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Natural Science Forum / Physics / Acoustics / May 2005Focus Sound?
I was looking at the diagram of a transducer where a piezo-ceramic disk is attached to a plastic concave lens; http://www.qmi-inc.com/AIRSCAN%20TRANSDUCERS,%20TECHNIQUE%20AND%20APPLIC ATIONS.htm supposedly to focus sound. I knew this was sometimes done to focus sound waves in water, but I didn't know it also works in air. Does anyone know if this can this work at any frequency, or just at ultrasonic ones, and is there any way to know how much of a sound increase is at the focus of such an arrangement? Ron -- "You see me now a veteran, of a thousand psychic wars. I've been living on the edge so long where the winds of limbo roar." > I was looking at the diagram of a transducer where a piezo-ceramic disk > is attached to a plastic concave lens; [quoted text clipped - 3 lines] > > supposedly to focus sound. The attachment of a plastic concave lens to the piezo-ceramic disk was a very early design that was used with readily available, inexpensive planar piezo-ceramic disks. Greater acoustic output is obtained with a custom focused piezo-ceramic element, especially when a quarter-wave impedance matching layer bonded to its front face. > I knew this was sometimes done to focus sound > waves in water, but I didn't know it also works in air. > Does anyone know > if this can this work at any frequency, or just at ultrasonic ones.... It will work at any frequency, but reasonable acoustic gain will be achieved only if the diameter of the element is large compared to the wavelength of the radiated sound. In the reference that you cited, the diameter of the element is 1" and its resonant freqeuncy is 400KHz, so the ratio of diameter to wavelength is approximately 30. >..... and > is there any way to know how much of a sound increase is at the focus of > such an arrangement? The increase would be the same as that obtained by a converging optical lens, but if you are operating at 20KHz, the increase issue is irrelevant unless your transducer is at least eight to twelve inches in diameter. > > I was looking at the diagram of a transducer where a piezo-ceramic disk > > is attached to a plastic concave lens; http://www.qmi-inc.com/AIRSCAN%20TRANSDUCERS,%20TECHNIQUE%20AND%20APPLIC > > ATIONS.htm > > [quoted text clipped - 24 lines] > lens, but if you are operating at 20KHz, the increase issue is irrelevant > unless your transducer is at least eight to twelve inches in diameter. Thanks, Ghost. I once had a half-inch curved piezo element that I paid big bucks for it, but in air, it put out very little acoustical energy. So it goes... ;-) Ron >> > I was looking at the diagram of a transducer where a piezo-ceramic > disk [quoted text clipped - 42 lines] > big bucks for it, but in air, it put out very little acoustical energy. > So it goes... ;-) Have you had any success building a resonant Terfenol-based Tonpilz transducer? Also, have you tried the approach used by American Technology Corp, using PVDF film stretched over a perforated plate with a vacuum on the rear side? In their white paper, www.atcsd.com/pdf/HSSWHTPAPERRevE.pdf they describe a 37KHz resonant transducer that is 1.75" in diameter with 85 9/64" holes arranged in a tight hexagonal pattern with a center-to-center spacing of 0.16." Using 28 micron PVDF film and 74Vp-p drive, they claim an output of 136dB SPL in air. With appropriate re-sizing of the holes you should be able to move the resonant frequency down to the 20KHz range, and if the damping is less at 20KHz than it is at 37KHz, you may be able to get close to 130dB SPL at 20KHz. > >> > I was looking at the diagram of a transducer where a piezo-ceramic > > disk > >> > is attached to a plastic concave lens; http://www.qmi-inc.com/AIRSCAN%20TRANSDUCERS,%20TECHNIQUE%20AND%20APPLIC > >> > ATIONS.htm > >> > [quoted text clipped - 50 lines] > if the damping is less at 20KHz than it is at 37KHz, you may be able to get > close to 130dB SPL at 20KHz. At first I got caught up with the idea of great acoustic power and simplicity of design when it came to a power supply, then I got to doing the math and realized that a 1 or 1.5" length Terfenol-D rod was going to have a resonance frequency way too high for my needs. It was the reason I had turned down magnetostrictive rods in the first place, but if I was dealing with a pure metallic material like nickel or cobalt, I could've made a 1" ring. Terfenol-D, however, is a very brittle alloy and not bendable or very machinable. So now it's back to using a piezo design. The lens and prism designs I just saw seem to offer what I need to finish my project if I can do a bit of clever engineering to get everything to fit in a small space. ;-) But thanks for the link to that article on the PVDF film; it was pretty interesting. Ron Does anyone know if it is possible to replace the plano-concave lens with a fresnel lens and still focus sound waves? Ron > > >> > I was looking at the diagram of a transducer where a > piezo-ceramic > > > disk > > >> > is attached to a plastic concave lens; http://www.qmi-inc.com/AIRSCAN%20TRANSDUCERS,%20TECHNIQUE%20AND%20APPLIC > > >> > ATIONS.htm > > >> > [quoted text clipped - 84 lines] > > Ron > Does anyone know if it is possible to replace the plano-concave lens > with a fresnel lens and still focus sound waves? > > Ron So long as the size of the wedges on the Fresnel are large compared with the wavelength (say 10 times), there is no reason why it wouldn't work acoustically. It wouldn't take long to write a simulation in Matlab or whatever, that would allow you to explore the effect of wedge size and wavelength, if that has not been done already, which it probably has. Cheers Greg Locock |
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