Adventure with Voigt Pipes by Herbert Jeschke Also see more in the NY Blast write up. For some time I was already tired of our stereo, I really did not use it much, but my daughter did and I liked what she listened to, only I could hardly stand the speakers threshing out the same bass-note when it got below 100 Hz somewhere. Surfing the inter-net for speaker projects, I looked at many but shied away from complicated vented boxes, transmission lines and crossovers with associated math. When I found the design of the Voigt Pipe at the site of the Lowther Club of Norway I was taken by the simplicity of it and the fact that it uses a single driver, much like a point source that would mirror the microphone that originally recorded the music of acoustical instruments. It also places the driver at ear-level of a sitting person placing the listener on a par with the performers. I didn't know how I would adapt to the steep price for Lowther drivers but went ahead anyway and build the Voigt Pipe cabinets thinking I might try some other full-range 8" drivers meanwhile. I used 3/4" chipboard that I had left over from a sub-flooring job. Along came summer 1999 and time passed with other predilections. In September came VALVE e-zine #4 with a glowing article about a small vented bookshelf speaker that used the Radio Shack #40-1354 drivers all of 5-1/2" big and famed them for their outstanding mid-frequency response. The article also said that the hardest material like cultured marble would be best for the boxes so I went out and got some 8" x 8" x 24" chimney tile (flue liner) and wanted to cut round holes in the sides of it. I have not found a good way to do it until today. Frustrated by that I still wanted to try these little 5-1/2 inchers when my eyes fell on the Voigt Pipes in a farther corner of my shop waiting a better day. The Pipes already had their 8" holes so I had to make a baffle to bridge the difference. Since it is a good idea to place the speakers off center in a baffle and because the scrap plywood I had offered that shape I made those baffles looking a little like wings pointing toward the middle of the sound stage. I thought that might help to fill in the void I sometimes sense in the middle of a stereo pair too far apart. That done using the fast method I hooked them to a stereo in my shop. The sound was stunning, I mean immediately better than anything I have listened to since the Triode Show in spring 1998. I got my wife to come down and put her in the chair at the sweet spot. She marveled and told me how you just know right away when you listen to some real good equipment. I did not change anything in 4 weeks. The Voigt Pipes are allowed in the living room now in spite of their unfinished surfaces and we enjoy them. They have 3 dimensionality and depth even when we are far away from the sweet spot. Midrange is just great and low end is amazing - probably due to the Voigt Pipe design, a horn design married to a transmission line. The RS drivers put out a lot. I need only 1 to 2 watt for loud listening levels on classical music. The cabinets might seem big to some but really they are only tall and take a square foot of floor. It is good to stand them a step away from any wall or more. They are good for fairly large rooms where you can spread them 8 to 10 foot apart and 3 foot away from the wall or more. Come December I build these little box speakers from VALVE #4 for my daughter to take to her dorm at college. I used just chipboard again but kept very close to the design given in the VALVE e-zine. That includes a pressure screw from the rear of the cabinet against the backside of the driver magnet. After Christmas we set up a shoot out between the speaker pairs, the Voigt Pipes and the Hawaii 5.0 boxes. The boxes sounded great, very fast and accurate, clear and happy without discoloration that I could hear. But somewhat the sound stage was attached to the speakers or not really free of it. The Voigt Pipes let it all out, loose and open almost boastful, creating with ease the impression of the hall the music was recorded in. Both sets were driven by the same amp using its A/B switches. The Voigt Pipes are significantly more efficient and the bass is almost unbelievable thunderous and follows the played tone accurate without preferring certain resonances. I did feel however that the Voigt Pipe would benefit from the same pressure screw arrangement as found in the box design and after installation we noted improvement of control in the low frequency end. Some day I might carefully add a little tweeter using a film cap to couple it in, just to see if I really miss it. The RS 40-1354 are said to go up to 15 kHz but not much further and I like the absence of that abrasive hiss some tweeters offer from digital recordings. Radio Shack SPECIFICATIONS for 5-1/4" HiFi Auto Speaker Cat. No. 40-1354A Impedance 8 Ohms Frequency response 50 - 14,000 Hz Free air resonance 50 Hz Efficiency (SPL at 1 Watt, 1 Meter) 90 dB Nominal power 20 Watt Maximum power 40 Watt Magnet weight 10 oz Speaker weight 30.6 oz SPECIFICATIONS posted on Bottlehead Forum by Greg on 1.20.2000 " Fs = 71.5 Vas = 7.3 liters Qts = .49 Qms = 3.85 and he notes: These should be enough to get you started. They seem to indicate a 14 liter ( .5 cu ft) box tuned to about 58 Hz. Good luck " Thank you for the posting Greg! Hints For Builders: from the Lowther Club of Norway: (partial quote) "The Voigt Pipe: The world's most easy-to-build High End Loudspeaker" A visually appealing speaker with good bass response down to 40 Hz. High sensitivity / efficiency. The Voigt Pipe acts as a cross between a horn and a transmission line. By tapering the pipe as shown, the resonances will spread over a wide range of frequencies. The Voigt Pipe is yet another of the late P.G.A.H. Voigt's audio inventions. Loosely stuff the upper half of the pipe with clean, long hair wool or equivalent damping material. The midrange can be tuned somewhat by experimenting with additional damping directly behind the driver. Dimensions are internal! Use 3/4" ply or MDF. Use Lowther drivers PM 6A, PM6C, PM7A. My drawing of the Voigt Pipe with dimensions in inches: When I built the Voigt Pipes I used good grade 3/4" chipboard, but now that I found out how well this design performs I must recommend a good grade plywood. I will build the next ones for a friend of 3/4" Baltic birch ply. The cabinet does vibrate and works as a secondary radiator to some extend, similar to a musical instrument. Look at the drawing with me. ALL DIMENSIONS ARE INTERNALLY MEASURED! The basic shape is a long wedge that stands on its smallest side. The front side is vertical, rectangular and carries the speaker's round hole and the rectangular vent hole in the bottom. CUT THESE HOLES AFTER ASSEMBLY! Only the side pieces are tapered or actually one rectangle cut diagonally into two triangles. Make these triangles first, that is your trickiest cut. Start with a rectangular piece of your wood 8-3/4" wide and at least 76" long, better longer if the cut should go wrong. Lay out the triangles on that board in such a way that your diagonal cut starts an inch or two away from one end of the board. You want to end up with the 8-3/4" by 71" triangle but cut the short rectangular crosscut after the diagonal cut is satisfactory. Clamp or nail the triangles together like a sandwich and plane or sand the triangles together so they end up having exact the same dimensions when taken apart again. Careful with the thin ends of the triangles! The rest is easy. The front panel is 8-3/4" plus twice the thickness of the triangle wide. It is 71" long unless you have to accommodate a slightly different dimension to fit your triangles. The back panel is as wide as the front panel but longer like 71-5/8", but it doesn't hurt to make it even an inch longer because it can be cut flush with the front panel at the top after assembly. The bottom plate butts against the four previous panels. It is square 8-3/4" plus twice material thickness by 8-3/4" plus twice the material thickness. The back panel should be miter-cut to fit the 7 degree angle with which it meets the bottom plate but I left it square cut and filled in the gap with silicon caulking. You will need silicon caulking at the top as well where the front and back meet at a sharp angle. When the glue is set and dry, cut the holes in the vertical front panel. NOTE the 7-7/8" by 4-7/8" vent-hole is not flush with the bottom but 3/8" higher than the inner bottom! If you have children, mount the speakers on a slap of stone or marble about two foot square to keep them from falling over easy. Do not put legs on the bottom of the speaker cabinets, because the rectangular vent hole uses the room's floor for extended bass radiation. The Pressure Screw This can be put in after you have already listened to your new speakers, but I found it audibly to great advantage. This is of course true because I used the RS drivers that have pressed steel bodies. Lowther drivers are likely to be rigid enough not to need that extra bracing. Same is probably true if you use cast frame speakers. I used 3/8" threaded rod for the screw and cut it to proper length after trying it in place. Drill a 1/2" hole behind the speaker through the back panel. It should be vertical to the front panel and coincide with the speaker axis. Take about a foot of 2" by 3" lumber and slice it in two pieces lengthwise but at an 7 degree angle. Cut each sliced piece 8-3/4" or a little less long. That braces the rear panel across its width and will carry the T-nut for the screw. Cut a 2" by 2" by 1-1/2" block and drill a 1/2" hole in its middle but only 2/3 into its thickness. Glue that and center it onto the rear of the speaker magnet. I used Silicon glue for that. Loosely assemble the whole pressure screw jig and find out the screw length you need. Cut it with a hacksaw and cut a slot for the screw driver in the outer end. Fix the speaker well into its hole now and then turn the pressure screw against the magnet with some feeling and judgment. Not too hard but hard enough to withstand all expected vibrations. It works like the sound-post in a violin and helps keeping the magnet rigidly mounted. Let me reiterate: The wings and the pressure screw plus the RS 40-1354 drivers are the only things that I added to the standard Voigt Pipe design from the site of the Lowther Club of Norway. Below the "Wings" which I call baffle is the standard 8" diameter hole for a 8" Lowther driver. It is beveled toward the inside because sound doesn't like sharp edges. On top of the 8" hole is the wing-shaped baffle: basically a rectangle 14-1/4 inch wide by 10-3/4 inch tall by 7/8 inch thick in my case. One of the vertical sides coincides with the vertical edge of the Voigt pipe. The driver is mounted in a 5-1/4 inch hole (beveled inside) which centers around a point 4-3/8 inch from the top edge of the baffle and 5-1/8 inch from the edge of the Voigt pipe, which coincides with the vertical centerline of the front of the Voigt pipe. The bottom edge of the rectangle farthest away from the driver is cut away in a gentle sloping curve tangential to the far vertical and to the bottom edge of the rectangle. I drew that curve by hand on one baffle and copied over to the other baffle. If you have difficulty with that shape take a piece of spring steel wire and bend it in a curve that touches the sides of the rectangle in a gentle slope almost coinciding with the sides near the corners. Hold it there and let your partner draw a line along the steel wire. I don't really think there is much to it and it would probably not sound much different with just a plain rectangle or whatever design you like. The baffle and speaker is mounted with drywall screws and sealed with latex caulk, the household kind. Currently I run my recently-built single ended triode amps (Svetlana 811-10) with my Voigt Pipes and really deem myself lucky to have arrived at a really good sounding system so easily. My professional past: Audio engineer for German TV and Film, Carpentry contractor, cabinet maker, audio engineer for archival recording in Philadelphia. Wishing you much fun building and enjoy the music. Happy listening! - Herbert Jeschke Top