event and Parts Express is a great company.

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