Steve Deckert
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Design notes: Jan 2003 - New Imperials
As usual the batteries in my camera are dead so pictures will be forthcoming. I wanted to take a few shots of the bent wood horn. I'm still experimenting with it. At the present time I have a different compression driver in each cabinet. The one on the right is an audiophile grade unit that goes up to 20K. The one on the left, is the Selenium that only goes out to 9K, but can be used down to 600 Hz.
I ended up leaving the left hand one in all this time because it actually sounds much better even though it lacks the top octave. Naturally I've decided the tone and timbre of the left side is something I can't live without but I've got to get more extension in the top.
Last week I discovered a significant rise in efficiency as I was varnishing the inside of the horn flare. This improved the situation somewhat with the left compression driver, but it will still have to either be replaced or supplemented with a super tweeter, something I hate to do.
Right now there is a natural balance of current between the three drivers in the cabinet and virtually no crossovers other then a single cap on the compression driver. Adding another component will probably break the synergy.
Terry and others have been waiting to see details on how the compression drivers are mounted onto these bent wood horns but at this point it's too early to say. My original idea, which I did, was to use a 3/4 board with a 1-3/8 hole threaded for the driver and mount it directly on the back of the lens. I used plywood at the time and it proved impossible to thread properly. Knowing I would have to resort to hardwood, and purchase a 1-3/8 fine thread tap I decided to cost of the tools may be less than ideal for the people planning to build a pair of these. To cover all the bases I decided to purchase some plastic horn lenses from a surplus catalog and cut the threaded flange off and simply glue it to the back of the bent wood horn. This is $5.00 verses the price of an expensive tap.
Tonight I re-installed the left compression driver by using part of a plastic lens as mentioned above. I cut the lens about 1.5 inches past the threaded collar because the square shape of the plastic lens matched perfectly with the square 1 x 1 inch opening in the back of the bent wood horn. I knew that the plastic in this high pressure area of the horn would raise the efficiency a bit, and should be technically superior to the original method of mounting the driver directly on the wood.
Interesting results. The horn did indeed pick up more efficiency. In fact with the varnish and now the plastic part, the sensitivity is now almost in danger of needing attenuation.
Listening test - prior to tonight, the horn on the left always had better tone then the horn on the right. Now with the plastic piece the entire speaker on the right hand side sounds better then the left. Prior to this the speaker on the left always sounded better because it has a calculated amounted of stuffing behind the woofers whereas the cabinet on the right does not.
The sound that is coming from the left horn is now unlistenable. It has that famous "horn" sound that makes you think your at a small live concert with a crappie sound man. I was surprised that it only took 1.5 inches of plastic to ruin the sound. Prior to this the horns sounded anything but like a horn, in fact I've been flirting with the idea of replacing my acustats with these speakers - in part because the highs are so good, open, free of any coloration, and non-beaming.
The conclusion is that the high pressure zone directly in front of the diaphragm is full of stress that is compounded by the first couple inches of the lens. All lenses are smooth, especially there. Controlled pressure release with pin holes, or specific dampening in the critical area of a horn determines the signature of the horn and effects every other aspect of its parameters. In the original method using a 3/4 board with the hole drilled in it so that the driver could screw in, the end grain of the plywood that was left because I couldn't thread the driver all the way in. The end grain absorbed the stress.
Next step is to restore the original driver mounting and loose the plastic piece. I knew from past experience that this was the most critical area of building a wood horn lens. I did not realize however what caused most commercial horn lenses to sound like PA systems until tonight. My conclusion is that if it is plastic, metal, fiberglass and does not trade a few dB with some carefully designed stress relief at the throat, it will sound like a horn. My bent wood horns do not sound like a horn, the sound like and in fact better than my electrostats. This is proof that the "horn" sound is really a byproduct of poor design.
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