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Natural Science Forum / Physics / Acoustics / October 2004Sound wave propagation in wind
Hi, Is it true that sound wave propagation is not affected by wind (in terms of a perfect wind at perfect speed). My idea was that it would be carried faster in the wind direction, and slower against the wind - actually returning without notice if it was reflected. Regards > Is it true that sound wave propagation is not affected by > wind (in terms of a perfect wind at perfect speed). > My idea was that it would be carried faster in the wind > direction, and slower against the wind - actually returning > without notice if it was reflected. Never heard of a perfect wind. You don't mean weather? Have a look: http://www2.sfu.ca/sonic-studio/handbook/Sound_Propagation.html http://www.squ1.com/index.php?http://www.squ1.com/sound/propagation.html http://www.met.uu.se/eng/forsk/noise.html http://citebase.eprints.org/cgi-bin/citations?id=oai:arXiv.org:astro-ph/9409048 http://www.madsci.org/posts/archives/feb2001/982775741.Ph.r.html http://www.phys.unt.edu/~matteson/1251-001/mwf17.ppt Cheers Eberhard Sengpiel German forum for microphone recordings and sound studio techniques http://www.sengpielaudio.com Thanks Eberhard, Lot's of good sources, especially the first one. As far as I can tell, my assumption was right. Regards >> Is it true that sound wave propagation is not affected by >> wind (in terms of a perfect wind at perfect speed). [quoted text clipped - 16 lines] > and sound studio techniques > http://www.sengpielaudio.com > Is it true that sound wave propagation is not affected by wind (in terms of > a perfect wind at perfect speed). > My idea was that it would be carried faster in the wind direction, and > slower against the wind - actually returning without notice if it was > reflected. A perfect wind would just reduce the transit time downwind, increase it upwind. But the most common problem is the usual wind gradient with altitude. The wind speed over the top of trees, terrain and buildings is faster than on the surface. This gradient bends the direction of travel of sound waves such that downwind, the sound that normally rises above our heads is actually returned to the surface, and is heard at great distances. Upwind, it simply rises further overhead, out of earshot. Similar "refraction" can happen at night, when the ground is cooled, cooling the air next to it. This time, the sound speed is reduced in cool air, but not at higher levels, say, 50 feet upwards, and the refraction occurs similar to that experienced downwind, to be heard at great distances. Angelo Campanella --------- www.CampanellaAcoustics.com --------- "I have simply studied carefully whatever I've undertaken, and tried to hold a reserve that would carry me through." - Charles A. Lindbergh. "As for background noise level; 35 dBA is a good classroom; 45 dBA is a sound masking system!" - Anthony K. Hoover >A perfect wind would just reduce the transit time downwind, increase it >upwind. [snip] If the source is stationary, a perfect wind does more interesting things than just change transit time. Think of a moving source in still air, then move to a frame of reference fixed to the source. Ken Plotkin >>A perfect wind would just reduce the transit time downwind, increase it >>upwind. [quoted text clipped - 6 lines] > Think of a moving source in still air, then move to a frame of > reference fixed to the source. To do the experiment properly you need to think of a moving source /and a receiver with the same velocity/ in still air. Cheers Greg Locock >To do the experiment properly you need to think of a moving source /and >a receiver with the same velocity/ in still air. What experiment? The receiver isn't as hard to contemplate as the source, since it's not doing anything. Another point to consider is that when the air is moving reciprocity does not necessarily apply. If there is a wind gradient, it definitely does not. Ken Plotkin Thanks Angelo, > A perfect wind would just reduce the transit time downwind, increase it > upwind. Great. My concern was actually more about the wave mechanics than "refraction". I saw a flash movie on the web with an experimental setup of a combined emitter/reciever and reflector placed at a distance from each other. like this: * B (Reflector) | | | * A (Emitter) + C (Reciever) ---> Now the experiment says that if we let the setup stay in a fixed position, the time it takes for a wave traveling from A->B->C is the same as if we let the whole setup move at a constant velocity to the right. They say that if the setup moves, the total distance that the wave have to travel is longer, but the time it takes is the same. But that means the wave is going faster than 341m/s which I don't understand. I thought it wasn't possible. Why are we then having a sonic boom if the wave can just go faster when we move ? My idea was that the wave is not going faster and the distance is the same, but what we pickup is not the same "part" of the wave. In a still setup the wave we pickup would be the uppermost front of an emitted wave (picture a circular wavefront). In a moving setup, we would pickup the upper/right front of the wave (the part of the wave that was actually going in the northeast direction). I don't know if I make myself clear. I can make some drawings of what I think and place them on a web page. Regards > My idea was that the wave is not going faster and the distance is the same, > but what we pickup is not the same "part" of the wave. In a still setup the That's the difference. If the source and receiver are traveling with a high speed, the direction of the wave that will be received is not normal to the path, but at some shallower angle. Although the subsequent path through space (not just the air) is longer, the moving air medium will contribute part of the velocity. As far as he air is concerned, normal propagation occurs. The extreme would be a thin layer of air, where the sound is projected ahead of the source, like two trains running in the same direction on two parallel tracks. The time to cross the narrow gap is but the distance divided by the same speed of sound in air. Ang. C. --------- www.CampanellaAcoustics.com --------- "I have simply studied carefully whatever I've undertaken, and tried to hold a reserve that would carry me through." - Charles A. Lindbergh. "As for background noise level; 35 dBA is a good classroom; 45 dBA is a sound masking system!" - Anthony K. Hoover > That's the difference. If the source and receiver are traveling with a > high speed, the direction of the wave that will be received is not normal [quoted text clipped - 6 lines] > but the distance divided by the same speed of sound in air. > Ang. C. So no matter what - like if we are moving in a airline, the sound propagation still occurs at 341m/s? Regards >That's the difference. If the source and receiver are traveling with a >high speed, the direction of the wave that will be received is not [quoted text clipped - 6 lines] >cross the narrow gap is but the distance divided by the same speed of >sound in air. In your last sentence, do you mean the normal distance across the gap? That's not right. Sound will propagate at an angle, the way you describe in the first part or your paragraph, so veritcal progress across the gap will be slower. When the medium is moving, it gets confusing to talk about the direction of the wave, since the rays are no longer perpendicular to the waves. It's not like you can draw quiescent rays and just convect them, either. Group and phase velocities are different. The setup Kasper describes is very illuminating. If the motion is supersonic, the reflected sound will never make it back to the receiver. Ken Plotkin > The setup Kasper describes is very illuminating. If the motion is > supersonic, the reflected sound will never make it back to the > receiver. Let's raise the ante: If two supersonic aircraft are flying above Mach 1 in parallel, how long will it take for sound to travel from one to the other? It obviously depends on whether they are inside or outside the other's bow wave. Ang. C. > In your last sentence, do you mean the normal distance across the gap? > That's not right. Sound will propagate at an angle, the way you > describe in the first part or your paragraph, so veritcal progress > across the gap will be slower. OK. I think I see the light. Consider two vehicles proceeding to the right at the same speed, less than the speed of sound, in still air. Their speed creates in effect a 'wind' in the reverse direction. Consider three fixed points A, B and C. A is the first vehicle's position at t=0. B is the second vehicle's position at t=0, and when vehicle B emits a sound pulse. Point C is further ahead on the vehicle A path, and where it is expected that the pulse will finally be heard by A. A spherical sound wave expands with point B as it center. When the radius of the sphere is equal to the distance from A to C, it will be heard by A. Clearly (finally to me!) the radius of the circle A-C is larger than the path separation. Therefore, sound traveling in a wind will always take longer to be heard than in still air; everywhere except for directions directly and almost directly downwind. Soooooooooo that must have been the basis for the Michaelson-Moreley experiment looking for the spatial 'aether' for E/M waves. They must have set up A and B in fixed locations (somewhere around Cleveland, OH). They then just sat and waited, and waited.... If the solar system was (is) moving through 'absolute' space, and all-space was filled with an 'aether' medium, then as we in our solar system cruise through it, an 'aether wind' must exist. Given that wind, the time of flight for E/M waves from A to B would vary according to our solar system motion direction and speed, but the aether wind speed as experienced on earth will vary with the seasons; with our changing orbital locations, so that periodic maxima and minima of time-of-flight from A to B should occur. As I recall, they used a rotating flat mirror at A where a beam of light from B made a round trip from B to A and back to B. Since the effect is symmetrically additive when traveling 'cross-wind' (delay both ways) and virtually cancels upwind and downwind, there should be a 6-month cycle evident on the time-of-flight data. My recollection is that they never found such periodic variations that were greater than the uncertainty of their measurements. Seems time to do it again with refined electrooptics.. Ang. C. [snip] > A spherical sound wave expands with point B as it center. When the >radius of the sphere is equal to the distance from A to C, it will be >heard by A. Clearly (finally to me!) the radius of the circle A-C is >larger than the path separation. Therefore, sound traveling in a wind >will always take longer to be heard than in still air; everywhere except >for directions directly and almost directly downwind. Yep - that's it. This is all formally laid out by Blokhintzev, in his papers that appeared in JASA in 1946. It can be pretty entertaining to try to get a back-of-the-envelope understanding of his results for moving media. >Soooooooooo that must have been the basis for the Michaelson-Moreley >experiment looking for the spatial 'aether' for E/M waves. They must >have set up A and B in fixed locations (somewhere around Cleveland, OH). >They then just sat and waited, and waited.... If the solar system was >(is) moving through 'absolute' space, and all-space was filled with an >'aether' medium, then as we in our solar system cruise through it, an [snip] That was the success of their experiment - that they did not detect the ether. That result (negative though it was) eventually sprang Einstein from his dead-end job at the patent office. Ken Plotkin > That was the success of their experiment - that they did not detect > the ether. That result (negative though it was) eventually sprang > Einstein from his dead-end job at the patent office. So much for Victorian physics. But now we can do the experiment with perhaps 100 times the precision. Any reports of it so far? Michaelson, I don't think, ever said that it was not there, only that he could not find any within his experimental error... That's what I have gleaned with pasts write-ups on the matter. I don't think I have seen the original paper... Ang. C. > So much for Victorian physics. But now we can do the experiment with > perhaps 100 times the precision. Any reports of it so far? Michaelson, I > I don't think I have seen the original paper... It's right here http://www.aip.org/history/gap/Michelson/Michelson.html The experiment has been repeated with lasers, which increases the precision by a large factor. The original MM experiment had large systematic errors that were never really addressed, AFAIK. Didier  SignatureDidier A Depireux ddepi001@umaryland.edu didier@isr.umd.edu 20 Penn Str - S218E http://neurobiology.umaryland.edu/depireux.htm Anatomy and Neurobiology Phone: 410-706-1272 (lab) University of Maryland -1273 (off) Baltimore MD 21201 USA Fax: 1-410-706-2512 > It's right here > http://www.aip.org/history/gap/Michelson/Michelson.html > The experiment has been repeated with lasers, which increases the precision > by a large factor. The original MM experiment had large systematic errors > that were never really addressed, AFAIK. Allow me to be the 'contrarian': 1- The velocity of light diminishes in a dense medium. Thus the speed of light in water is about 2/3 that in air. 2- Although space is "empty", it's not completely empty. Thus to the extent that we can assign a density to space, a medium exists there, and with that medium an infinitesimal reduction of the speed of light can occur, and there will be a delay of the sort we seek. Ang. C. "Angelo Campanella" <a.campanella@att.net> wrote in message news:ulCbd.699278$Gx4.70636@bgtnsc04-> > Allow me to be the 'contrarian': Please do. > 1- The velocity of light diminishes in a dense medium. Thus the speed of > light in water is about 2/3 that in air. [quoted text clipped - 3 lines] > with that medium an infinitesimal reduction of the speed of light can > occur, and there will be a delay of the sort we seek. Anyone interested in this should read "The Golem: What Everyone Should know About Science" by Harry Collins & Trevor Pinch. It deals in detail with the uncertainties of the original experiment, as well as being a thoughtful survey of experimental science in general. And for about $5 secondhand on ABE, the book is a lot better value than a Sunday newspaper. SignatureTony Woolf My e-mail address has no hypen - but please don't use it, reply to the group. > It's right here > http://www.aip.org/history/gap/Michelson/Michelson.html > The experiment has been repeated with lasers, which increases the precision > by a large factor. The original MM experiment had large systematic errors > that were never really addressed, AFAIK. Ah well, someone did look at their error bars after all, http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=AJPIAS00005 0000011000987000001&idtype=cvips&gifs=yes And the experiment was repeated, using lasers etc, as mentioned in http://www.aip.org/pnu/2002/590.html Didier SignatureDidier A Depireux ddepi001@umaryland.edu didier@isr.umd.edu 20 Penn Str - S218E http://neurobiology.umaryland.edu/depireux.htm Anatomy and Neurobiology Phone: 410-706-1272 (lab) University of Maryland -1273 (off) Baltimore MD 21201 USA Fax: 1-410-706-2512 Since the sound wave is spherical, there will be a component of the wave velocity in the direction of A's motion. Does this then mean that there will be somewhat of a doppler shift (to lower frequencies) when the sound is heard at the upstream point C? Tom Correction. The direction of the frequency change of this doppler effect depends on the velocity difference between A and the wave velocity component in the direction of A's velocity. It now seems to me that the component velocity will be less than A's velocity, otherwise A would not catch the wave. Thus, there should be a doppler shift to higher frequency? Tom |
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