Voigt pipes were a creation of P.G.A.H. Voigt in the 1930's which are similar to the concept of a transmission line, except the speaker is placed near the center of the horn-shaped pipe. Since this is intended to be a full-range, single speaker sound source, the type of speaker chosen is rather critical. One of the favorites is the Radio Shack #40-1354A, a very versatile driver that had a whizzer cone to aid the high frequencies, and good overall tonal balance for a mere $10 -- until they discontinued it. Some very excellent Voigt pipes were constructed for well under $50. The triangular nature of the Voigt pipe causes its resonant frequencies to be spread across the midrange bands, and the length of the pipe accentuates the bass frequencies very well. Because of this, the Voigt pipe can produce a very wide frequency range with very natural tonal balance.

• After deciding that I needed some speakers in my basement (I had 12 tube amplifiers, an ancient pair of cheap two-way speakers with one woofer that literally melted when a 6550 failed, and a test pair of little 6.5" speaker boxes!), I decided that the Voigt pipe would be the way to go. I had my eye (but not my wallet) on a pair of $2400, polished aluminum D2-al2's from O'heocha Design, but decided to build a simple Voigt pipe instead. I already had two 5" drivers that I hoped would do the job. They are not full range drivers and I have no idea what their Qts is. They don't have great high frequency extension, but I didn't mind because I wanted to use my plasma tweeters for the highs. They do have awesome low frequency characteristic that I hoped the Voigt pipe would bring out.
• I went to Home Depot and got a very nice sheet of 3/4" Birch Hardwood plywood for $42. I was bent on building both pipes from one sheet, so mine are only 7.5" wide and 9.5" deep (I wanted to preserve the 7 degree angle that Jeschke used). They stand ~74" tall at the moment (with the bottom attached) and I located the speaker center at 39" above the floor. I used liquid nails and tub caulk to glue and seal the boxes, with a few nails in the back panel.
• I put a few nails inside the pipe to hold the stuffing in there. I bought a 5lb bag of Acousta Stuf from Parts Express, as well as some gold binding posts.
• I used ~1.2 lbs of stuffing per pipe in the upper half.
• The sound was surprisingly good for a pair of midbasses. The high frequency extension was a little weak, but thats what my plasma tweeters are for. The midrange was very full and clean, and the bass was strong down to about 50Hz. I heard details in music I had never heard before. The biggest problem was doppler distortion at high volume levels (~95dB) due to the excessively heavy cone. It makes trumpets sound a little like they are under water when a large bass note hits (like the Olympic Fanfare, Starwars theme, Star Trek theme from the Telarc CD I used). The cure would be for me to get a subwoofer. I have my eye on a Pheonix Gold Cyclone Sub, which is made for car audio, but is the most bizzare speaker I have ever seen (it looks like a washing machine agitator, and works like an Archemedian screw compressor).
• I have been listening to these for several hours now and they sound really really good with the plasma tweeters. The midrange definition and presence is amazing. The bass doesn't quite come out and grab you, but it's definetely there, and very accurate and well-defined. I need a sub for the lower frequencies. I want to listen to them all day and night. I have gotten used to the level of bass they produce: I think it's not that they are weak, but that I am used to listening to bass a few dB louder than it should be in my car (need to combat road and wind noise by increasing bass). I think these pipes are the best sounding speakers I have heard for under $100, and my friend says they sound better than most big 3-way speakers he's heard. I love the single-speaker wide-bandwidth sound now. The fast transient response of such wide-bandwidth systems just makes the sound so smooth, like a well built horn-loaded speaker. My system diagram.