The Doppler effect - geek question

[Simpo Two]

Driving past a reversing lorry today I was surprised to hear that the 'beep beep' reversing noise dropped by almost exactly a semitone as I went past. I was doing about 35mph and this made me wonder if the Doppler shift is proportional to speed. For example, an octave represents a doubling in frequency, so would I need to travel at 16 x 35mph = 560mph to get a drop in tone of an octave? How do you calculate Doppler shift anyway?

[stifler]

You must have a very good ear to be able to tell how much it has dropped by.

[TheEnd]

yep, it's relative speed related.

the frequency is speed/ wavelength.

the wavelength doesn't change, but your speed relative to it will, so it makes the frequency you hear change.

speed in this case is the speed of sound, but it does the same with light, and you get things changing colour.

[Huntsman]

Simpo Two said:

Driving past a reversing lorry today I was surprised to hear that the 'beep beep' reversing noise dropped by almost exactly a semitone as I went past. I was doing about 35mph and this made me wonder if the Doppler shift is proportional to speed. For example, an octave represents a doubling in frequency, so would I need to travel at 16 x 35mph = 560mph to get a drop in tone of an octave? How do you calculate Doppler shift anyway?

Extra geek points to that man!

[tribbles]

If it was a semitone, then I suspect you need to be travelling 12 times as fast, not 16 times.

[Cara Van Man]

We have Doppler and his bloody theories to thank for GATSO cameras.

Git!

[Eric Mc]

Also useful for working out the speed of expansion of the universe - and the operation of GPS systems.

Very useful findings by Mr Doppler.

[Cara Van Man]

One of the best example to hear the doppler effect is when ambwilans drives past.

Listen to the apparent change in note as it approaches then recedes.

[rhinochopig]

Eric Mc said:

Also useful for working out the speed of expansion of the universe - and the operation of GPS systems.

Very useful findings by Mr Doppler.

Personally I'm not impressed by Mr Doppler. Small boys have been going neeeeeeeeeeeaaaaaaaaaaaaaawwwwwwwwwwwwwwwww since cars were invented to mimic the sound of a passing race car. If a five year old boy can discover it, it can't be that hard.

[Simpo Two]

stifler said:

You must have a very good ear to be able to tell how much it has dropped by.

Actually intervals aren't that big a deal - any Grade 1 piano student can recognise most of them. The hard bit is identifying or singing a particular note.

I did however later realise that although there are eight notes in an octave, there are not 16 semitones. Funny thing, music!

I shall now attenpt to prove my theory by going past at 560mph and seeing if it really does make an octave

[Martial Arts Man]

Cara Van Man said:

One of the best example to hear the doppler effect is when ambwilans drives past.

Listen to the apparent change in note as it approaches then recedes.

Or any other vehicle's engine note.

Only on PH could this thread have come about

[tribbles]

Simpo Two said:

I did however later realise that although there are eight notes in an octave, there are not 16 semitones. Funny thing, music!

I shall now attenpt to prove my theory by going past at 560mph and seeing if it really does make an octave

420mph. 12 semitones in an octave (which is what I was alluding to).

[Simpo Two]

tribbles said:

Simpo Two said:

I did however later realise that although there are eight notes in an octave, there are not 16 semitones. Funny thing, music!

I shall now attenpt to prove my theory by going past at 560mph and seeing if it really does make an octave

420mph. 12 semitones in an octave (which is what I was alluding to).

Sorry, yes. I haven't had any coffee yet!

[Eric Mc]

rhinochopig said:

Eric Mc said:

Also useful for working out the speed of expansion of the universe - and the operation of GPS systems.

Very useful findings by Mr Doppler.

Personally I'm not impressed by Mr Doppler. Small boys have been going neeeeeeeeeeeaaaaaaaaaaaaaawwwwwwwwwwwwwwwww since cars were invented to mimic the sound of a passing race car. If a five year old boy can discover it, it can't be that hard.

Mr Doppler didn't "discover" it - he explained it. I doubt if most 5 year olds could do that.

He also worked it out in the 19th century where the only object likely to provide a good example of the Doppler Effect was a train whistle.

[timmytortoise]

Didn't he set up an experiment with a string quartet(might have been a brass band??) on an open railway carriage & listened as it went past?
Why didn't he just put the band on a platform & lean out of the window as he went past?
TT
PS. Doesn't apply to Audi / Peugeot diesels - way too quiet!

[Simpo Two]

timmytortoise said:

Didn't he set up an experiment with a string quartet(might have been a brass band??) on an open railway carriage & listened as it went past?

Peter Ustinov, in a brilliant programme many years ago, used a similar concept to explain Einstein's theory of relativity - except he needed a clock not a microphone, and the train had to go awfully fast!

[ThatPhilBrettGuy]

This is why I try to go past them at the speed of sound. Yes. it's more annoying as to approach them, but you can't hear them at all afterwards

[Dogwatch]

timmytortoise said:

Why didn't he just put the band on a platform & lean out of the window as he went past?

Because in The Good Old Days they had notices telling you not to lean out of the window!!

Helf 'n Safety you know...

[PD9]

Eleventy is the answer