D.I.Y. 50 Hz bi-radial front horns - diyAudio
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Old 29th June 2004, 06:18 PM   #1
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Default D.I.Y. 50 Hz bi-radial front horns

Hi everyone.

I am a newcomer here and posted a picture of my horn in the "How big is too big?" thread yesterday to which Keyne replied with a few questions and request for more info and pics.

Here I shall explain how I came to make these behemoths and show some pictures of the build process.

I built these speakers about 3 years ago as a winter project to celebrate my renewed interest in hi-end audio after more than a decade sabbatical where I was off being a radio astronomer and listening to no music at all....well except for the music of the spheres.

I work with microwave dish antennas and had a 3 meter (10 foot) diameter fiberglass radome that is used as a snow and environmental shield for a dish antenna. I was looking at it one day and conceived that it had about the right shape that If I cut four petals out of it I could use these petals in their form to make a horn loudspeaker. Here is a picture that shows the flare profile of my radome next to the horn flare profile of the Altec 210 theater horn. Note that my horn has a longer, less radical throat expansion. The fiberglass had to be reinforced with steel mesh and automotive fiberglassing compound before I cut the radome structure apart or the profile would have been lost (the shape was very flimsy) so this was done in my shop as the first step. I ended up laying a total of 12 gallons of fiberglass, short strand polyseter resin on the four petals as well as imbedding steel mesh and 40 lbs of lead shot (for shot gun shells) in the resin (10 lbs. per petal), to strengthen and dampen the petals to the same sonic characteristics of the rest of the cabinets that are made from well braced 3/4" MDF. All of the major resonance nodes in the large areas of the cabinet sidewalls were mapped with a test driver, audio oscillator and amplifier, and then dealt with effectively with steel cross brackets bolted across the horn between the flat side walls (which are at a 2:1 flare angle to avoid standing waves between them unlike the Altec 210 which has parallel sides). Sheet lead was glued to the steel cross braces to dampen their own ringing tendencies. The resultant horns have a very high forward gain with negligible cabinet emission. Except for an energy storage issue around 350 Hz that I must still address (I believe it is a standing wave along the length of the sidewalls between mouth and driver), I am very pleased with these full-range speakers that were specifically sized for my home theater. The horns each employ a single 12" coaxial driver from Selenium. This is a true 'through the woofer magnet" dual driver design just like the legendary Altec 604 coaxial loudspeaker. The tweeter is a 2" voice coil, titanium dome compression driver.

In answer to one of Keyne's questions, the coaxial comes with it's own passive crossover bolted to the side and crosses at ~2K I believe. I do not hear the transition in this cabinet. It is very smooth. The cabinet operates extremely well from 50 Hz in open air testing below where it unloads like a switch over only a couple of Hz, all the way up to the upper limit of the compression driver, about 18 KHz IIRC. With corner boost my bass in the room is probably lower than 50 Hz which explains why it sounds adequate as I use them without a subwoofer.

The rear chamber and drivers have to be unbolted to reduce the size of these behemoths to allow them to fit through my stairwell to get them into the house. Even so they are a very tight fit. Knowing that bigger is better in horns, I made these as physically large as possibly I could. I didn't want the situation later where I would have had second thoughts and said to myself, "I should have made them bigger!" :wink: They weigh about 350 lbs. each. Serious theater speakers!

With their very high efficiency over 100 dB they are ideally suited to use with or test very modest powered SET amplifiers that I am experimenting with.

How do they sound? Well, except for the annoying energy storage issue which makes them sound bloated and colored around 350 Hz, they are lightning fast, do not loose coherence up close like the Altec A-7's that I also own and which are totally useless in a living room IMO), and are wide range enough that I do not use a subwoofer with them. Their simple 8 ohm load is very tube amplifier friendly. I should mention that they sound like crap on SS power but very impressive on vacuum tube power. I was using them on a homebrew triode wired 6CA7 P-P amp with 14 watts/ch., and they really rock. While absolutely amazing for home theater they were built with the intention of being my main hi-fi music speakers as well. Except for the 350 Hz issue they are quite nice.

Keyne observed that the rear chamber looked kind of large. Since I do not know how to calculate such a thing I went with the Vas figure for the driver which is about 3.5 cu.ft. I figured to err on the large side and it would be possible to make the volume smaller if necessary. The chamber is lined on all surfces except the baffle with 4" pink fiberglass insulation. Any suggestions on tuning this chamber would be appreciated. Oh yes, the horn throat is 4 feet long in response to Keyne's question. The driver is fully open to the ~ size matched horn throat. No acoustic LP filter coupling inotherwords.

I have not been idle and in the time since building these however have come up with a vastly different speaker design which IMO blows these horns away for music and are just as adequate for HT with much lower bass response as a bonus. Those may become the subject of future discussions. They are a dipole phased array/TL hybrid containing 24 drivers each, and are big. I have an interest in marketing them or a version of them under the trademark name ULTOR TM . I am making another radically designed offshoot begun just a week ago based on what I have learned from the first set which I have high expectations for.
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Old 29th June 2004, 06:32 PM   #2
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Major inspiration: "Hey...I can make a set of horn speakers from this!"
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Old 29th June 2004, 06:34 PM   #3
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Mapping out and re-inforcing the areas to become horn flare petals with 2"x2" steel mesh and short strand fiberglass/polyester autobody resin. The radome was flimsy and would have lost the exact flare profile if reinforcement was not donwe prior to cutting.
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Old 29th June 2004, 06:35 PM   #4
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Fitting the fiberrglass petals.
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Old 29th June 2004, 06:39 PM   #5
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Here I am drilling the hole for the electrical connector.
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Old 29th June 2004, 06:40 PM   #6
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Here is a picture of the back of the Selenium 12CO1P coaxial driver and the inside of the rear chamber.
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Old 29th June 2004, 06:44 PM   #7
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Here is a pic of the inside of the finished horn underneath. I decided to put some absorber on the outside of the baffle to discourage any sound bouncing around this cavity that was formed by the nature of the cabinet construction. I do not know if it helps but it can't hurt. Note how the fiberglass petals are attached to the 3/4" MDF sidewalls with pieces of Glastic angle.
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Old 29th June 2004, 06:55 PM   #8
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Here is a pic of the driver installed and some of the cross braces visible.

Note the visible reflection of the driver as a ghost driver image on each side of the actual driver. This is a concern as it means there are two virtual drivers in the horn creating their own set of sidelobes in the main beam pattern created by this horn. Luckily the HF drivers are small horns (hidden behind the acoustically transparent dustcap of the Selenium coaxial) and this HF horn projects the HF above crossover at ~2 KHz keeping it from hitting the cabinet sidewalls. Thus the HF dispersion above crossover exiting the cabinet is quite clean. There are sidelobes from the upper level of spectrum made by the 12" woofer. It is not really an issue in actual listening tests but if I place 1" rockwool on the insides of the two flat side walls of the horn to dampen the 350 Hz standing wave I have from mouth to driver, that absorber will also attenuate the virtual driver reflections IMO, improving the main lobe response of the overall horn in azimuth.

I should mention that the front-to-back ratio on the pattern in the room of these horns is amazingly high. Standing between the horns practically all you hear is the reflection from the back wall of the room some 40 feet away! I had to build huge pyramidical absorbers to terminate the far end of the room as a result before I could listen to threse horns. The auditorium delay effect was too much to bear!
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Old 29th June 2004, 06:59 PM   #9
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A shot of the side during construction. I employed welded frame of 1-1/8" square angle iron on the mouth of the rear chambers to facilitate bolting to the main cabinet with 1/4-20 bolts and a compressible gasket. 1x1" angle with tapped holes is attached on each vertical side of the mouth for handling protection, reinforcement and a way to bolt the three front cross braces in place.
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Old 29th June 2004, 07:02 PM   #10
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Here is another reflection photo which has valuable clues about HF reflection paths inside the horn which can be contributing negative sonic effects.
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