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How to re-size a sound's wave length

  1. Aug 10, 2014 #1
    In the field of acoustical physics the mathematical relationship (wavelength (lambda) = speed of sound (velocity) / frequency (cycles per sec)) indicates that lambda (wavelength) would increase if the speed of sound were increased while the frequency (cps) is held constant (e.g., 100 Hz).

    Setting aside (for the moment) the empirical issues surrounding how one increases the speed of sound while holding frequency constant, my question relates to understanding the pressure mechanics that would have to underlie such an increase in wavelength.

    I’m looking for an intuitive physical explanation that starts with the idea that wave length describes the distance required for the pattern of pressure increases (condensation) and decreases (rarefaction) to repeat its self.

    Doesn’t an increase in wave length mean that somewhere in this process the leading edge of the pressure wave must have selectively and spatially jumped forward relative to the trailing wave?

    Really ! – How could that happen?
     
  2. jcsd
  3. Aug 10, 2014 #2

    Nugatory

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    Staff: Mentor

    The speed of sound is different in different mediums, so the way to increase the speed of sound and hence the wavelength is to get your sound wave to stop moving through air and start moving through something else instead.

    In this case, the leading edge of the pressure wave really does jump ahead relative to the trailing edge. The leading edge is traveling through the air, reaches the surface of the new medium, and starts propagating through the new medium at the new higher speed. Meanwhile, the trailing edge is still traveling through air, so is propagating at the slower speed and falls behind. When the trailing edge reaches the new medium, it also propagates at the faster speed so stops falling behind, but by then the wavelength has already been stretched out.
     
  4. Aug 10, 2014 #3
    Wavelength changes happen at the boundaries between materials. Frequency never changes. For example, maybe a rock hammer is tapping out a regular beat underwater. It does so with a frequency of say 10 Hz. In the water, the wavelength would then be sound speed / frequency or 150 m (1497/10). Now the energy travels as a disturbance through the water up to the surface. Just because the wave has reached the surface doesnt mean the rock hammer isnt hitting the rock 10 times a second. The frequency is the same. However, since sound speed in air is different, the wavelength changes 331/10 (33 m). These are probably unrealistic numbers but you get the idea. Nothing about the material changes what is happening with the source of the wave (frequency), that would be weird/crazy if it did.
     
  5. Aug 11, 2014 #4
    Thanks Guys - All your responses agree with my privately held rational - Thanks again
     
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