This is a small speaker which I designed as an improved clone of a Bang&Olufsen designer speaker. Unlike the B&O model, however, the Tube Speaker is made of real metal and sounds really good. It uses two 11-cm Seas-Excel mid-woofers (yes, the ones with the magnesium membranes) and a Scan-Speak D2905/930000 soft-dome tweeter mounted in a d’Appolito arrangement. The metallic outside is backed with a 2-cm thick layer of sand-filled epoxy, which results in an extremely «dead» enclosure.
Since these speakers are only 15cm wide I had to use very small woofers. The only good woofers in this size (about 12cm max. outer diameter) I could find at the time were the Seas Excel drivers. Their outer diamter is about 11cm and there’s even a model with the famous magnesium-membrane by Seas. Because these woofers are so small I had to use at least two of them in each speaker to get reasonable bass-response – so why not mounting them in d’Appolito style? I never built a d’Appolito speakers before so I decided to try it. Well, it was worth it: the sound qualtiy and the 3D imaging of the Tube-Speakers turned out the be very good!
The magnesium membranes have a very strong resonance peak at at about 11 kHz. This is way higher thant the cross-over frequency, so this resonance is not a problem in this design. The tweeter should be of the same good quality as the mid-woofers. I ended up using the ScanSpeak 9300 tweeter, befcaus it is a very good tweeter and it’s not too difficult to design a suitable cross-over filter network for it.
The design of the metal enclosure is where the Tube Speakers are different from every other speaker I have come across so far. But when you get off the beaten track, things can get rough…
The first major problem was to get a suitable metal tube. I called a metal-working companies, who told me that a suitable tube with cut-outs and a flat face plate to mount the drivers would cost a fortune. That means two fortunes for a stereo pair. Finally, a friend who works as a metal-working teacher took up the project and built the tubes for me. Thank you Erwin!
The second major problem with a metallic enclosure is that it would resonate like a church bell, very bad for a speaker cabinet. My first idea to solve this problem was to fill the metallic tube with concrete from the inside, but I was afraid the concrete wouldn’t stick to the metal. So, what else? Epoxy is really sticky… so I mixed epoxy with as much sand as possible. This gives a very heavy and solid material after the epoxy is hard. I put about 2 cm of my epoxy-sand mixture on the inside of the metal-tube. To do this I put a carton-tube inside the metal-tube. The carton-tube’s outer radius was about 2 cm less than the radius of the outer metal-tube which left the required gap to pour my mixture in. Epoxy has some disadvantages, though. Firstly, it is expensive. And secondly, it gets very hot when hardening and after becoming hard it cools down again which also means it contracts a little. This contraction caused the epoxy-sand-mix to loose contact to the metal-tube so I had to pour some more epoxy into these gaps. The result of my epoxy treatment is a very rigid and perfectly air-tight speaker-cabinet with virtually no vibrational resonances.
Crossover filter network
I used a 3rd order low-pass filter for the woofers (both woofers in parallel) and a 2nd order high-pass filter for the tweeter. Together with the natural low-frequency roll of the tweeter, this results in a 3rd order slopes in the frequency responses of both the woofers and tweeter, as required for a school-book d’Appolito design. The cross over frequency is about 2 kHz. The figure below shows the frequency response (measured with MacSpeaker using an MLS-signal), which is very flat (±1.5 dB between 500–8000 Hz, and ±3 dB throughout the entire frequency range of the measurement).
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