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A question about the speed of EM and mechanical waves

  1. Dec 15, 2011 #1
    I have known for many years that the speed of sound (usually quoted ≈340 m/s) and the speed of light (usually quoted ≈3*10^8 m/s) are vastly different. Doing some reading, I would seem to conclude that part of the reason for this is the fact that sound is a mechanical wave, propagated through whatever medium it exists in, whereas light is an electromagnetic wave. One key fact seems to be that EM waves can propagate in a vacuum, whereas mechanical waves cannot. To quote a rather corny phrase, "In space, no one can hear you scream."

    What I am wondering, and I would appreciate someone pointing me in the right direction for information, is the following:

    Waves, be they mechanical or EM, are measured by much the same characteristics, by what I see (wavelength, frequency, amplitude, etc.). That is not to say that they are the same thing, of course, but they would at least appear to be highly similar. I note that when I see a diagram of the electromagnetic spectrum, they usually deem to stop with low frequency/long wavelength radio waves. With mechanical waves, the typically quoted range of human hearing is 20 Hz to 20 KHz. Ultrasonic transducers typically operate in the range of 40-50 KHz (and at these frequencies, they still quote the speed of sound when calculating distance, which is what they are typically used for). This brings the two ranges within oh, say, 50-100 KHz of each other.

    So what happens in this intermediate range? Do we hit a point where some energy state suddenly changes, and now we have left the speed of sound behind and now we are suddenly traveling at the speed of light? Is there some sort of gradual shift where I see the speeds shifting from one to the other?

    Or is it a question of their propagation? Sound is usually created by, say, a vibrating membrane, whereas EM is typically produced via vibrating electrons? If that is the case, would it theoretically be possible to create mechanical waves in, say, the 500 KHz range? If so, would they be of any use, or would they not propagate well in air due to their high frequency?
  2. jcsd
  3. Dec 15, 2011 #2


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    Gold Member

    the type of wave has nothing to do with the frequency (you could have a mechanical wave an an electromagnetic wave of the same frequency). The type of wave depends on what's oscillating. If neutral matter is oscillating, you have just a mechanical wave. If a charged particle is oscillating, you get electromagnetic waves. Plasmas are an example that have both kinds of waves interfering with each other... since a particle can have both charge and mass, it can be susceptible to both kinds of waves.

    But notice that a single electron oscillating back and forth will generate an EM wave, while a mechanical wave is energy propagating through an ensemble of particles and manifesting motion. They're quite different kinds of things.
  4. Dec 15, 2011 #3


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    lxman, your post reads as if light and sound are the same phenomena just with different frequencies. They aren't. They are completely different. So there is no transitional region where you go from one to the other. And the radio frequency range extends down to single digit Hz, so they do overlap: http://en.wikipedia.org/wiki/Extremely_low_frequency

    Higher frequency mechanical vibration becomes difficult because unless the amplitude is very low, you risk ripping apart the medium you are trying to transmit through.
    Last edited: Dec 15, 2011
  5. Dec 15, 2011 #4
    The mechanical waves are not limited to the kHz range. Not even from a practical point of view.
    Ultrasonic transducers for industrial and medical applications operate in MHz range. 20-50 MHz are quite usual.
    Mechanical waves in solids may have, frequencies comparable with these of the visible light.
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