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Are Faraday waves acoustic waves?

  1. Aug 6, 2013 #1
    Hi,
    I am confusing if capillary-gravity waves (Faraday waves) belong to a type of acoustic waves stricto sensu? What's the difference between Faraday standing waves and acoustic standing waves (interference) at the the air-liquid interface.

    Thanks

    harmonyU
     
  2. jcsd
  3. Aug 7, 2013 #2

    Bobbywhy

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    Gold Member

    In order to distinguish between Faraday waves and acoustic standing waves read these two descriptions:

    "Faraday waves, also known as Faraday ripples, named after Michael Faraday, are nonlinear standing waves that appear on liquids enclosed by a vibrating receptacle. When the vibration frequency exceeds a critical value, the flat hydrostatic surface becomes unstable. This is known as the Faraday instability. Faraday first described them in an appendix to an article in the Philosophical Transactions of the Royal Society of London in 1831.[1][2]
    If a layer of liquid is placed on top of a vertically oscillating piston, a pattern of standing waves appears which oscillates at half the driving frequency, given certain criteria of instability. This relates to the problem of parametric resonance. The waves can take the form of stripes, close-packed hexagons, or even squares or quasiperiodic patterns. Faraday waves are commonly observed as fine stripes on the surface of wine in a wineglass that is ringing like a bell. Faraday waves also explain the 'fountain' phenomenon on a singing bowl."
    http://en.wikipedia.org/wiki/Faraday_wave


    Sound waves
    "Standing waves are also observed in physical media such as strings and columns of air. Any waves traveling along the medium will reflect back when they reach the end. This effect is most noticeable in musical instruments where, at various multiples of a vibrating string or air column's natural frequency, a standing wave is created, allowing harmonics to be identified. Nodes occur at fixed ends and anti-nodes at open ends. If fixed at only one end, only odd-numbered harmonics are available. At the open end of a pipe the anti-node will not be exactly at the end as it is altered by its contact with the air and so end correction is used to place it exactly. The density of a string will affect the frequency at which harmonics will be produced; the greater the density the lower the frequency needs to be to produce a standing wave of the same harmonic."
    http://en.wikipedia.org/wiki/Standing_wave

    For further study of both phenomena, including Faraday waves on a liquid with a surfacant layer (2.) see these references:

    1. http://www.physicsclassroom.com/class/sound/u11l4c.cfm
    2. http://www.scielo.org.ar/scielo.php?pid=S0327-07932005000100009&script=sci_arttext
    3. http://doc.utwente.nl/80679/
     
  4. Aug 7, 2013 #3

    sophiecentaur

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    The wavelength of a standing wave (any wave) will depend upon the speed and the frequency. The speed of surface waves is much lower than the speed of longitudinal waves (in liquids and solids) or transverse waves (solids only). The speed of a wave relates to the density of the medium and the modulus or restoring force for a given displacement. For a surface wave, the restoring force only depends upon gravity and its density - nothing like as strong a force as in a solid like rubber or steel, when it is distorted. For this reason, a low frequency buzzing can cause standing waves with a wavelength of, perhaps a couple of centimetres. Also, the amplitude of the waves can be much greater than for vibrations in a solid, so these standing waves are much 'bigger' and impressive than what you get on a solid. You can get transverse waves on a metal plate but they need something like sand in order to reveal the nodes and antinodes and the excitation frequencies tend to be way up in the audo frequency bands. See this link on Chladni plates
     
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