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
> below the loudspeaker, connected to oscilloscope. The usual exercise
> consists of changing the water level to find the resonant lengths of
> the air column tones of different frequency, thus finding the speed
> of sound and the end correction. The oscilloscope is used to observe
> when the sound from the tube is loudest.
>
> One notices that with the water just a centimetre or two below the
> resonant position, the position apparently not affected by the
> frequency, the trace goes flat, suggesting destructive interference.
> I have a student looking for a detailed explanation, but he has run
> out of ideas.
Some ideas:
1) Does it depend on how far the microphone is below
the loudspeaker? Can you reposition the microphone and
see what happens? Multiple microphones simultaneously?
2) You can get additional information by using the
*speaker* as a point of measurement. You could drive
the speaker with a constant-voltage source and measure
the current, or (probably better) drive the speaker with
a constant-current source and measure the voltage.
Hint: easy way to make a moderately high impedance
constant-current source:
audio source -->
audio amplifier -->
1:10 step-up transformer -->
10:1 voltage-divider, in which the lower
leg is the speaker itself.
If you want to get fancy, better circuits are available.
Emitter follower.....
3a) Plot out the whole resonant response curve. Locating
the peak of this curve is interesting, but it's not the
only interesting thing.
3b) Pay attention to the *phase* as well as magnitude of
the response. If you can possibly lay hands on a synchronous
detector ("lockin amplifier") use that to record the in-phase
and quadrature phasor components. Otherwise just eyeball it.