Measuring the Velocity of Sound

1. Department of Physics Measuring the Velocity of Sound Li Xianglian

2. Contents • Objectives • Pre-lab Questions • Introduction & Apparatus • Procedure & Experiment • Problems

3. Objectives • Learn how to measure the speed of sound. • Practice using an oscilloscope. • Learn to manipulate the evenly spaced data with the method of successive subtraction.

4. Pre-lab Questions • Which determines the speed of sound, the source frequency or the medium? • The distance between successive nodes or antinodes in a standing wave is a measure of what?

5. Sound is a kind of mechanical wave. Like any other waves, the wavelength , wave speed u and wave frequency f are related to by . Thus, given the f , we need only measure the in order to determine the speed u of sound. 3. Introduction & Apparatus Sound wave as a longitudinal wave

6. In this experiment, we use the so-called resonance method to measure the wavelength of the sound wave. If a sound wave propagates in a limited region, it will be reflected at the border, and the reflected wave would interfere with the original one, producing a standing superposed wave. Demonstration of a standing sound wave

7. One of the characteristics of a standing wave is that there are successive nodes or antinodes at intervals of half of the wave-length of the traveling waves. Distribution of nodes (N) and antinodes (A) in standing waves Therefore, if we get the distance of two adjacent anti-nodes, we can have the wavelength .

8. The apparatus to be used is shown as follows:

9. 4. Procedure & Experiment 4.1. Connect the wires as in the following figure.

10. 4.2. Set the oscilloscope to show the signal from S2. 4.3. Check to be sure that the plates of the two transducers are parallel. 4.4. Set the signal generator to produce a sinusoidal signal and choose an appro-priate frequency of the signal such that the crystal transducers resonate. Record the frequency value f on your notebook.

11. 4.5. Move S1 slowly and unidirectionally away from S2, and observe the received signal on the screen. Record the location one by one (as shown in the following table) where the antinodes of the standing wave appear.

12. 4.6. Calculate the sound speed in air. At first, calculate the mean intervals of the adjacent antinodes using the method of successive subtraction as follows: Then calculate the sound speed in air according to .

13. 4.7. Measure the speed of sound in water. Use the lower pair of transducers instead of the upper ones (you need reconnect some of the wires). Fill the tank half full with water and place the transducers into the water. 4.8. Repeat steps 4.3-4.5 and calculate the sound speed in water the same way as that in step 4.6. 4.9. Calculate the theoretical value of the sound speed at room temperature in air,

14. where both T1 and T2are absolute temper-atures. And the theoretical value of the sound speed in air at and one atmos-phere pressure is given to be 330m/s. Compare it with the measured . using

15. Problems • What determines the pitch of a sound wave? The source, or the medium? • Where does sound travel faster? In air, or in water? End