[phoenix-project] 5.3 Velocity of Sound Experiment
- From: Georges Khaznadar <georges.khaznadar@xxxxxxx>
- To: ozhan fenerci <o_fenerci@xxxxxxxxxxxx>
- Date: Fri, 12 Jun 2015 18:53:13 +0200
Hello Ozhan,
ozhan fenerci a écrit :
5.3) Velocity of Sound Experiment
I am doing the experiment 5.3 of ExpEYES Junior manual, which aims at
determining the velocity of sound by phase differences. The experiment
doesn't give a consistent data. For example, the manual claims that there
should be 2 cm change in distance when the the phase changed from 176 to 102.
For example in my experiment, the change in distance from 4cm to 6cm gives
the change in phase from 188 to 152; from 6cm to 8cm gives the change in
phase from 131 to 70 degree. In the first case, 2cm change gives 36 degree
phase difference; In second case, 2 cm change gives 61 degree difference.
Calculating a velocity of sound gives two different results, which are far
away from each other. When I did the experiment, I followed the methods as
mentioned in lab manual such as keeping the mic and the Piezo on the same
axis.
Have you checked the frequency of the emitted sound? I suppose so.
Please can you try to make a short experiment: slowly varying the piezo
generator distance, and watching the resulting phase information.
The theory predicts that when you plot the phase vs. the distance, you
shoud get a sawtooth-like plot (which means: a linear plot, modulo 360°)
Is that theoritical prediction fulfilled? Or do you get at least a part
of a linear plot?
Most known pitfalls are the possible creation of stationary wave
patterns between the sound emitter and the sound probe, due to sound
reflection. To avoid them, you may consider surrounding your microphone
and the other parts of the experiment zone with sound-absorbing
materials. You may try: velvet, small carpets, and other materials sold
as sound absorbers.
If both the piezo emitter and the expeyes box are lying on a plain
table, the sound is reflected on its surface (which is roughly
equivalent to having two emitters, the second one mirrored by the table
surface). Small irregularities of the table surface can slightly change
the orientation of the sound emitter (and of its acoustic image),
resulting in a deviation of the interference pattern, which has a
consequence on the sensed phase. Put a smooth cloth on the table's
surface to disminish this effect.
The wavelength of the feature sound (3500 Hz) is near 10 cm. If you want
to try distances like 2, 4, 6 cm between emitter and microphone, chances
are that strong reflections due to the body of the expeyes box near the
microphone yeld interferences. I imagine two different counter-measures:
- prefer distances like 12, 14, and 16 cm: you should get the same
results (with less amplitude), and attenuation due to the distance
between the emitter and the probe in free air disminishes the
interference phenomenon
- put some absorbing stuff near the microphone
Best regards, Georges.
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