Re: Standing waves

[Ludwik Kowalski <kowalskil@MAIL.MONTCLAIR.EDU>]

My message keeps bouncing. We will probably see
all three versions eventually. But I want to see replies
before the thread is forgotten. Sorry for repostings.

REPOSTING A MESSAGE POSTED YESTERDAY

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, Bernard Cleyet wrote:

> I disagree w/ just about everyone, except JD's dissipation / Q is
> important.
>
> 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
> amplitude
> 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.