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Generation of Ultrasound in the Megahertz region

Discussion in 'Electronic Basics' started by Joe, Mar 30, 2007.

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  1. Joe

    Joe Guest

    Hello to the group,

    Does anyone know how I could generate sound waves in the Megahertz
    region? I would like to do a demonstration for my class on the Bragg
    Effect (a non linear optical effect where a high frequency sound wave
    will couple with a laser beam and cause the optical beam to diffract
    and even possibly deflect, in the right medium).

    It is part of our study of non linear optical effects in different
    media. The only acoustic transducers I have been able to find are the
    standard 24khz and 40Khz transducers used for rangefinding, alarms,
    etc. Those frequencies are not high enough to cause the coupling
    effect I was hoping to demonstrate.

    I visited this site:

    I have also googled for :Bragg Effect, photoelastic effect, Bragg
    scattering, acousto optic effect an have come up empty as far as a
    practical way to demonstrate the effect.

    I am wondering about using an off the shelf crystal at, say, a few
    megahertz, removing the metal casing around it, and trying that. I
    know they are cheap enough, but not sure if it would work. I know that
    the higher the frequency, the thinner the 'slice' of quartz inside. I
    thought I would run it by the group first and see if anyone knows
    about this or maybe has done it before. Any info, links, or references
    are welcome.

  2. amdx

    amdx Guest

    Hi Joe,
    I'm not sure how far you can propogate sound (in air) in the megahertz
    The only graph I can find is here,
    But, Figure 5 only goes to 240khz. The formula above it says you would have
    21db of loss per foot in air.
  3. Medical ultrasound uses frequencies in the 1 to 20 MHz area, so there
    are transducers available, but they might be pricey. At those
    frequencies, it would have to be water or some liquid medium with a
    high enough speed of sound, not air.

    If you try to use an ordinary quartz crystal in water, I think you'd
    have to insulate it electrically in something that would pass the
    sound waves, since the crystal is high impedance/voltage. It may also
    be that applying a high intensity drive would shatter the crystal.

    There are ultrasonic cell disintegrators used in biology labs that put
    out very high intensity sound into water (hundreds of watts), but they
    work in the 20-25 kHz range. If the Bragg effect depends on high
    intensity, they might be more effective than rangefinders, and maybe
    you could borrow one from the biology department.
  4. Radium

    Radium Guest


    I have a relevant question

    What is the highest-frequency of ultrasound that can propagate through
    air on earth's atmosphere?


  5. I think your timezone is wrong. Or else your clock is. :) BTW, Radium is
    a troll. He will keep asking short little questions in order to keep people
    making long winded responses. And then say stupid things like "I dind't
    understand that, could you rephrase it" etc..... Play along if you like,
    but I thought I should let you know. :)
  6. Jamie

    Jamie Guest

    well, sound travels at aprox 770 mph, that would be the choking point.
    anything below that was be greatly suppressed until you start moving
    more more down away from the 770 factor.
    Also remember, temperature and movement plays a roll on it, look
    around for the data. I'm sure you can find it somewhere.
  7. This is a bit OT, but hey it's Friday

    Many moons ago when I was a programmer, I worked in St Louis for a stock
    brokerage company. They had these bad-ass laser printers that printed on
    continuous forms paper at a rate of 30"/second. No matter what size fonts
    or lines/inch, the paper always moved at the same speed. Amazing machine
    (Siemens 0777 Laserdrucken (sp?)), especially the stacker. At any rate,
    there was some documentation laying around and it included a theory of
    operation. Well how could anyone resist looking at that. ;-)

    Now to the point. The laser went thru a "lens" (might have been quartz, I
    don't recall). The lens was somehow vibrated/modulated by several audio
    range frequencys. This had the effect of splitting the beam into multiples
    that, IIRC, came out parallel. By turning a tone on or off, the beam would
    reciprocate at that position. This allowed the laser to scan the drum 6
    times as fast since it was drawing with multiple beams. Pretty smart IMO.
    It's been 20 years so please forgive my rusty memory on the details. :)

    The fuser/stacker was a seperate module so the paper travelled several feet
    in the open with the print on the paper only being held in place by static
    electricity. People couldn't resist not touching it when the door was open,
    but don't touch the edge. Not many places can you get a paper cut that
    results in stitches. I wish I had video of it in operation. The stacker
    was just incredible. Paddle wheels, flappers and air puffing jets to coax
    the continuous form paper into fan-folding back up. The key to getting the
    stacker to work was to feed the first two pages thru the fuser pre-folded.
    That way you could tell how it had to fold back up. After the fuser
    "ironed" the perforations, you couldn't tell which way it originally went.
    But the paper knew. ;-) It had just enough memory that if you didn't start
    it correctly it would mess up every 100 pages or so.
  8. John  Larkin

    John Larkin Guest

    I've always wondered how much a regular quartz crystal would scatter a
    laser beam when driven hard. Some crystals have a quartz disc with
    plated electrodes in the center and a clear rim, so you could un-can
    one, shoot the laser through the clear part, and build a nasty
    oscillator that drives it insanely hard. These crystals mostly run
    shear mode.

  9. Joe

    Joe Guest

    Thank you all for the responses. This demonstration would not be
    carried out in air. For precisely the reasons you cited. Sound does
    travel about 5 times faster in water, about 1500 meters/sec. In fact,
    I was thinking of using water, just because it is so abundant, and
    behaves non linearly with light and sound together.

    Yes, I know about Massa Corp. they are about 10 miles from where I
    live. They build sonar and sonar imaging arrays, mostly for the Navy.

    About quartz crystal. It is also a non linear medium. When a laser
    hits it, depending on what angle it is to the crystal axis, it
    exhibits birefringence (ie, 2 different indices of refraction), so you
    will see 2 beams emerge. It's really cool to watch. 1 beam in , 2 come
    out. They are polarized oppositely also, if they enter the crystal
    unpolarized. Otherwise, they can become circularly polarized, or
    elliptically polarized. When you modulate it at audio frequencies, all
    kinds of cool stuff can happen. I never saw one get split into 6 beams
    tho, but it wouldn't surprise me.

    Fish finders usually use 2 frequencies, around 50Khz and 160Khz, but
    that is not high enough. They would be waterproof at least.

    The equation for the Bragg scattering has only the angle of
    diffraction, the speed of sound, and the frequency in it. I don't
    think amplitude makes much difference.

    Maybe I will order some quartz crystals and take them apart. What's to
    lose? They are cheap enough.

    I think I'll mosey on over to the alt.sci.lasers forum. Maybe Sam or
    someone over there has an idea.

    Thanks again for the responses,

  10. Radium

    Radium Guest

    1 m/s = 2.2369 mph

    344.226384728865841119406321248156 m/s = 770 mph

    Wave velocity = frequency x wavelength

    344.226384728865841119406321248156 m/s = 1 Hz X
    344.226384728865841119406321248156 m

    I still don't get it? What is the max frequency possible? Seems like
    their is no limit, yet there is. This is extremely confusing.

    Just what is the highest-ultrasonic frequency in which acoustic wave
    can travel through air on earths atmosphere if the speed of sound in
    770 mph????

  11. See if you can find one of the old FT-243 military type crystals.
    They were built for rugged service in tube equipment, and at higher
    drive than today's crystals.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  12. Bob Masta

    Bob Masta Guest

    Hmm, I wonder if the medical profession ever uses disposable
    ultrasound transducers? This may sound outrageous, but they use
    other disposable transducers, such as pressure transducers for blood,
    which are much cheaper in the disposable form than the research-grade
    form. It is cheaper to thow away a $50 disposable than to sterilize a
    $1000 unit.

    You might think you could snag a disposable unit on its way out to
    the medical waste bin, but there would be liability issues if they
    let you do that. (What if it had AIDS-contaminated blood on it?)

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Scope, Spectrum, Spectrogram, Signal Generator
    Science with your sound card!
  13. redbelly

    redbelly Guest

    It sounds like you are looking for an acoustooptic modulator, or AOM.
    They are sold commercially; try a google search for more info.
  14. MassiveProng

    MassiveProng Guest

    No such thing, dingledorf.
  15. Oh gawd, not again. Why don't you just check things out before going off
    like that? There is most certainly ultrasound in the MHz range. Just go to
    any OB/GYN clinic and ask to see the ultrasound machine. Have someone read
    to you what is clearly written on the wand/probe/transducer. 3MHz

    Guess what, there are radio waves in the kHz range. Spooky huh?
  16. Ron Capik

    Ron Capik Guest


    You might want to look up some of the surface acoustic waves
    work at Bell Labs done in the 70's. Some of the authors to check
    would be Gary Boyd, Larry Coldren and P.K.Tien.


    Ron Capik
  17. Ron Capik

    Ron Capik Guest

    Ever hear of an optical phonon?


    Ron Capik
  18. redbelly

    redbelly Guest

    Devices are commercially available. Googling acousto-optic, I
    eventually found:

    RF drivers:

    AOM modules for deflecting laser beams:

    Also of possible interest:

  19. MassiveProng

    MassiveProng Guest

    Crunchy WinterGreen LifeSavers.
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