Independence of speed of sound from motions of source and receptor
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From: Richard Dobson on 22 Mar 2010 14:00 On 21/03/2010 21:35, Arindam Banerjee wrote: ... >> "Science of Thunder >> Lightning has a diameter of 1-2 inches (2-5 cm) and can heat air to >> 70,000� F (39,000� C) in a few milliseconds. Ninety percent of the >> electrical energy of lightning is released in the form of heat, which >> is quickly dissipated into the atmosphere. Less than 1% of lightning's >> energy is converted into sound and the rest released in the form of >> light. A SUDDEN INCREASE IN PRESSURE AND TEMPERATURE CAUSES SURROUNDNG >> AIR TO EXPAND VIOLENTLY AT A RATE FASTER THAN THE SPEED OF SOUND, >> SIMILAR TO A SONIC BOOM. THE SHOCK WAVE EXTENDS OUTWARD FOR THE FIRST >> 30 FEET (10 M), AFTER WHICH IT BECOMES AN ORDINARY SOUND WAVE CALLED >> THUNDER . > > Fine. So it is obvious here that the sources of sound are moving in > all directions from the lightning bolt. Let us say, like masses of > playing pianos (playing all the while) rapidly moving away in all > directions. A perfect natural phenomenon to indicate whether or not > sound from moving sources send their sound waves at different speeds > to the static receiver who sees the lightning. > I was wondering how this topic was proceeding. What a delightful image - sort of acoustic shrapnel, a Bosendorfer here, a Steinway (yes! blow them up!) there, and a toy piano, being lighter, ~way~ over there. Except of course the "sources of sound" are not moving. The source of the sound is the explosion causing a rapidly expanding pressure wave (an acoustic tsunami in effect) which if you stand too close will blow you off your feet, or worse. In signal processing terms it is an "impulse". A whip crack does must the same thing on a smaller scale (the crack being a mini sonic boom). The impulse response is of course a combination of the response of the air itself, plus echoes from the environment. ... > > I have already written enough on this subject. Now, all I have to do > is to prove my ideas with new experiments. That will take some time, > however, as I will do it entirely on my own and with just my own > resources. I would like to thank you for your very kind interest, and > your most polite discussions on this most vital subject. > Well, it seems it may not be either difficult or expensive. The critical factor is the timing. You just need to arrange for a moving sound to be emitted simultaneously with a static source at the same position, in a way that can be measured very precisely. Have a fast-moving car (Bugatti Veyron or somesuch), sounding its horn, drive through a large glass window (or other suitably noisy non-lethal target). The horn ~stops~ exactly at the moment of impact (a trigger on the front of the car can easily be rigged up to do this). The horn is moving, so the speed of sound is (according to the hypothesis) faster; the glass is static so the sound of it smashing will be emitted more slowly. Record it all from the static observer position. The proof will be that the sound of the horn (which is arranged to stop instantly at the moment of impact) ~overlaps~ the sound of the glass, because it has arrived that little bit earlier. A control test can easily be set up to identify any echoes from the environment that need to to be eliminated in post-processing. Sounds expensive? Now, there is a TV program in the UK called "Top Gear" who just love doing things like that (when they are not blowing up caravans). They would I am sure do the whole thing for free. Or perhaps the USA "Mythbusters" team, likewise. Get it on TV, that's the surest route to scientific immortality! Nothing to do with Doppler of course; but you already knew that. Richard Dobson
From: Richard Dobson on 22 Mar 2010 16:34 On 22/03/2010 19:55, tg wrote: ... >> Well, it seems it may not be either difficult or expensive. The critical >> factor is the timing. You just need to arrange for a moving sound to be >> emitted simultaneously with a static source at the same position, in a >> way that can be measured very precisely. Have a fast-moving car (Bugatti >> Veyron or somesuch), sounding its horn, drive through a large glass >> window (or other suitably noisy non-lethal target). The horn ~stops~ >> exactly at the moment of impact (a trigger on the front of the car can >> easily be rigged up to do this). The horn is moving, so the speed of >> sound is (according to the hypothesis) faster; the glass is static so >> the sound of it smashing will be emitted more slowly. > > No, the glass is moving as the car. > There will be some glass that directs ahead, but plenty that will direct laterally (not least since the car will not present a perfectly flat face to it); and in any case the subject at issue is not the speed of the glass, but the ~sound~ of the impact. Relatively little of the energy of the car will have been absorbed (given it is travelling at up to 200MPH and is considerably heavier),so the car will carry on virtually unchecked, and no piece of glass will of itself be a ~source~ of sound other than randomly, and at very low level, through serendipitous collisions with other pieces of glass. But you are right; it is not a serious proposition, since of course (excluding duly noted temperature and other effects) the truth is that the speed of sound in air is constant, and Doppler effect is caused by relative motion between source and observer. Arindam for reasons only he can tell, seeks to prove otherwise. Since no physicist can plausibly suggest an experiment that will prove his "theory" (while any number of demonstrations can be proposed that will falsify it, playful or otherwise; as is hopefully done in schools across the globe on a regular basis), we will perforce have to await Arindam's own demonstration and judge appropriately. Given his stated opinion of himself, one wonders whether he would accept peer review from anyone at all. Richard Dobson
From: Richard Dobson on 22 Mar 2010 20:18 On 22/03/2010 22:20, Zinnic wrote: ... > > It is hard for me to accept that this has not been demonstrated > previously by classic investigation of the different Doppler effects > engendered by motions of emitter versus receiver. > I guess it is so well established that sound speeds depend only on the > states of its media, that discussions are limited to explanation of > the theory without giving details of the experiments whereby the > theory was first established. Nowadays it would be a snap > demonstration for current techniques in radar and sonar and probably > is regularly confirmed during routine instrument calibration. It is indeed very well-established, over so many centuries that the identity of the person who first measured it is lost in the mists of history. One site suggests Marin Mersenne was (among) the first: http://asa.aip.org/pierce.html One of the best sites to visit, to read about the general principles of wave propagation in various elastic media (i.e. there is nothing intrinsically special about air other than that we breathe it) is here: http://hyperphysics.phy-astr.gsu.edu/HBASE/hframe.html Note that while we casually talk about "the speed of sound" as though there is a something called "sound" that travels through the air, it is, strictly speaking, ~wave propagation~ in a medium, and the site above shows general formulae for wave propagation in a variety of gases, including the famous example of helium. See especially the page on Transverse Waves. Note that the Doppler effect is an everyday familiar process, that hardly needs demonstrating, as such, given that such things as fast noisy vehicles are ubiquitous. It is exploited musically in the "Leslie" rotating speaker system associated with the Hammond organ, to apply a vibrato effect to the sound; as good an example as any. It is the sort of thing that would be a good demonstrator for schools (teachers have been known to swing small loudspeakers around their heads, hoping the wires are sufficiently strongly attached!); but other systems have of course been devised ad libitum for such purposes, according to the creativity of the designers; including spectacular machines involving fire, water, etc. There is a famous 19th-Century wooden wave machine in the archives of the Royal Institution (London) that demonstrates wave propagation very clearly to young children, but which apparently is also good enough to demonstrate science at degree level too. So formally speaking - sound is not a ~thing~ that "moves through air" (though that is a perfectly reasonable description for everyday purposes); it is an example of transverse wave propagation through an elastic medium, which in turn has definable and measureable properties including temperature, humidity, density, etc. Richard Dobson
From: Richard Dobson on 22 Mar 2010 20:39 Correction! On 23/03/2010 00:18, Richard Dobson wrote: ... .... > including the famous example of helium. See especially the page on > Transverse Waves. > >... >..it is an example of transverse wave propagation ... should both be ~Longitudinal~ waves. Thinking about strings too much! Richard Dobson
From: Richard Dobson on 23 Mar 2010 06:25
On 23/03/2010 10:04, Zinnic wrote: ... > Thank you for the additional links, great source of fundamental > physics theory. Again I can find only the theory/math for the Doppler > effects but no references to the original research by which it was > demonstrated. Guess I will have to delve into the history of science > to find it. May as well start here: http://en.wikipedia.org/wiki/Doppler_effect The Doppler effect is a classic example of the Conan Doyle phrase to the effect "we see but do not observe". The Doppler effect has been evident to our ears since we evolved ears; but sometimes it takes a scientist to ~observe~ it precisely and document it - which is where the theory/math comes in. It never really needed to be demonstrated (in the sense of something not seen in daily life), but it needed to be explained; and hence made both predictable and a means to perform measurements (e.g. optical red shift). That said, boys of a certain age have been able with remarkable consistency to capture the essence of the Doppler effect in mimicking the sound of a racing car speeding past them. Richard Dobson |