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How do I build a underwater Infrasonic (10 -30 Hz) projector?

Discussion in 'Audio' started by William P Hilmes, Jul 4, 2018.

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  1. William P Hilmes

    William P Hilmes

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    May 10, 2018
    I am a beginner in my electronic ability. I want to study the effect of sound waves in the 10 - 30 Hz range on submersed aquatic plants. To do this I will need to build a underwater infrasonic projector. I have started my lab with a two channel signal generator and a two channel Oscilloscope. The problem is that all the suppliers who sell Piezoelectric Ceramics components are designed for much higher frequencies (>1000 Hz). That is my starting material to build an underwater transducer. I will need to look at many frequencies in this range so I will need my transducer to be tunable. Any suggestion or ideas would be appreciated. Thank you!
     
  2. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012

    Forget about using piezoelectric ceramic transducers for 10 to 30 Hz infrasonic sound waves in water. You need a much more powerful transducer capable of moving a significant mass of water under controlled excitation. A magnetostrictive transducer will do the job nicely.

    To build one you need to acquire a magnetostrictive rod and wind a solenoid coil around it. When the coil is energized, the rod will change length (this is the magnetostrictive effect). So the signal you need to energize the coil with is in the 10 to 30 Hz range. My best guess (pure rectal extraction) is you need something on the order of a kilowatt of power to get any appreciable "audio" power into the water. You will also need an impedance-matching horn attached to the magnetostrictive rod to efficiently couple its energy into the water. How to build that is an exercise left for the student.

    The frequencies are low enough that you might consider "beefing up" an ordinary loudspeaker to use as a transducer. I would try coating the speaker cone with a water-proof substance, and coating the spider supporting the voice coil and speaker cone with a flexible water-proof adhesive, perhaps clear RTV. You need to also water-proof the outer cone support where the cone attaches to the speaker basket. Finally, the normally air-insulated copper braid used to connect the voice coil to the external speaker terminals needs to be coated with a water-proof substance... perhaps something akin to rubber cement, but tougher. Experiment to find out what works for you.

    Good luck with your project, but I must confess that it sounds like woo-woo science to me.
     
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  3. Hopup

    Hopup

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    Jul 5, 2015
    You could just install a vibrating element to the "aquarium" wall, the wall made of flexible material. This would probably generate enough vibrations to reach the plants.

    I would not expect anything major to happen using this frequency range for plants especially if you are just trying randomly frequencies between this range.
     
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  4. kellys_eye

    kellys_eye

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    1,146
    Jun 25, 2010
    10-30Hz is only 600-1800 rpm of a motor - fitted with an eccentric cam and operating against a flexible metal disk may get you somewhere.....
     
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  5. FuZZ1L0G1C

    FuZZ1L0G1C

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    Mar 25, 2014
    A motor-driven piston, top end open with plastic or metal rod attached to piston face.
    Free end of rod fixed to large rubber diaphragm, sealed onto cutout hole of tank.
    I once built the above from an old car-tyre-inflation compressor pump, as plastic piston-rings were worn out.
    The vessel used was a photographic developing tank, 40cm x 40cm x 50cm deep, with a rubber bath-mat (thick flexible rubber) silcon caulk-sealed to the tank then clamped tight with 30cm aluminium flanges and SS machine screws.
    Using a PWM / SCR circuit at 12V DC, I was able to go from barely moving (0.5Hz up to maximum cycles, probably 30-40 Hz or so (back then, I had no means of accurately measuring the frequency.
    Worked several years as a cleaning-tank 'agitator' in workshop.
    Using a mike and 'scope, or sound-card and Audacity, you could calibrate one or two rotary dial(s) of potentiometers showing coarse/fine frequency settings.
    PS- advise you tap a thread in top of piston for a threaded rod, 4-5 mm or so, as violent vibration kicks epoxied joints loose.
     
  6. William P Hilmes

    William P Hilmes

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    May 10, 2018
    Thank you for the advice. Your help is appreciated.
     
  7. Hopup

    Hopup

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    Jul 5, 2015
    Also instead of the eccentric cam you could possibly use subwoofer/low frequency speaker element which is inverted in the sealed box. One side of the box would be the water tank having the diaphragm fitted in it to act as passive radiator.

    This would be more accurate and easier to control than the cam but has obviously much less output power. In my experience you need only very little power to reach the plants or other organic matter so this way might work too.
     
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  8. William P Hilmes

    William P Hilmes

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    0
    May 10, 2018
    Thank you for the help. I will stay in touch as I progress.
     
  9. William P Hilmes

    William P Hilmes

    17
    0
    May 10, 2018
    Hevans1944
    I need your advice. For my project I was unable to buy a magnetostrictive rod. All the sources were from China and I had to buy at least 50 Kg of material. Any suggestion of where I can buy one rod (18" L x 3/4"dia)?
    Also I tried to buy a Magnetostrictive transducer without success. Every time I told them I wanted to use it in the 10- 30 Hz range that said they had no products for sale in the range. Any suggestion of who might sell such a product?

    My field is Chemistry, not electronics. Any help you can provide would be appreciated. Is there any old electrical device where I might be able to harvest a rod? Thank you for starting me on the right path.


    [mod note: moved post from its separate thread to this thread]
     
    Last edited by a moderator: Jul 10, 2018
  10. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    There is nothing magical about a magnetostrictive rod. A nickel or a nickel alloy rod, or even a soft-iron rod should work. There are also rare-earth alloys, Terfenol-D for example, that work better, but the Chinese have pretty much cornered the market on rare-earth elements. Prices are increasing.

    The biggest problem is coupling the linear expansion and contraction of the rod to your water environment. A practical diameter rod might be only one inch diameter, and with a reasonable magnetizing current may only increase or decrease in length by a few thousandths of an inch. You will need to couple this to a horn that "magnifies" and impedance-matches the actuator rod to the water tank load. You need the services of an acoustical engineer (not me) to figure out how to do that.

    I only suggested magnetostrictive transducers because you were going down the wrong path to generating infrasonic waves in water with piezoelectric transducers, which typically resonate at high kilohertz (ultrasonic) frequencies and have very limited ranges of motion. Others have posted reasonable suggestions for alternative means of producing infrasonic waves based on conventional loudspeaker technology, i.e., a voice coil moving in an axial magnetic field. I would try some of these suggestions first before engineering a new magnetostrictive transducer.

    The 10 Hz to 30 Hz range is just below the low-frequency limits of human hearing, but this range of frequencies resonates well with the adult chest cavity and can easily be felt while other, higher, frequencies are simultaneously heard from large "woofer" loudspeakers. A lot of air is being moved by these speakers, but air is compressible whereas water basically is not. So you need to move an actual volume of dense water back and forth with your transducer, which is going to be a power-hungry task if you want a large variation is pressure.

    There is a commercial audio transducer development that uses Terfenol-D mangnetostrictive rods to drive large surfaces at low frequencies. Unlike voice-coil based loudspeakers, their transducer moves a much larger area over a much smaller distance to make sound. It might be worth your while to contact them and explain what you want to do. They are marketing their "driver" as an attention-getter, turning (for example) display windows into loudspeakers to attract passersby. I see no reason why their devices couldn't be re-purposed to your needs. The fact that this new company chose to use magnetostrictive transducers probably is related to the mass of the things they are driving. They emphasize that the driver exciting the "sound producing" surface makes very small motions, but there is nothing mentioned about the physics involved. You should probably look into that.
     
    Last edited: Jul 15, 2018
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