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Re: resonance tube



"10:1 voltage-divider, in which the lower
leg is the speaker itself."

Yes, is a constant current source, as the current is limited by a
constant resistance* (only, therefore, constant impedance, also).
However, the point is to unload the speaker, so it is not damped by the
source. Unfortunately, one must also measure the impedance of the
speaker away from the the water tube, so its (the speaker's)
resonance(s) may be found and ignored.

* As JD states, >10 X the impedance of the speaker. More if one wants
a quantitative result.

BTW a much less expensive method of detecting phase (than a PAR,
Stanford Research, etc.) is to use a center tapped secondary
transformer. In this case the primary is driven by the speaker,
unloaded (isolated) by a cathode follower (emitter follower for the
modern, op. amp. for the "really" modern). The output is compared to
the source (oscillator output) using another transformer, Its
secondary is in series with other's center tap and the center of a
balanced rectifier (the other legs of the rectifier go the the secondary
of the first transformer). The output of the rectifier is proportional
to the input signal and the polarity indicates the phase.
Unfortunately, unless the signal (and the reference) are constant, this
device does not give the phase angle, only whether it leads or lags the
reference. This device was invented for use with strain gauge bridges.

If your library hasn't discarded Hill's Electronics in Engineering,
(because it's so old, 1949) look on page 247 for the circuit and
description.

bc

John S. Denker wrote:

On 09/16/2003 02:24 AM, Mark Sylvester wrote:
> The setup: 1.5m transparent tube about 10 cm diameter, set vertically
> so that water can be introduced into or drained from the bottom.
> Loudspeaker mounted over the open upper end; small microphone just


cut