Re: Waves

[brian whatcott <inet@INTELLISYS.NET>]

> At 09:51 AM 7/4/00 -0700, Leigh Palmer wrote:
> > ... Your wire rope
> > had a tension which varied due to gravity, the tension being much
> > higher at the upper end than at the lower.


> At 00:56 7/5/00 +0200, Mark Sylvester replied:
> Indeed. I used to take people caving. In one particular cave we'd descend a
> 70 metre rope hanging done in a huge bell-shaped chamber, open to the sky.
> If there were physics students in the group we'd be sure to send a short
> wave-train up the rope by shaking the bottom end a few times....


> >  The high pitched precursor
> > to the main pulse is likely due to the transmission of longitudinal
> > modes in the cable at speeds higher than those of the transverse mode.


Though there was some nit-picking about the possibilities of transverse
waves being responsible for the auditory precursor vice compression waves
after this earlier conversation, I ran into another shock excited audible
response this evening, for which only a compression wave will presumably
fit, the medium being air.

 We had obtained an example of this year's hot fireworks selection,
called an artillery shell and drove south of town to let off this celebrated
device. It consists of a ball (actually one buys a set of twelve) with
a 30 cm fuze, and a projector tube, mounted vertically.

One 'lights the touch-paper and retires' as the quaint instructions used
to say on the fireworks of my youth.

 These artillery shells set off a propellant charge and then explode in a
star burst at one or two hundred feet. (It turns out that most aerial shells
burst well under 500 feet, as one can demonstrate by flying fairly low on
July 4th evening: at 1500 ft., one is invariably looking downwards
to see aerial shells...)

This evening, we chose a country lane between crop fields to let them off.

Each shell first lifted off and banged with a shower of stars, and then
there was a distinct sustained echo at a few hundred Hertz, reminiscent
of the proton relaxation audio signal available from a jar of magnetized
water, but much lower than the 2 kHz note available in that case.

I wondered what was being excited.
My first suspicion was the diffraction grating due a ploughed field.
 So I climbed down the roadside ditch and up to the field to look into it.

There was a crop at least three feet tall, of close set leafy plants.
 Without a hand lamp I could see no great pattern in it, and I imagined
the plants would provide a rather effective attenuation.

There were no tall structures, besides the utility poles marching down
the side of the lane. The note lasted perhaps 2 seconds or less, and
descended one or two semitones in the first half second, then held on
the lower note.
 This was another possible indicator of some geometrical effect of
ground intereference, I suppose.

Do you think the furrows of perhaps 70 cm wavelength would account for
this echo?


brian whatcott <inet@intellisys.net>
Altus OK