Module 6

What Causes Sound Waves?

Student Guide

Robert Culbertson, Scott Surgent, Ami Carlon

INTRODUCTION

Wave motion is a fundamental aspect of nature. We can perceive waves through our senses as sound, visible light, and rumbles (vibrations), to name a few common examples. Waves can be visible, as in waves in a piece of vibrating string or as ripples on the surface of water. Some kinds of waves invisible, such as sound waves. But all waves behave in the same way. What causes the sensation we recognize as sound? What must happen for sound to be created? How is noise different from a melody? How are different tones generated? What carries sound across a room? How fast does sound travel?

OBJECTIVES

To determine the speed that sound waves travel.

MATERIALS: Part I

mechanical vibrator	rods (at least 2)
wave function generator	leads
table clamps (at least 3)	elastic string, 1 meter

PROCEDURE: Part I

Note: The mechanical vibrators can only accept a limited range of frequency. You may damage the vibrator if the frequency is too high. A frequency of 100 Hz is considered high. When the Wave Function Generator is turned on, it is set at 1000 Hz by default, hence the need to adjust the amplitude knob to min at the start.

Obtain the materials and set up the apparatus as in the illustration. Be sure to tie the string securely and to allow slight slack (it is not necessary, and probably harmful to the vibrator, to have the string taut).
Before turning on the Wave Function Generator, turn the amplitude knob to min. Turn on the Wave Function Generator, and adjust the frequency knob to near zero (it won't go to exactly zero). Ready the vibrator by unlocking the switch.
Adjust the amplitude knob to near max. Slowly adjust the frequency knob and watch the behavior of the string. Experiment, and describe in your notes anything of interest that you observe.
Consider adding or subtracting length or slackness to the string, and repeat your observations.
Can you make any conclusions based on your observations? By adjusting some of the apparatus' parameters, do your conclusions remain the same? Explain why or why not.

MATERIALS: Part II

wave function generator	Electrical leads, 1/8 inch jack
small speaker	CBL or MBL
Macintosh computer	Tube (mail tube is acceptable)
software?	tubes of varying length, several
TI-83 calculator	microphone probe
PHYSICS program(TI-83)	MICROPHONE program (TI-83)
Sound 4.5 program

PROCEDURE: Part II

Obtain the materials and set up the apparatus as in the illustration.
(CBL) Ready the CBL, and run the program PHYSICS. Follow the directions on your calculator. Select MICROPHONE when asked to do so.
(MBL) Ready the < a href="http://www.ti.com">MBL, and run the Sound4.5 program. Suggestions: Under Collect, select Triggering and turn it off. Adjust the y-axis (Sound-Pressure) to a range of -100 to 100.
Adjust the amplitude knob to min, turn on the Wave Function Generator. In this exploration, keep the amplitude fairly low. You can adjust the volume using the volume switch on the speaker.
Adjust the frequency knob, and view the graphs generated on your viewscreen. Make note of anything interesting, and explore various settings.
Use the CBL (or MBL) and microphone probe to determine the speed of sound. After you have selected MICROPHONE on the TI-83, select WAVEFORM/TRIGGER, and follow the directions.
Hold the microphone at one end of the tube, and block the other end. Create a dull sound (snapping your fingers is excellent), and view the resulting graphs. Repeat to get a feel for the program and the microphone's sensitivities.
Use TRACE to explore the behavior of the graph on your TI-83.
On the MBL, set the Triggering on. Do the same actions as for the CBL.
From the data that you have collected find the speed of sound.