Hello,
I am looking for some theoretical help on an infra sound subwoofer idea I am thinking about trying out.
I am software engineer and an inveterate stuff builder, but I don't have a ton of knowledge with the finer points of loudspeaker design theory. I was hoping maybe someone would be up for answering some quick questions before I start screwing things together.
So you may have heard of this thing: Eminent Technology TRW-17 Rotary Subwoofer | Home Theater
It is very cool, a functionally infinite excursion woofer. And very expensive. And somewhat difficult to build. Judging by the forums, there have been several attempts by more capable tinkerers than myself. My background in software and hardware control systems has me thinking of taking a different tack to aim for the same result.
I am thinking of taking a higher pressure blower, something like this: Residential Inflatable Blower by Blast Zone - Bounce Houses Now
And running it constantly (outdoors or in the garage or the next room or something) while I am watching movies or whatever. Then valving it with a servo and something like this: D690A1002 - Honeywell D690A1002 - 6" Single Blade Round Damper Maybe a diversion instead of a flat valve could keep the required servo torque down and make it more responsive.
I could easily make a cheap micro controller convert an analog sound signal amplitude to a valve openness percentage, and a cheap stepper motor aught to be able to drive the valve up to maybe 20 or 30 hz.
So far so good.
Here are my concerns, that maybe someone (or a few someones) could take a whack at:
- This would only be a half wave. Unless I used two blowers, one sucking. I wonder if it would matter at all at such low frequencies. If the positive side of the wave were doubled in amplitude compared to the signal, I am not sure there would be any meaningful difference at all.
- I am afraid the wavefront would be too defused to really do anything. A coil can slam a speaker cone out pretty hard and fast with a thousand watts behind it and produce some good compression. I wonder how high the static pressure stopped up behind the valve would have to be to be able to get meaningful sound pressure when the valve was opened. I worry the air would only just be starting to bother to get moving through the newly opened duct when the valve closed again.
- I am worried about fan noise and hissing and such. I thought about putting a batting stuffed folded horn or something like that on the output side of this thing to tame some of that. Which may prove how little I know about this stuff
It is encouraging to me that the rotary woofer is able to set up enough pressure to be meaningful, since its just a fan.
The other approach I have considered is just skipping to compressed air and something like a tiny ball valve controlled by a servo motor. No question of possible sound pressure there, is there a good way to deal with the hiss caused by high velocities at the nozzle? And what is the difference between a horn and a whistle here? I imagine an infrasonic whistle would have to be very big, and a remarkably bad idea. I would probably filter anything under maybe 10 or 12 hz completely anyway.
The whole point here would be to avoid a 30kw system with a dozen 18" woofers. I dont want huge pressure levels, just enough to feel it in my chest a little. I don't know what kind of pressure level that takes. I cant help wondering if cone speakers just aren't really the tools for the job. At such low frequencies, we are getting well within the range of other more powerful and efficient, if much less delicate, computerized mechanical approaches.
I am looking for some theoretical help on an infra sound subwoofer idea I am thinking about trying out.
I am software engineer and an inveterate stuff builder, but I don't have a ton of knowledge with the finer points of loudspeaker design theory. I was hoping maybe someone would be up for answering some quick questions before I start screwing things together.
So you may have heard of this thing: Eminent Technology TRW-17 Rotary Subwoofer | Home Theater
It is very cool, a functionally infinite excursion woofer. And very expensive. And somewhat difficult to build. Judging by the forums, there have been several attempts by more capable tinkerers than myself. My background in software and hardware control systems has me thinking of taking a different tack to aim for the same result.
I am thinking of taking a higher pressure blower, something like this: Residential Inflatable Blower by Blast Zone - Bounce Houses Now
And running it constantly (outdoors or in the garage or the next room or something) while I am watching movies or whatever. Then valving it with a servo and something like this: D690A1002 - Honeywell D690A1002 - 6" Single Blade Round Damper Maybe a diversion instead of a flat valve could keep the required servo torque down and make it more responsive.
I could easily make a cheap micro controller convert an analog sound signal amplitude to a valve openness percentage, and a cheap stepper motor aught to be able to drive the valve up to maybe 20 or 30 hz.
So far so good.
Here are my concerns, that maybe someone (or a few someones) could take a whack at:
- This would only be a half wave. Unless I used two blowers, one sucking. I wonder if it would matter at all at such low frequencies. If the positive side of the wave were doubled in amplitude compared to the signal, I am not sure there would be any meaningful difference at all.
- I am afraid the wavefront would be too defused to really do anything. A coil can slam a speaker cone out pretty hard and fast with a thousand watts behind it and produce some good compression. I wonder how high the static pressure stopped up behind the valve would have to be to be able to get meaningful sound pressure when the valve was opened. I worry the air would only just be starting to bother to get moving through the newly opened duct when the valve closed again.
- I am worried about fan noise and hissing and such. I thought about putting a batting stuffed folded horn or something like that on the output side of this thing to tame some of that. Which may prove how little I know about this stuff
It is encouraging to me that the rotary woofer is able to set up enough pressure to be meaningful, since its just a fan.
The other approach I have considered is just skipping to compressed air and something like a tiny ball valve controlled by a servo motor. No question of possible sound pressure there, is there a good way to deal with the hiss caused by high velocities at the nozzle? And what is the difference between a horn and a whistle here? I imagine an infrasonic whistle would have to be very big, and a remarkably bad idea. I would probably filter anything under maybe 10 or 12 hz completely anyway.
The whole point here would be to avoid a 30kw system with a dozen 18" woofers. I dont want huge pressure levels, just enough to feel it in my chest a little. I don't know what kind of pressure level that takes. I cant help wondering if cone speakers just aren't really the tools for the job. At such low frequencies, we are getting well within the range of other more powerful and efficient, if much less delicate, computerized mechanical approaches.