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: Air resistance



This is from a message which did not bounce to me for some reason.

.... the data seem to be enhanced. When I am looking at a distance from
a stationary object it appears to be a constant at the level 1 mm. The
wavelength of the ultrasound used is 8 mm. How come the individual
distances do not fluctuate at the level of couple of mm?

The naive assumption that the accuracy of each sample can not be better
than one lambda may not be correct. The distance is determined by d=v*TT,
where TT is the round trip travel time of the "sound particle". The period
of one oscillation, equal to 1/f=1/40000 s, is certainly very long in
comparison with possible small errors of electronic time measurements. Can
the departure and arival times be identified with an accuracy which is much
better than one period? The accuracy slightly better than one ten'th of a
period would produce the constancy of distancies at the level of 1mm.

Now some speculations. Suppose that sound particles, actually wave trains,
are well-shaped, that is the first sinusoidal cycle is exactly the same as
the next one, etc. In that case the departure and arrival times can be
electronically recognized, for example, as instances of first cross-overs
(from big plus to big minus). How accurately defined are the moments of
cross-over? It depends on the accuracy of the frequency f. Without doing
any Fourier analysis I expect the spectrum of f to be sufficiently narrow;
the length of wave trains was probably chosen to make this happen.

Ludwik Kowalski
P.S.
Brian, 1200 W was for heating, not for blowing. I have no idea what the
air temperatures are when the hair dryier is used for a couple of seconds
each time. I do have a student who expressed interest in the subject of
our concern and I hope she is serious. Perhaps she will be able to tell
us sothing new during the spring semester.