| MadSci Network: Physics |
What you need to do is set up the pulse (or signal) generator so that
the pulse of signal from it (the same one that powers the speaker) triggers
the oscilloscope. You can then measure the time it takes for the sound to
arrive at the microphone by timing the pulse that you see on the
oscilloscope that is after the start of the trigger. Here's a sketch:
Notice that I show only one wire from the signal generator to the oscilloscope. You might actually have to have a second wire there, to connect the grounds of the two instruments.
This scheme depends on the fact that the speed of the electrical signals is much faster than the speed of sound. This is, in fact, true. The speed of the electrical signals is the same as the speed of light, which is about 3x108 meters per second, while the speed of sound in air is roughly 3.3x102 meters per second. So there will be very little signal delay in the wires, which is a good thing!
The distance between the speaker and the microphone, D, will have to be measured, and the more accurately you can measure it the better your determination of the speed of sound will be. For the speaker you might consider the middle of the cone of paper to be the surface to measure to, unless you have a piezoelectric (crystal) speaker which would be better, in which case the front surface of the crystal will suffice. Also, you will need to estimate how far back from the front of the microphone the active element is, even though in my simple sketch I show the distance being measured to the front of the microphone's case.
Also, the farther apart the microphone is from the speaker the less trouble the measurement using the oscilloscope will probably be.
The speed of the sound is, then,
D / t
where D is the measured distance and t is the time interval from the start
of the trigger (the time at which the signal goes to the speaker) to the time of
arrival of the signal from the microphone.
Well, I hope this helps you!!!!
John Link, MadSci Physicist
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