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Sound Waves

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Measurement of the speed of sound in air by two methods: (1) using time of flight measurements and (2) using the wave properties of sound.

In the first method, a microphone is set a known distance from a speaker. The microphone is connected to one channel of an oscilloscope and the speaker is connected to the external trigger of the scope. When the speaker is caused to emit a sound pulse it triggers the scope. The sound pulse is detected some time later by the microphone. The time it took for the pulse to travel from the speaker to the microphone is read directly from the scope. This is done for a variety of speaker-microphone separations and the d vs. t data is plotted. The slope of the line is the speed of sound.

In the second method, a speaker at one end of a lucite tube is connected to a sine wave generator and to one channel of an oscilloscope. A microphone on a movable aluminum rod in the tube is connected to the other channel of the scope. With the scope set to display A vs. B, the user can detect positions in the tube where the wave emitted by the speaker and the wave received by the microphone are in phase. The distance between in phase positions can be measured using a vernier-meter stick combination attached to the movable aluminum rod. The frequency of the sound wave is measured by a frequency counter attached to the signal generator. The speed of sound is then computed by multiplying the wavelength times the frequency. Different gases can be put into the sound tube.

Course Level

Student Handouts


Number of set-ups available:

Per lab station:

Sound pulse setup:

1 2 meter meterstick
1 oscilloscope
4 banana leads
1 microphone/stand
1 speaker/line cord
1 BNC cable
2 BNC to banana adaptor (male)
1 roll of masking tape

Traveling wave setup:

1 lucite sound tube
1 oscillator
1 oscilloscope
1 frequency counter
banana leads
1 BNC T-connector BNC cables
BNC to banana lead adaptors
1 balloon/rubber tube

Class materials:

1 tank of He gas/regulator and hose
1 tank of CO2 gas/regulator and hose
1 long rubber tube (long enough to reach all stations)

Time of Flight Measurement

sound pulse apparatus

Wave Properties Measurement

travelling wave apparatus apparatus

Circuit Diagram

circuit diagram


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Site contact: largent@Dartmouth.EDU

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