Allan Hancock College



Allan Hancock College

Physics Lab

Sound Waves in Tubes

Purpose: To observe, model and compare the behavior of longitudinal sound waves in air.

Equipment: Function generator, glass tubes.

Procedure: Measure the speed of sound waves in a tube as described below:

2. Sound waves in a tube open at both ends, (fixed length). Open Science Workshop, using a speaker attached to the power supply and a sound sensor plugged into an analog circuit. Use the power supply as a signal generator in sine mode and set it to ~ 0.5 – 1.0 Volts to keep the sound fairly low. Place the speaker at one end of the tube, and the sound sensor at the other. Open an oscilloscope display for the sound sensor, but also plug the power supply into one of the channels. This way you can see how your source amplitude compares to your resonant amplitude. You might also open an FFT display for the sensor for an accurate reading of the frequency.

Measure the length of the tube L. Set the function generator to a fairly low frequency approximately 100 Hz and gradually increase it until you find a resonance. A resonance occurs when the sensor amplitude increases dramatically. Find about 5 resonant frequencies.

fn = n v/2L

This is a linear equation. Graph f vs. n/2L. The graph should be linear and the slope of the line will be the speed of sound in air. Compare your experimental value with

vair = 331 m/s + 0.59 (Temp oC) m/s.

3. Sound waves in a tube closed at one end, (variable length). Put the cork and plunger into one end of the tube. Set the speaker facing the open end of the tube, but about 4 – 6 inches away. Set the sound sensor right at the tube opening. Set up Science workshop as before in experiment 3.

Using an approximate value of 343 m/s for the speed of sound in air, choose a frequency that will generate sound waves with a wavelength λ < 15 or 20 cm, about 1500 – 3000 Hz. Set the signal generator to this frequency and using the oscilloscope as before monitor the system for resonance.

Start with the plunger pulled all the way back, so the tube has its maximum length. Measure the effective length of the tube. Now vary the frequency up or down until a resonance is found.

Now, keeping the frequency constant, vary the length of the tube by pushing in the plunger slowly. Measure the effective length of the tube for each resonance.

What is the wavelength? Draw a diagram of the successive tube configurations and convince yourself that the change in length is equal to λ/2. Use

v = λf

to find the speed of sound in air. Compare this value to

vair = 331 m/s + 0.59 (Temp oC) m/s.

Repeat the experiment for a different frequency.

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download