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Re: Answer to Steve & driver stuff

Hi Steve

I had a chance to look at what was needed so far as a driver for the horn you were talking about. You mentioned an exponential horn with a 4 inch dia throat, 1060 sq / in mouth and nine foot path length.
Given the 60 to 600Hz bandwidth and that horn (one that size) I found a number of “off the shelf” drivers which should do.
This will result in a predicted sensitivity of about 106 – 107 dB 1W 1M average, the response ripple will be small (about + - 1 dB) and overall, should be pretty flat (not humped).
The ball park maximum level at 60Hz, at Xmax, about 124 dB more or less.
Below Xmax, distortion should be very low.
The driver needs the following conditions to work properly.
Rear volume, about 5.5 L
Front volume, about .4 L (this is the total volume trapped under the cone when mounted to a flat baffle board, between the radiator and throat).
Unless your trapped volume is larger than .4 L, then don’t waste your time on a phase plug.
If you need more volume, use a spacer board of the appropriate thickness.
If the edge suspension protrudes forward of the mounting plane, cut a groove into the baffle for the edge suspension /cone clearance at / past xmax.

Any of the following drivers I found in the Madisound catalogue would work nearly identically (according to physical properties).
They all seem to have well behaved cones and should be safe at about a 2:1 compression ratio. Rightfully making them bass compression drivers.

Peerless 7” CSX woofer
Vifa PL18wo-0908
Vifa XT18wo-09-08
Vifa M18wo-09-08

The Peerless has slightly more excursion limited output and rolls off a bit more above 600Hz. Fwiw.

You must have a ridged air tight rear volume, the approach I use here is to depend a lot more on the rear enclosure compliance for the total compliance than tradition.
This produces lower harmonic distortion than using a stiffer suspension as the air spring is much more linear than the mechanical suspension.
It also must be the “right size” in order for the reactance annulling to work properly.

This driver / horn should have a flat response but a Voltages sensitivity that suggests an efficiency of about 20-25%.
Should one have a lot of cash they wish to burn up (which it would seem there are such audiophiles), then one can squeeze out a bit more with a cost is no object driver.

This effort raises the efficiency up to 60%, which would raise the sensitivity to a shade over 109 dB 1w1M.
I wouldn’t expect this could be built, at least with any excursion.

Fs = 65 Hz
Qe = .15
Qm = 6 or more
Vas = 836cu/in
Front Volume = 22 cu/in
Rear Volume = 110cu/in
Le = low enough to have R/L corner above 700Hz

Anyway, try one of the cheap ones, they ought to do what you want.

So far as folding horn etc, you can fold a horn with no ill effects what so ever but only when the acoustic dimensions are small enough. If you benefit from a corner reflector acoustically , you no longer have a horn operating in its coherent mode.
The 600Hz upper limit, I assume should be the coherent range of operation, this sets a number of horn dimensions to a smallish value.
For example, if you have parallel walls, that’s no problem UNTIL you reach the frequency where that spacing is ½ wl or multiple ½ wl’s, where you have a deep mode transfer notch you can’t eq.
What does the horn your building look like, I mean is there a drawing somewhere.

Hope this helps, I know these drivers aren’t exotic, but acoustically, they appear to be about right for this job.
If you guys get a chance to try one I’d be curious what you think.

Tom Danley



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