Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Dopler effect

On 23 Jan 1998 11:06 Ari Hamalainen <aohamala@penger.helsinki.fi> wrote:

... The difficulty of this kind of experiment [observing Dopler shift
of f with a tuning fork] rises from the process that is used to calculate
the spectrum of the sound, that is, the discrete Fourier transform. The
resolution of the spectrum is the inverse of the sample length. If you
have for example 0.2 second sample, then you get only 5 Hz resolution
for the spectrum. For ULI DOS software the resolution is often even
worse (I have not tried it with the new Logger Pro, though).

I only used Vernier's microphone setup once to observe the spectrum of
frequencies and I do not remember details.

1) Is it possible to zoom on a narrow range of f, say from 254 to 260,
in order to observe a small shift at 256 Hz?

2) What prevents you from increasing the sample length to 2 seconds?
Is it only a matter of memory of your computer?

Me and our students have tried several times the experiment, both by
moving a tuning fork or a piezoelectric buzzer by hand, or by putting
the sound source on a cart and pushing it fast. The results have been
that the frequency shift to the right direction can be seen, but the
proportionality of the shift and the velocity can not be verified.

Quite recently, I finally managed to make a Doppler shift experiment
that gives reasonably good quantitative results too. I attached a piezo
buzzer (4.5 kHz, VERY loud end irritating) to the end of the vertical
rotating bar of a Pasco Rotating Platform (#ME-8951). I rotated the system
with a small DC motor at speeds up to 10 rev/s. The anqular velocity was
measured with a photogate. I used a Vernier microphone and a ULI//
interface, at 10 000 S/s sampling rate, and at maximum sample length,
0.74 s. By measuring the widening of the frequency peaks, I was able to
show quite plausibly that Df ~ v. I used my own software for data analysis,
but I can't see why it would not be possible with standard ULI programs too.

One might be also be interested in these:

* Briotta, D. et al. The appropriate use of microcomputers in
undergraduate physics labs. Am. J. Phys. 55 (10), October 1987. ...

* Wisman, R. et al. Experimental data frequency measurement with a PC.
Am. J. Phys. 58 (6). June 1992. ...