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Hands-On-Physics:
SOUND - Extension #1
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SOUND INTENSITY IN THE OPEN
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This extension involves measurements of sound intensity. For this investigation
you will need to build a Sound Level Meter.
You will also need a sound source. The tone generator you built
in the Core Project will serve quite well if you add a speaker.
INTRODUCTION
Like many topics in physics, the relationship between the intensity of a sound
wave and the distance from its source is subtle and potentially complicated. The
models presented here reflect a number of simplifying assumptions including a
point source, a uniform medium, and no dissipation of wave energy. As a result,
this model is inaccurate in some real world situations. However, as a first
approximation, it is quite useful and appropriate in a wide variety of
circumstances.
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. Analysis of the question yields 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.
While the broad relationship is clear, the details are not so obvious. Does
sound intensity decrease in a linear fashion where each additional increment of
distance produces the same drop in intensity? Or is the intensity decrease with
distance nonlinear? For instance, is there gradual decrease in intensity at
first followed by a more rapid decline at greater distances?
EXPERIMENT
In an open space,measure the sound intensity directly in front of a speaker at a variety of distances from that
speaker. When you complete this section, you should have a table of distances
and corresponding sound intensities. You may be able to establish a mathematical formual
that models the relationship between sound intensity and distance from the source.
ISSUES TO CONSIDER
- Placement of your speaker
For this experiment, "open space" means outside (that's why the
equipment is battery-powered) or as large a room as possible.
Within your "open space", place your apparatus in a position that
minimizes the effect of reflected sounds and sounds produced by
other experimenters. Remember, you are trying to measure the
sound intensity produced directly by your speaker, not from other
sources.
- 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.
- Measuring intensity
Use the sound-level meter to measure the intensity of sound at
each of the distances you have chosen. Your voltmeter will not
show decibels (special sound intensity units), it will show plain volts.
That's OK, because you do not need to compare readings with other
experimeters who might be using different sound-level meters.
The voltmeter reading at each distance is proportional to the sound intensity there.
Record the voltage at each distance
- How many data points? over what distance?
Your goal is to discover the functional relationship between
distance and sound intensity for your speaker. Your result will be
ambiguous if too few points are taken over a distance that is too
small. You will be working too hard if too many points are taken
over a distance that is too large. This issue confronts scientists
every time they conduct an experiment. Try to develop a rational
strategy for confronting this problem.
- Modeling
Make a graph of values, using distance as the independant variable
(horizontal axis), and intensity as the dependent variable.
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