High Efficiency Speaker Asylum: Re: The Ultimate Subwoofers ? by tomservo Need speakers that can rock with just one watt? You found da place.

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Posted by tomservo (M ) on August 22, 2005 at 12:10:27

In Reply to: Re: The Ultimate Subwoofers ? posted by weltersys on August 20, 2005 at 23:09:03:

Hi Art, All
I know of the “valve” speaker although it was used differently.
Back in the Intersonics days I spent a lot of time at MSFC in Huntsville Ala.
Next door was Redstone arsenal, the place the Saturn 5 engine was developed.
In several places away from the base itself there were towers with a large horn on top.
I asked what they were for and got the explanation from our C.O. at NASA.
Anyway, here is what they were for.
Big Rockets make a lot of sound, one day while testing a single Saturn 5 there was a thermal inversion aloft.
Sound velocity is proportional to temperature, with a warm layer aloft, the sound from the test was gradually curved back to earth. That happened in a town about 20 miles away where the C.O. was living and in fact was at the grocery store and saw the windows bulging in and out.
The amount of damage that was done (broken windows etc, after several tests.) was sufficient to force NASA to “do something”.
That solution was to put a number of these high power horns on towers near Redstone AND to put microphones in the surrounding towns.
Before any further testing, these horns were sounded and the microphones red back to see if it was “safe” to shoot (safe being no thermal inversion).

The sources were a “flow modulator”, what I would call a class A valve in that it is half open at “no signal”. A voice coil motor moved the valve, either opening further or closing off the valve relative to the no signal condition.
The sources on the towers were made by LTV-Ling, the former industrial branch of Altec Lansing and now a defense contractor. I could not find my data sheets or the exact unit on line but I did find a similar source made by Wyle Labs.

http://www.wylelabs.com/sp1b.html

At the time, Wyle Labs (who were located kind of across the highway from MSFC and Redstone arsenal) did all the vibration testing on our payloads.
They also had the largest tube amplifier I have ever seen, it was something like 250KW, it had two huge output tubes and BIG driver tubes, water cooled and took up an entire wall in a good sized room.
This drove the shaker tables and as one fellow described, with a sheets of plywood attached to make a “radiator” and moved down its track to the dock door, did a fair job broadcasting Christmas carols.

Anyway, the “down side” of these flow modulators was the noise from the ~ 30 psi air flow (supplied by a big diesel air compressor ) limited the signal to noise ratio (the sound it makes with no signal compared to the sound it makes at max drive level) to only about 10 dB.
This was the problem I faced when designing the sonic boom simulator for GTRI.
They needed a source to study the acoustic signature of the space plane NASA was working on, given its large size, the sonic boom was going to be a doozey. An “N” shaped wave with a 3 Hz period.

This simulator system needed to produce 132 dB from 5 kHz, down to 3 HZ, outdoors, 2 meters from the wall of an old house next to the air base outside Atlanta. This output required the equivalent displacement of a piston 12 feet wide and 8 feet high, moving 18inches peak to peak at 3 Hz.

My solution was to use a “push pull” valve and lower the pressure to about ¼ psi.
Given the job it had to do, it required an air source moving about 50 cu mtrs per second and this required a special 3 phase service to be run to the site for the 12, 5HP fans.
This flow modulator had vastly more air flow volume and a much lower pressure, combined with the class”A” push pull valve, increased the signal to noise by about 100 (20 dB) compared to the high pressure versions.
For such a valve to be used in the home which is a contained space, it is worth considering that an air pressure of only 1/100 PSI is 132 dB SPL or 1.6 pounds per square foot applied to your walls ceiling and floors.
At the old house in Atlanta, with 132 dB available (on one wall) and using a slow sine sweep with the TEF machine, I was able to find the fundamental resonance of the test wall. In the middle where the window was, the pressure could move it in and out about a foot and a half peak to peak at 4.5 Hz.
No one of our group was able to stay in the house during this (in fear) and later when a loud CRUNCH came from somewhere below the floor, we stopped teasing the old house.
I’ll never forget the feeling of standing between the house and the system with the “pickle” button that made the “N” signal. It was the kind of KAAABOOM, very satisfying to do in rapid succession until you saturate and have to get away.
The actual use of the system the customer had in mind involved observing dishes and household stuff as well as test subjects when it went off with no warning.
Best Regards,

Tom Danley

Danley Sound Labs

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