## Hands-On-Physics: SOUND - Extension #2

### SOUND INTENSITY IN A CLOSED SPACE

In a closed cardboard tube, measure the sound intensity at various distances from a speaker fastened to one end.

This extension involves measurements of sound intensity in a closed space. You will need learn about the superpostion of waves and build experimental apparatus for investigating standing sound waves.

#### INTRODUCTION

Generally speaking, the intensity of a sound wave diminishes as it moves away from its source. This is obvious from experience. You can reduce noise levels by walking away from the source. An investigation of this question in extensions #1 will yield a similar conclusion. When a sound wave leaves its source, its total energy is set. As the wave front spreads out, the energy associated with the wave must also spread out. Intensity is a measure of energy density, so spread out energy means low intensity, and concentrated energy is high intensity.

This is what we expect if the sound wave is free to spread out. What if sound is enclosed in a containter? Does sound intensity decrease in a linear fashion where each additional increment of distance produces the same drop in intensity? What is the pattern of sound intensity as you move away from the source in a closed tube?

#### EXPERIMENT

To adjust for differences in the length and diameter of various cardboard tubes and in the frequency of various tone generators, the length of the tube must be "set" with the plunger. In this investigation, try several different plunger postions. Look for patterns. When you complete this extension, you should have a table of distances and corresponding sound intensities. A graph might clarify any pattern in your data.

#### ISSUES TO CONSIDER

1. Measuring distances:
You are responsible for designing a method for measuring the distance from the speaker to the microphone. Depending on the equipment available in your laboratory, this can be done in many ways.

2. How many data points?
A decision about the number of data points to be collected is required. You need a rational strategy for confronting this problem.

3. Is there a pattern?
Scientific investigations search for patterns. What pattern(s) did you discover?

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