Re: [Phys-l] sound stuff

["John Clement" <clement@hal-pc.org>]

I think there was a sign error in the original response, or a
source/observer confusion.  The frequency of an advancing source goes to
infinity as the speed approaches the speed of sound.  However when you have
a moving observer retreating from the source, the perceived frequency of the
source goes to zero as the speed of the observer approaches the speed of
sound.

John M. Clement
Houston, TX

> 	Concerning your response to the Doppler
> effect question posed by Anthony L., the frequency
> of the sound wave detected by an observer as the
> car retreated at the speed of sound would actually
> be one-half the emitted frequency (not a frequency
> approaching zero).  It's interesting to note that
> with the relativistic Doppler effect an object that
> recedes with a speed approaching "c" would have an
> observed frequency approaching zero.
>
> Dave S.
>
>
> > 2) If a car sounds a horn. If you hear 450 Hz as it approaches and 350
> Hz
> > as it recedes, the actual frequency is about 394 Hz (not 400 Hz).
> > Conceptually, why isn't the Doppler effect result symmetric for when
> the
> > sound (or observer) is approaching/receding the observer (sound)?
>
> Because 1/x is not a linear function of x.
>
> Consider the extreme case of the car moving very near Mach 1.
> The sound of the retreating car goes to frequency=0, a 100% change.
> The sound of the advancing car goes to frequency=infinity, a much larger
> change.