>>>>> "Jerry" == Jerry Avins <jya@ieee.org> writes:Jerry> Abhijit wrote: >> >> Jerry and all >> >> I always thought (similar to what Ray thought) that the frequency strictly >> depends on the relative motion (and not on who is moving). Jerry> You were wrong in detail. For speeds that are low relative to the speed Jerry> of sound, there is little observable difference. >> >> Do you mean to say that a moving source will "produce" different frequency >> in different directions? Jerry> The wavelength in the air around a moving source is different in Jerry> different directions. You knew that, but you apparently didn't know you Jerry> knew. Stationary observers to the front and rear of the moving source Jerry> hear different frequencies. Ok, then I am confused. Yes, the wavelength around a moving source is different in different directions. But if the source is moving directly away from me, I get a certain wavelength. If I move directly away from a source, the wavelength will be different? I'm assuming no non-linearities in the sound wave, etc, because the sound is very loud or because the speed is very high. So, if I were blindfolded, I could tell if I were in a train moving towards a whistle instead of the whistle moving towards me? Ray
Is the Doppler effect really linear?
Started by ●August 19, 2003
Reply by ●August 20, 20032003-08-20
Reply by ●August 20, 20032003-08-20
Raymond Toy wrote:>...> > So, if I were blindfolded, I could tell if I were in a train moving > towards a whistle instead of the whistle moving towards me? >Only if you knew the original pitch and the speed very accurately. Let's say that the train is moving at Mach .2 (fast, but possible) and its whistle has a frequency of f. when the train is approaching, its whistle's frequency sounds like f/(1 - .2), or 1.25f. If the train is in the station and you are on another approaching it at Mach .2, the pitch you would hear would be f(1 + .2) or 1.2f. The nonlinearity that Clay wrote of -- I hadn't thought of it explicitly before, but its tickling was the reason I started this thread -- is that the magnitude of the pitch shift from a moving source is different for the same speeds to and away. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●August 20, 20032003-08-20
"Raymond Toy" <toy@rtp.ericsson.se> wrote in message news:4nbrukypln.fsf@edgedsp4.rtp.ericsson.se...> So, if I were blindfolded, I could tell if I were in a train moving > towards a whistle instead of the whistle moving towards me?Raymond, Yeap you can tell if you had precise velocity and frequency info! However, for low velocities, they asymptotically become the same. 1/(1-x) = 1+x+x^2 +x^3+... approx = 1+x (x small means x^2 real small so ignore it and higher order terms) So the denominator term for low velocities looks a lot like the numerator term. In realitivity, however we have simply f' = f*sqrt( (c-v)/(c+v) ) This is found by applying the contraint that all observers measure the same phase at the same point in space. If you have a moving medium, then you have to add velocities (use Einstein's formula). Fresnel did some neat experiments with measuring the speed of light in moving water, and he devised a drag coefficient. This become readily understood via realitivity. Clay> > Ray
Reply by ●August 21, 20032003-08-21
"Jerry Avins" <jya@ieee.org> schrieb im Newsbeitrag news:3F442930.1ADB903@ieee.org...> Raymond Toy wrote: > > > ... > > > > So, if I were blindfolded, I could tell if I were in a train moving > > towards a whistle instead of the whistle moving towards me? > > > Only if you knew the original pitch and the speed very accurately. Let's > say that the train is moving at Mach .2 (fast, but possible) and its > whistle has a frequency of f. when the train is approaching, its > whistle's frequency sounds like f/(1 - .2), or 1.25f. If the train is in > the station and you are on another approaching it at Mach .2, the pitch > you would hear would be f(1 + .2) or 1.2f. The nonlinearity that Clay > wrote of -- I hadn't thought of it explicitly before, but its tickling > was the reason I started this thread -- is that the magnitude of the > pitch shift from a moving source is different for the same speeds to and > away.I think you are terribly wrong here. Isn't it the relative motion of R and T? Assuming the Speed of R and T relative to the Medium is much lower than the propagation speed in it. v = relative speed, positive is towards c = propagation speed in medium f = emitting frequency f (c+v) f' = ------- c And you can even get negative frequencies at v <= -c ! This could be interpreted that R will never get it. Raymund Hofmann __ I will travel at the speed of sound is the last thing we heard of him. A funny sign reading: "Attention! I am traveling at the speed of light."
Reply by ●August 21, 20032003-08-21
>>>>> "Clay" == Clay S Turner <physicsNOOOOSPPPPAMMMM@bellsouth.net> writes:Clay> "Raymond Toy" <toy@rtp.ericsson.se> wrote in message Clay> news:4nbrukypln.fsf@edgedsp4.rtp.ericsson.se... >> So, if I were blindfolded, I could tell if I were in a train moving >> towards a whistle instead of the whistle moving towards me? Clay> Raymond, Clay> Yeap you can tell if you had precise velocity and frequency info! Ok, I dug up my old physics book. This example is explained clearly there. The 2 situations are different because how the waves arrive are different. So, yes, you really could tell which was moving. But not if it were a beam of light instead of a whistle. :-) Ray -- A little learning is a dangerous thing.
Reply by ●August 21, 20032003-08-21
"raymund hofmann" <filter001@desinformation.de> wrote in message news:bi25nc$4n3$1@online.de...> And you can even get negative frequencies at v <= -c ! > This could be interpreted that R will never get it. >Raymond, Assume the source has been emitting the sound for some time before you go flying through the wave field. And yes if your speed exceeds the speed of sound in the medium, you will get negative frequencies. Each successive wavecrest that you pass through is older than the one before it - this is the negative of the negative frequency interpretation in this case. Note this can't happen with realitivistic waves. A ship moving left at 0.5c relative to you and another ship moving to the right at 0.5c relative to you. Are they moving at c relative to each other? Einstein says no. He gives 0.8c as the relative velocity between them. Even if they are each moving at c relative to you, they are moving at c relative to each other also! Clay
Reply by ●August 21, 20032003-08-21
raymund hofmann wrote:> > "Jerry Avins" <jya@ieee.org> schrieb im Newsbeitrag > news:3F442930.1ADB903@ieee.org... > > Raymond Toy wrote: > > > > > ... > > > > > > So, if I were blindfolded, I could tell if I were in a train moving > > > towards a whistle instead of the whistle moving towards me? > > > > > Only if you knew the original pitch and the speed very accurately. Let's > > say that the train is moving at Mach .2 (fast, but possible) and its > > whistle has a frequency of f. when the train is approaching, its > > whistle's frequency sounds like f/(1 - .2), or 1.25f. If the train is in > > the station and you are on another approaching it at Mach .2, the pitch > > you would hear would be f(1 + .2) or 1.2f. The nonlinearity that Clay > > wrote of -- I hadn't thought of it explicitly before, but its tickling > > was the reason I started this thread -- is that the magnitude of the > > pitch shift from a moving source is different for the same speeds to and > > away. > > I think you are terribly wrong here. > > Isn't it the relative motion of R and T? Assuming the Speed of R and T relative > to the Medium is much lower than the propagation speed in it. > > v = relative speed, positive is towards > c = propagation speed in medium > f = emitting frequency > > f (c+v) > f' = ------- > c > > And you can even get negative frequencies at v <= -c ! > This could be interpreted that R will never get it. > > Raymund Hofmann > > __ > I will travel at the speed of sound is the last thing we heard of him. > A funny sign reading: "Attention! I am traveling at the speed of light."Raymund, The formula you give, f (c+v) f' = -------, c is appropriate for approaching a stationary source. For a stationary observer approached by the source, the correct formula is fc f' = ------- . (c-v) These formulae assume steady motions, including that of the air. Jerry P.S. If your behind an object moving away from you at or faster than the speed of sound, you will hear sounds from it. If a stationary object emits sound and you move away from it at or faster than the speed of sound, you will never hear it. The cases are not equivalent. -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●August 21, 20032003-08-21
raymund hofmann wrote:> > "Jerry Avins" <jya@ieee.org> schrieb im Newsbeitrag > news:3F442930.1ADB903@ieee.org... > > Raymond Toy wrote: > > > > > ... > > > > > > So, if I were blindfolded, I could tell if I were in a train moving > > > towards a whistle instead of the whistle moving towards me? > > > > > Only if you knew the original pitch and the speed very accurately. Let's > > say that the train is moving at Mach .2 (fast, but possible) and its > > whistle has a frequency of f. when the train is approaching, its > > whistle's frequency sounds like f/(1 - .2), or 1.25f. If the train is in > > the station and you are on another approaching it at Mach .2, the pitch > > you would hear would be f(1 + .2) or 1.2f. The nonlinearity that Clay > > wrote of -- I hadn't thought of it explicitly before, but its tickling > > was the reason I started this thread -- is that the magnitude of the > > pitch shift from a moving source is different for the same speeds to and > > away. > > I think you are terribly wrong here. > > Isn't it the relative motion of R and T? Assuming the Speed of R and Trelative> to the Medium is much lower than the propagation speed in it. > > v = relative speed, positive is towards > c = propagation speed in medium > f = emitting frequency > > f (c+v) > f' = ------- > c > > And you can even get negative frequencies at v <= -c ! > This could be interpreted that R will never get it. > > Raymund Hofmann > > __ > I will travel at the speed of sound is the last thing we heard of him. > A funny sign reading: "Attention! I am traveling at the speed of light."Raymund, The formula you give, f (c+v) f' = -------, c is appropriate for approaching a stationary source. For a stationary observer approached by the source, the correct formula is fc f' = ------- . (c-v) These formulae assume steady motions, including that of the air. Jerry P.S. If you're behind an object moving away from you at or faster than the speed of sound, you will hear sounds from it. If a stationary object emits sound and you move away from it at or faster than the speed of sound, you will never hear it. The cases are not equivalent. -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●August 21, 20032003-08-21
On Wed, 20 Aug 2003 22:06:40 -0400, Jerry Avins <jya@ieee.org> wrote:>Raymond Toy wrote: >> > ... >> >> So, if I were blindfolded, I could tell if I were in a train moving >> towards a whistle instead of the whistle moving towards me? >> >Only if you knew the original pitch and the speed very accurately. Let's >say that the train is moving at Mach .2 (fast, but possible) and its >whistle has a frequency of f. when the train is approaching, its >whistle's frequency sounds like f/(1 - .2), or 1.25f. If the train is in >the station and you are on another approaching it at Mach .2, the pitch >you would hear would be f(1 + .2) or 1.2f. The nonlinearity that Clay >wrote of -- I hadn't thought of it explicitly before, but its tickling >was the reason I started this thread -- is that the magnitude of the >pitch shift from a moving source is different for the same speeds to and >away. > >JerryI'm not sure if that's really a nonlinearity, since the cases are different. Writing the expression as you did with the terms for the emitter and receiver separated clarifies the issue, I think. There's actually a lot of information about the situation that can be obtained merely by observing the Doppler history of things moving around you (this was the topic of my MS thesis). Back in the day I set out to show that one could even determine how far away the emitter was by the Doppler history observed at a single sensor (i.e., microphone). After some initial failed attempts I told my advisor that I was going to drop that parameter from my analysis, and he emphatically replied in his Korean accent, "No! Must do range!" I then set out to prove that range could not be determined in this manner and after a weekend hunched over a pad of engineering paper I had an algorithm for determining range from the Doppler observation. It worked pretty well. So my effort to prove that it couldn't be done failed, too. Eric Jacobsen Minister of Algorithms, Intel Corp. My opinions may not be Intel's opinions. http://www.ericjacobsen.org
Reply by ●August 21, 20032003-08-21
Eric Jacobsen wrote:> On Wed, 20 Aug 2003 22:06:40 -0400, Jerry Avins <jya@ieee.org> wrote: > > >>Raymond Toy wrote: >> >> ... >> >>>So, if I were blindfolded, I could tell if I were in a train moving >>>towards a whistle instead of the whistle moving towards me? >>> >> >>Only if you knew the original pitch and the speed very accurately. Let's >>say that the train is moving at Mach .2 (fast, but possible) and its >>whistle has a frequency of f. when the train is approaching, its >>whistle's frequency sounds like f/(1 - .2), or 1.25f. If the train is in >>the station and you are on another approaching it at Mach .2, the pitch >>you would hear would be f(1 + .2) or 1.2f. The nonlinearity that Clay >>wrote of -- I hadn't thought of it explicitly before, but its tickling >>was the reason I started this thread -- is that the magnitude of the >>pitch shift from a moving source is different for the same speeds to and >>away. >> >>Jerry > > > I'm not sure if that's really a nonlinearity, since the cases are > different. Writing the expression as you did with the terms for the > emitter and receiver separated clarifies the issue, I think. > > There's actually a lot of information about the situation that can be > obtained merely by observing the Doppler history of things moving > around you (this was the topic of my MS thesis). Back in the day I > set out to show that one could even determine how far away the emitter > was by the Doppler history observed at a single sensor (i.e., > microphone). After some initial failed attempts I told my advisor > that I was going to drop that parameter from my analysis, and he > emphatically replied in his Korean accent, "No! Must do range!" I > then set out to prove that range could not be determined in this > manner and after a weekend hunched over a pad of engineering paper I > had an algorithm for determining range from the Doppler observation. > It worked pretty well. > > So my effort to prove that it couldn't be done failed, too. > > > Eric Jacobsen > Minister of Algorithms, Intel Corp. > My opinions may not be Intel's opinions. > http://www.ericjacobsen.orgThere's a derivation in Quinn and Hannan's "The Estimation and Tracking of Frequency"
