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Re: Standing waves


1) In observing standing waves I noticed that the
oscillating string was often hesitant between showing
either the node or the antinode at the vibrating rod.

2) I also found the dissipation and the Q explanation
by JohnD very useful. But I do not remember seeing
a textbook in which the Q aspect or the dissipation
aspect are discussed.

3) If a wave is defined as a "moving shape" then
a standing wave is not a wave. On the other hand,
mathematically it is a superposition of two waves.
Interesting, the mathematical sum of two waves
is not a physics wave. This looks like a paradox.
A different name, such as "oscillating string" would
probably be better than "standing wave."
Ludwik Kowalski

On Thursday, Feb 12, 2004, at 02:45 America/New_York, Bernard Cleyet

I disagree w/ just about everyone, except JD's dissipation / Q is

I decided to follow up on my accusation and found deep in my garage the
same driver (Pasco) and a Pasco osc. / amp.

Immediately it is obvious, no surprise (long experience with building
hi-fi speaker systems), that the amplitude varies w/
freq. So I (by eye * -- this is rough) adjusted the amp's gain so the
amplitude of the driver's piston was constant and measured the
of the loops of the Pasco supplied bungee chord. Doing so results in a
much closer to constancy of the loops' amplitude. ~ 12 (two loops) =>
100 (12 loops) Hz. This required nearly full power at 100 Hz and
somewhat less at 12. (~ 1/5th the current, see below.)

This was after taking an inordinate time to determine the internal
resistance of the P/S and the impedance ** of the driver (0.1 => 0.2
Ohms and ~ 6 => 32 Ohms) I measured the driver's by finding the
resistance that halved the EMF applied to the driver. The P/s's EMF is
remarkably constant. In use I inserted a 0.1 Ohm in series to measure
the current. The driver's resonance is thus heavily damped, but still
very evident (~50 Hz.)

* I know where the strobe is, but that later, if ever.
** As expected inserting the plug (string clamp) changes the resonance
freq. and Q, the string has little effect, in or out of resonance.


Ludwik Kowalski wrote:

I think I found the answer shortly after
the send button was pressed. But is it
satisfactory (see below)?

For identical amplitudes the string would
have to be longer with 9 loops than with
one loop. For a rubber band (of low k) the
amplitudes would not be as different as
for a common string.