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Natural Science Forum / Physics / Acoustics / August 2007Two, Better Than One?
Can somebody here tell me why two piezo discs aren't used to get more sound out of the unimorph design? Ron > Can somebody here tell me why two piezo discs aren't used to get more > sound out of the unimorph design? > > Ron If I understand your question, I think it is done sometimes. It's called a bimorph. Search on that term and see if it answers your question.  SignatureDave dvt at psu dot edu Everyone confesses that exertion which brings out all the powers of body and mind is the best thing for us; but most people do all they can to get rid of it, and as a general rule nobody does much more than circumstances drive them to do. -Harriet Beecher Stowe, abolitionist and novelist (1811-1896) > > Can somebody here tell me why two piezo discs aren't used to get more > > sound out of the unimorph design? [quoted text clipped - 13 lines] > circumstances drive them to do. -Harriet Beecher Stowe, abolitionist and > novelist (1811-1896) No, I've known about bimorphs for a long time. They are usually rectangular and are called "benders" cuz they have a center vane of brass and are designed to usually be used as strain sensors rather than acoustical transducers. On the other hand, some piezo speakers use two very thin piezo plates bonded to the sides of a piece of brass, but that's not the same thing as using two p. discs in series bonded to one side of a plate. I've never seen it done in the literature, but I never have heard a reason for why it's *not* being done either. I've seen that kind of arrangement used in ultrasonic cleaners where two plates are bonded together with a high tensile bolt with a steel mass in back and an aluminum mass in front, then the whole thing bonded to the metal bottom of the cleaner-- but once again, that's not the same as making a "unimorph" with two (or more) discs. Ron >>> Can somebody here tell me why two piezo discs aren't used to get more >>> sound out of the unimorph design? >>> Ron >> If I understand your question, I think it is done sometimes. It's called >> a bimorph. Search on that term and see if it answers your question. >> Everyone confesses that exertion which brings out all the powers of body >> and mind is the best thing for us; but most people do all they can to >> get rid of it, and as a general rule nobody does much more than >> circumstances drive them to do. -Harriet Beecher Stowe, abolitionist and >> novelist (1811-1896) > No, I've known about bimorphs for a long time. They are usually > rectangular and are called "benders" cuz they have a center vane of [quoted text clipped - 12 lines] > cleaner-- but once again, that's not the same as making > a "unimorph" with two (or more) discs. I'm afraid I don't understand what you're asking about. Some companies sell disks with piezo on both sides [1]. But you suggest that both piezos should be on one side of the disk and connected in series. Are the piezos in your suggested arrangement stacked atop each other? I think a picture would be worth the proverbial kwords. By the way, Ralph Woollett might be a good search term if you're interested in flexural disks. http://www.americanpiezo.com/products_services/Bimorph_Sheet.pdf SignatureDave dvt at psu dot edu > >>> Can somebody here tell me why two piezo discs aren't used to get more > >>> sound out of the unimorph design? [quoted text clipped - 39 lines] > > - Show quoted text - Transducers made with stacks are called Tompilz transducers. They are most commonly used for low frequency, ultrasonic transducers for use under water. But those typically use somewhat thicker rings and are bonded to thicker masses of aluminum. I can see the design theory behind unimorphs; they produce a good amount of sound for comparatively little power. But maybe I'm dense, cuz I don't understand why only one disc is used when other transducer designs use two or more. While it's very difficult to make one by hand, it wouldn't be a lot of trouble for a manufacture to make such a design. Ron > Transducers made with stacks are called Tompilz transducers. They are > most commonly used for low frequency, ultrasonic transducers for use > under water. But those typically use somewhat thicker rings and are [quoted text clipped - 6 lines] > hand, it wouldn't be a lot of trouble for a manufacture to make such a > design. I /think/ I understand your question now. Replacing one large piezoceramic piece with two half-size pieces does not typically increase power output. If you replace one piezoceramic plate with two stacked plates, each half the thickness of the original, and you connect them in parallel, the electrical impedance changes. So for the same output, you need half as much voltage but twice as much current. There is no improvement in efficiency. The reduced impedance might be useful for matching to your electronics, though. If you connect these two thin layers in series, there is no change in the transducer. The above discussion assumes that the joint between the layers is ideal. As Angelo mentioned, the joint is rarely ideal and increasing the number of layers typically reduces efficiency slightly. It's also more expensive, which might be the reason that unimorphs use only one layer. The above discussion is applicable to unimorphs, bimorphs, and tonpilz transducers alike. SignatureDave dvt at psu dot edu Everyone confesses that exertion which brings out all the powers of body and mind is the best thing for us; but most people do all they can to get rid of it, and as a general rule nobody does much more than circumstances drive them to do. -Harriet Beecher Stowe, abolitionist and novelist (1811-1896) > > Transducers made with stacks are called Tompilz transducers. They are > [quoted text clipped - 38 lines] > circumstances drive them to do. -Harriet Beecher Stowe, abolitionist and > novelist (1811-1896) Hi, Dave; With stacks, electrical concerns take second place to power output in terms of force. Thanks, now I answered my own question: two or more discs aren't used becuse while they produce more *force,* they don't produce any more deflection, hence no greater sound output. Bummer. The way my tiny brain is hard-wired, I always equate force with sound intensity, but the universe has other ideas. You can have greater force, but not a greater degree of deflection. It's not only for electrical reasons that the piezo elements in unimorphs are very, very, thin but becauce they produce a much highr degree of deflection than a thicker disc. Ron > With stacks, electrical concerns take second place to power output in > terms of force. Thanks, now I answered my own question: two or more > discs aren't used becuse while they produce more *force,* they don't > produce any more deflection, hence no greater sound output. Bummer. Sorry to burst your bubble, but for a unimorph construction, force and deflection are proportional. Consequently, for a unimorph construction, if a piezo disc produces more force, it also produces more deflection. Furthermore, for a unimorph construction, if the total thickness of stacked piezo discs is constant, the relationship between force and displacement will be the same, irespective of the number of discs in the stack. > The way my tiny brain is hard-wired, I always equate force with sound > intensity, but the universe has other ideas. You can have greater > force, but not a greater degree of deflection. False. > It's not only for > electrical reasons that the piezo elements in unimorphs are very, > very, thin but becauce they produce a much highr degree of deflection > than a thicker disc. It's a bit more complicated than that because the deflection of a unimorph depends on the force produced by the radial expansion of the piezo element as well as on the bending stiffness of the metal disc to which it is bonded. A very thin piezo disc which is bonded to a thick metal disc will produce very little deflection. Additionally, there is the issue of resonant frequency which depends on the total thickness of the piezo disc and the metal disc to which it is bonded. > > With stacks, electrical concerns take second place to power output in > > terms of force. Thanks, now I answered my own question: two or more [quoted text clipped - 8 lines] > displacement will be the same, irespective of the number of discs in the > stack. I have dyslexia, so I may have gotten it backwards, but that's not what I've read in all those treatises on transducers. Point me towards something that agrees with what you said and I'll be the first to admit I got it wrong. :-) Ron > Can somebody here tell me why two piezo discs aren't used to get more > sound out of the unimorph design? These transducers are not deployed like a common loudspeaker. Rather, they are ditven at their resonance frequency. The unimorph eigenmode is like a 2D xylophone mode where the node (nil motion) is a circle of about 80% diameter of the total wafer. Sometimes they are mounted (constrained) at their edge for a slightky lower frequency. They are usually used as "beepers" from 1 kHz through maybe 3.5 kHz. Thicker wafers will have a different resonance frequency. And thicker plates often have a complex array of resonances not harmonically related. Check a text book on the vibration of bars and plates. These usually can be bought in any thickness desired since they are all cast from powder stock. Also, when joining two discs, the quality of the bond is extremely important. Any dissipation will ruin the efficiency. Recall that constrained layer damping uses identical geometry and analogous materials. It's a crap-shoot. Buy some and try it. Then tell us the results.... Angelo Campanella >. It's a crap-shoot. Buy some and try it. Then tell us the > results.... I had serously thought about it, and bought a dozen thin discs but I couldn't manage to connect them right. As you say, try it: it's extremely dffcult for some schmuck in his bedroom to manufacture several perfectly round brass discs with a suitable tab for making the connections and gluing them onto a fragile piece of ceramic nor is it easy to knock off a 1" diameter brass disc without a punch and other equipment an ocassional hobbyist doesn't usually have. This is why vry few people roll their own piezo speakers and things. Ron >>. It's a crap-shoot. Buy some and try it. Then tell us the >>results.... [quoted text clipped - 6 lines] > other equipment an ocassional hobbyist doesn't usually have. This is > why vry few people roll their own piezo speakers and things. Many decades ago, My masters thesis project was to rebuild a St. Claire vibrator, a 6" long 4" diameter give or take, solid aluminum bar that resonated at 15 kHz, driven by a kilowatt electroniic amplifier. Well, I managed to do that OK. But I was also committed to make a higher frequency unit, half as long, to run at 30 kHz. Well, the suspension, a "girdle" at mid-point. was difficult to do at 15 kHz; I had to freeze the bar in dry ice and and roast the girdle before quick-as-a-bunny assembly (the crap shoot). Needless to day I went through serval girdles, many tries, managing some data at the anomolous (then) modes at 35 kHz and 60 kHz (75ish also I think), but that suspesion usually blew before I could get much high frequency data. The "rub" was that the girdle had to precisely sit on a raised equatorial rim, ostensibly on a definite line that was very thin...dings and misalignment, perhaps uneven machining by others as I had to order new girdles on several occasions, all could cause friction which destroyed the Q. Net results are shown in my 1980 JASA article on same. The message is that acoustic transducers are unique devices that are not to be trivialized. One could say that fabrication of musical instruments and these various transducers have a lot in common; it's an art. Angelo Campanella > >>. It's a crap-shoot. Buy some and try it. Then tell us the > >>results.... [quoted text clipped - 11 lines] > instruments and these various transducers have a lot in common; it's an > art. I'm working on a Vulcan harp and finding out the hard way howmuch instrument making is more of an art than a science. But isn't always so much easier to manufacture something big than something small and thin? ;-) Ron On 8/27/07 1:55 AM, in article 1188204955.969257.255910@r34g2000hsd.googlegroups.com, "Ron" <ryon@quik.com> wrote: > Can somebody here tell me why two piezo discs aren't used to get more > sound out of the unimorph design? Sometimes they are. Bill SignatureSupport the troops. Impeach Bush. Oh, I forgot about Cheney. |
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