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What is the highestfrequency sound that can exist in Earth's atmosphere? 
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#1
Nov1906, 05:00 AM

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Hi:
What is the highestfrequency sound that can exist in Earth's atmosphere? AFAIK, there is no limit, but I would like some clarification. Is it possible for a puresinewave tone of 140 dB, 10^10,000 Hz [i.e. 10tothepower 10,000 Hz; or 10 followed by 10,000 zeros] to exist on Earth's atmosphere? What determines the upper limit of highfrequency in the air? Thanks, Radium 


#2
Nov1906, 05:00 AM

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There is a natural cutoff set by the average distance between two air
molecules, I guess. Everything with wavelengths shorter than that will not propagate any more. Radium schrieb: > Hi: > > What is the highestfrequency sound that can exist in Earth's > atmosphere? > > AFAIK, there is no limit, but I would like some clarification. > > Is it possible for a puresinewave tone of 140 dB, 10^10,000 Hz [i.e. > 10tothepower 10,000 > Hz; or 10 followed by 10,000 zeros] to exist on Earth's atmosphere? > > What determines the upper limit of highfrequency in the air? > > Thanks, > > Radium 


#3
Nov1906, 05:00 AM

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Radium wrote:
> Hi: > > What is the highestfrequency sound that can exist in Earth's > atmosphere? > > AFAIK, there is no limit, but I would like some clarification. Of course there is a limit. > What determines the upper limit of highfrequency in the air? Sound is associated with (relativly low amplitude) pressure waves in air. However, such vibrations are only possible under conditions where air can be approximated as a continuous fluid. This approximation breaks down when the wavelength of the wave becomes comparable with the mean free path of the molecules that make up the atmosphere. The mean free path is roughly the average distance between collisions of atmospheric molecules. It is collisions between molecules that keep air in local equilibrium, which allows the continuum approximation. Below the mean free path scale, hydrodynamic equations (which also describe propagation of sound waves) are no longer sufficient and we must resort to a molecular description. Consequently, coherent pressure waves are impossible if the wave is supposed to make several oscillations before the front of the wave has even collied with a sufficient number of molecules to make the surrounding air move. The mean free path in air at sea level is about 0.1 micron[1]. Let's take the speed of sound to be roughly 300 m/s. Then the upper limit on sustainable sound frequencies is f_max = (300 m/s) / (10^7 m) = 3*10^9 Hz = 3 GHz. Incidentally, there should be a limit on the maximum amplitude of sound in the athmosphere as well. The first obstacle would be nonlinearities of the hydrodynamic equations that become important when the wave amplitude deviates strongly from equilibrium. Nonlinear effects usually mix different harmonics. But coherent waves may still be possible. The larger the amplitude of sound wave the larger the pressure differences that build up between the crests of the wave. At sufficiently high pressures, air can change phase, while at sufficiently low pressures, it can become so rarified that the continuum fluid approximation fails as well. Both of these properties destroy the hydrodynamic approximation. I'll leave someone more knowledgeable in gas dynamics to estimate this upper bound. Hope this helps. Igor 


#4
Nov1906, 05:00 AM

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What is the highestfrequency sound that can exist in Earth's atmosphere?
Radium wrote: > Hi: > > What is the highestfrequency sound that can exist in Earth's > atmosphere? > > AFAIK, there is no limit, but I would like some clarification. > > Is it possible for a puresinewave tone of 140 dB, 10^10,000 Hz [i.e. > 10tothepower 10,000 > Hz; or 10 followed by 10,000 zeros] to exist on Earth's atmosphere? > > What determines the upper limit of highfrequency in the air? > > Thanks, > > Radium No. The highest (effective) frequency that can exist is linked to the mean free path. This at sea level is about 10^7m. Hence the highest frequency (even loosly defined as sound) is 340e7 Hz. Even that is stretching a point. http://www.kayelaby.npl.co.uk/genera...2_4/2_4_1.html gives a general account of attenuation. At 100kHz we have about 1800dB/km in fairly dry air. Attenuation on a simple model goes up as f^2. Range of 1000kHz therefore bein of the order of 10m (1.8dB/m). At 1MHz we have 10cm. At 10MHz 100 microns. In my book 100MHz+ can't really exist in air.  Ian Parker 


#5
Nov2206, 05:00 AM

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Radium wrote:
> Hi: > > What is the highestfrequency sound that can exist in Earth's > atmosphere? > > AFAIK, there is no limit, but I would like some clarification. > > Is it possible for a puresinewave tone of 140 dB, 10^10,000 Hz [i.e. > 10tothepower 10,000 > Hz; or 10 followed by 10,000 zeros] to exist on Earth's atmosphere? > > What determines the upper limit of highfrequency in the air? > Well, if the wavelength is shorter than the mean spacing between air molecules there's an obvious problem, but I don't know if anything gets in the way before that...  Boo 


#6
Nov2206, 05:00 AM

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Igor Khavkine wrote:
> The mean free path in air at sea level is about 0.1 micron[1]. Sorry, I forgot to give the reference. [1]http://amsglossary.allenpress.com/glossary/search?id=meanfreepath1 Igor 


#7
Nov2206, 05:00 AM

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René Meyer wrote:
> There is a natural cutoff set by the average distance between two air > molecules, I guess. Everything with wavelengths shorter than that will > not propagate any more. So the closer the distance between the two air molecules, the higher the frequency that can propagate?? 


#8
Nov2506, 05:00 AM

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Radium wrote:
> Hi: > > What is the highestfrequency sound that can exist in Earth's > atmosphere? > > AFAIK, there is no limit, but I would like some clarification. > > Is it possible for a puresinewave tone of 140 dB, 10^10,000 Hz [i.e. > 10tothepower 10,000 > Hz; or 10 followed by 10,000 zeros] to exist on Earth's atmosphere? > > What determines the upper limit of highfrequency in the air? > > Thanks, > > Radium > In the context of the de Broglie Hypothesis: http://en.wikipedia.org/wiki/De_Broglie_hypothesis "The second de Broglie equation relates the frequency of a particle to the kinetic energy" So an individual air molecule frequency approaches infinity as its velocity approaches the speed of light. Richard 


#9
Nov2706, 05:00 AM

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Boo wrote:
> Radium wrote: > > Hi: > > > > What is the highestfrequency sound that can exist in Earth's > > atmosphere? > > > > AFAIK, there is no limit, but I would like some clarification. > > > > Is it possible for a puresinewave tone of 140 dB, 10^10,000 Hz [i.e. > > 10tothepower 10,000 > > Hz; or 10 followed by 10,000 zeros] to exist on Earth's atmosphere? > > > > What determines the upper limit of highfrequency in the air? > > > > Well, if the wavelength is shorter than the mean spacing between air molecules > there's an obvious problem, but I don't know if anything gets in the way before > that... > >  > Boo Not sure why I feel compelled to chip in ... but a supersonic pulse, like from lightning and such ... could, and very likely would contain higher frequencies than GHz as an FT of the pulse. Though not technically "audible" by the human ear, it could still be characterized by a group of mechanical wavefunctions. And while we are on the subject, you could super heat the air, indefinitly, to the point of first disociating the molecules, then geting up to stiping the electrons from the nuclei and getting, well, plasma. That's where I'd have to leave off as I don't know much about high energy physics. 


#10
Nov2806, 05:00 AM

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>>> What determines the upper limit of highfrequency in the air?
>>> >> Well, if the wavelength is shorter than the mean spacing between air molecules >> there's an obvious problem, but I don't know if anything gets in the way before >> that... > > Not sure why I feel compelled to chip in ... but a supersonic pulse, > like from lightning and such ... could, and very likely would contain > higher frequencies than GHz as an FT of the pulse. But you can't put an arbitrary shaped pulse into a band limited transmission medium, so the pulse shapes you can transport are determined by the high frequency cutoff, not viceversa.  Boo 


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