Tom Danley: Hi everyone! Moderator: Tom, maybe you can start off with a bit of history - how did you become interested in loudspeakers and low-frequency sound in particular. Tom: I became interested in music because of grandfather's hi-fi, and when I was 9 my grandfather let me go into the pipe loft in church, and that started the fascination with low frequency sound - I didn't know whether to run or stay - but I stayed! And I built speakers since the 6th grade. Most of my jobs have been in electronics and loudspeakers and stereo related things have been my hobby, so when I went to work at Intersonics. The owner of the company was a hi-fi buff and, after a number of conversations, I told him about an idea for a speaker with a motor. The third prototype was good enough to demonstrate. After that he said I could pursue a small side business and that's how I got started. Another Dave: Were the pipe bellows the inspiration for the Servodrive speakers? Tom: Not directly - it was effect of the low frequency sound that really got me Grampa: What was Intersonics' core business? Tom: They were an experimental flight hardware contractor for NASA www.nasa.gov. Most of our work involved acoustic levitation and some electromagnetic levitation. The idea with levitation is that you can support an object in mid air without any kind of physical contact while it is melted or processed some way, usually at high temperature. My job was to build the control electronics, parts of the actual flight hardware and develop the sound sources for the levitation. Another Dave: An application of Bernoulli's Principle, or something different? Tom: Yes, it is the Bernoulli force that causes acoustic levitation once the sound level is high enough. Charlie Hughes: What sort of power output & freq for the levitation, and what mass were you supporting? Tom: Most of the levitation sources that I made ran at 21 KHz - a typical source that could produce a sound pressure of 165 dB @ 12 inches. An array of six can produce in excess of 175 dB at the center - which is enough to light a cigarette with acoustic friction. Charlie Hughes: An array of six what? Tom: Oops! An array of six sound sources. Charlie Hughes: What were the sources, generally? Tom: This was a resonant piezo-electric device. It consisted of 1/4 wave stub on the back and a half wave resonant on the front and a 1/4-wave transformer and flexural radiator. I don't have the patent number handy but I can send it to you. You could make one from the drawing on the patent. Charlie Hughes: Tom, I've read a bit about your adventures in Egypt measuring chambers of the Pyramids with the TEF. Care to elaborate on what you measured and how? Tom: This has to be one of the weirdest measurement things I've ever done. Originally the producer of the movie "Mystery of the Sphinx" asked me if I had any idea why the inside of the Great Pyramid sounded so strange. Having never been there, I had no idea but about four years later he called me up and asked me if I would like to go and find out. What I saw were the normal the room mode resonances from absolutely rigid stone room, but also what I found were a number of low frequency components that were present without the test signal, which, I suspect, were Helmholtz resonances caused by the wind blowing across the entry tunnel. One thing that was interesting - over a number of octaves, some of the resonant frequencies fell into the pattern that makes an F sharp chord. Of course, the movie people jumped on this as being proof of ancient voodoo or something. jack arnott: Tom, what would happen to the wedge speaker if it had an uneven number of mids, IE, three so the horn could sit flatter on the floor? Tom: I believe there would not be very much difference, although I have not actually tried that. Another Dave: Were the resonances in the pyramids simply because the room dimensions were mathematically related? Tom: Yes, they seem to be - one of things that was interesting was that the position of the sarcophagus and its resonances, in some cases, were coupled to the resonances of the room as well. Weogo: Tom, when ya gonna make a lower output td-1 style box with maybe a couple 8-in speakers and a 1-in? Tom: Stayed tuned, there's more coming - perhaps in the next year. Weogo: I like the idea of the even frequency response of horns for near-field use. You know I'm paying attention! Tom: So do we. Grampa: But don't you need to match the Q of the horns carefully so they sound good at short distances? Tom: The biggest problem is avoiding discontinuities as you go from one range to another, if I understand your question correctly. Grampa: Certain manufacturers' all-horn-loaded boxes sound AWFUL at close range, and not just because they're loud. Tom: Well, one thing that can cause that is having radically difference directivities at different frequencies. In this situation you'll find at some distance the speaker will balance properly, but closer or further, the tone balance will change. Weogo: Discontinuities between what ranges? I'm lost here… Tom: I thought what you were asking about was the transition from high to mid, and mid to low. In a normal speaker, these transitions are where the different ranges usually interfere with each other. Grampa: Yes, that's what I meant - at close range some all horn-loaded boxes just "bite" at certain frequencies, but balance out at a greater distance. Tom: When you're up close the effect of separation of the different
ranges is greater than if you are far away.
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