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Does it work both ways ?

  1. Dec 28, 2011 #1

    A quick question for you guys, one that I can't figure out, but no doubt someone in here will be able to explain to me.

    I'm curious as to how a speaker works {yea, I know the basics}, but I can't figure out if lets say a bass speaker was placed in the middle a length of pipe, that green plastic drainage pipe I have in mind, and then the speaker was driven.

    Would the amount of sound / pressure coming out of each end of the pipe be the same ?

    One of the things I'm trying to get my head around is, is most or all of of the sound made on the compressive forward stroke ?

    Thanks for your time and consideration, it's appreciated.
  2. jcsd
  3. Dec 28, 2011 #2


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    So let me get this straight. You're envisioning a speaker that is not enclosed like a normal speaker is and is free to push air on both ends? If so, yes, the output would be the same going out of both ends. I'm not very keen on speakers so I'm not sure how doable such a speaker design would be.
  4. Dec 28, 2011 #3


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    Canada Bob, Welcome to Physics Forums!

    No, most or all of the sound is NOT made on the compressive forward stroke. The measured sound intensity is a result of equal parts of the compression and rarefaction strokes. These pressure waves are caused by the vibration of the speaker diaphragm.

    Sound is a variation in pressure. A region of increased pressure on a sound wave is called a compression (or condensation). A region of decreased pressure on a sound wave is called a rarefaction (or dilation). The best graphic showing this is here: http://physics.info/sound/

    For a great overview of speakers of all kinds, including enclosures see this wiki article: http://en.wikipedia.org/wiki/Loudspeaker
    Last edited: Dec 28, 2011
  5. Dec 29, 2011 #4
    Hello Bobbywhy & Pengwuino.

    Yea, that was the idea, to have just the speaker itself {no enclosure} in the middle of a pipe.

    I had it in mind that maybe more force went out on the compression phase, but, what do I know, well I know a what I needed to know now, thanks to you guys...
    Having said that, is there a "speaker" or some such related thing that produces a "pulse" in one direction ?

    Leads me to another question then, I have a little project in mind, but, it's not really sound that I need to produce, it's pressure {yea, I know}, inside a pipe where most if not all the energy goes in one direction.

    I was hoping to do this using a bass speaker, and maybe that's still workable as I could close one end of the pipe and let the rebounding energy bounce back from the closed end ? would there be anything that I'd need to avoid there ? or anyway of tuning the pipe by having the speaker at some optimum position ?

    The object of my little project is to produce {if possible} a couple of lbs/sq inch from the end of the tube, and from what I have read on the www it takes an incredible amount of Db's to produce {can I call it} a workable pressure for my device.

    Maybe I should forget the plastic pipe and find myself a brass canon ;) the problem there would be I'm looking to produce rapid pulses of just a few lbs/sq inch.

    Thanks for the links Bobbywhy, valued and appreciated, I'm at least one step further forward thanks to the time you guys have afforded me.

    Canada Bob.
  6. Dec 29, 2011 #5


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    Canada Bob, Note: High power sonar transmitters use piezo-electric barium titanate ceramic crystal transducers. A short high voltage electrical signal, say 10 milliseconds in duration (the pulsewidth) and of some frequency, say 10 Kilohertz, is applied to the ceramic. The ceramic then vibrates at its resonant frequency and that motion is transferred to the medium of water as sound energy. Now, if you looked with an oscilloscope at that pulse you would see it contains many “cycles” of the 10 Kilohertz. The acoustic waves in the water are, as you already know, compressions and rarefactions.

    So it is not really sound that you want to generate…it is a pressure pulse! I imagine you could use a “woofer” speaker and apply to it an electrical pulse of one polarity only. That is, for example, from off to some high positive voltage instantaneously. One would have to experiment to discover the correct polarity (direction). That would drive the “voice coil” and diaphragm in one direction only and create the single pressure pulse. But speakers are designed to vibrate, that is, move both back and forth, so once you drive it forward and turn off the drive signal it would try to return (backward) to its rest position. That rearward motion tends to create a rarefaction.

    If you had a piston in a cylinder and hit the backside of the piston with a hammer(know as an "impulse"), it would create a pressure pulse which would travel at the speed of sound (around 344 meters/second in air) out of the open end of the tube. Perhaps you could use a solenoid to act in place of the hammer and a diaphragm in place of the piston. Send the electrical “energize” command to the solenoid, its plunger slams into the membrane, and out goes your pressure pulse!

    Measurement of the amplitude of the output pulse would be difficult. I imagine if you had some smoke in the tube and illuminated it you might be able to photograph the pulse as it compresses the smoke particles and then calculate the pressure, but this last is just speculation.
  7. Dec 29, 2011 #6
    Thanks for the above Bobby, appreciated...

    I remember a few years ago seeing some sort of "canon" that sent out a pulse of air, or maybe it was sound, it didn't seem t have any useful application just a fun experiment.

    Anyway, I have a project in mind that maybe you can help me with, I'll send you a PM {hope you don't mind}.

    Thanks again.

    Canada Bob.
  8. Dec 29, 2011 #7


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    A "carbide cannon" was a great invention. We teenagers put in some carbide crystals, added water, and then ignited it (acetlyene?) with a flint spark. The output was a HUGE acoustic pulse (explosion)!
  9. Dec 29, 2011 #8


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    Vortex cannon?


    A larger version was demonstrated on the tonight show, but I can't find the video. Youtube has other votex cannon videos.
  10. Dec 29, 2011 #9
    I think that was the one I had in mind, thanks for the link...

    Canada Bob.
  11. Dec 29, 2011 #10
    That might do the job for me Bobby :smile:

    What I'm looking to produce is a pressure/pulse, something not dissimilar to the recoil of a canon, without all the weight involved or any explosives :eek:

    Canada Bob.
  12. Dec 29, 2011 #11


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    Can you just use a spring-loaded piston? Sort of a big version of the mechanism in a spring-loaded pellet rifle...
  13. Dec 29, 2011 #12
    Part of the problem Berkeman is that I need a device that will pulsate {with some force} at least 50 times per minute and possibly much faster than that, up to the speed that a bass speaker would vibrate at.

    The goal would be to have the pulse/pressure go in one direction, as described by Bobbywhy in the earlier post where he describes a single polarity pulse, not sure if that would create the forceful "recoil" {sort of} that I'm looking for...

    Canada Bob.
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