Category: Science > Earth Sciences
Asked by: racecar-ga
List Price: $5.00
29 Oct 2002 16:50 PST
Expires: 28 Nov 2002 16:50 PST
Question ID: 92601
Ocean waves breaking on the beach commonly arrive in sets. A wavetrain with say 5-10 more or less evenly spaced waves will be followed by a quiet spell, then another set. Why? Dispersion may seem to offer an explanation (it has been offered to me before in response to this question): longer waves travel faster, so waves get sorted by wavelength as they traverse the ocean from their generation point, and the long swell from a storm may reach shore hours, or even days, before the shorter, slower waves. However, this does not, to my mind, constitute an explanation of the phenomenon, at least not a complete one. Unless the wavelengths stimulated by a given storm are somehow discretized, it seems that waves from the storm should arrive continuously, with gradually increasing wavelength.
Re: Wave sets
Answered By: drdavid-ga on 29 Oct 2002 20:09 PST
The phenomenon you describe is characteristic of so-called "beat" frequencies, which occur when you add two waves of slightly different frequencies. You get an amplitude modulation at the difference frequency. Piano tuners use this phenomenon to help them tune strings (they listen for the beat frequency, which sounds like a pulsing of the sound). The same phenomenon also occurs in ocean waves (and therefore beach waves). If slightly different frequency waves are simultaneously present (very common) then beat frequencies will be generated and waves will appear to come in "sets." You can see this illustrated and discussed further at "Wave Motion" http://www.media.uwe.ac.uk/masoud/projects/water/wave.htm You might also be interested in the discussion of ocean waves and the data on ocean waves at the National Data Buoy center: What causes ocean surface waves? http://www.ndbc.noaa.gov/educate/pacwave.shtml If I've misunderstood the phenomena you are seeking to explain, feel free to request a clarification. Google Search: ocean wave frequency ://www.google.com/search?q=ocean+wave+frequency beach wave frequency ://www.google.com/search?q=beach+wave+frequency beach wave beat frequency ://www.google.com/search?q=beach+wave+beat+frequency
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