HPP Activity A50v1
Can you hear me?
Exploration
Let's explore just how good your hearing is. Go to the web page "Sound Levels" in the course
web. Turn the volume on your computer speakers up to the half way point. Choose a person to
listen to the sounds.
GE 1.
1. The test subject should listen to the 6 sounds. Can you hear all of the
sounds?
2. Order the sounds from softest to loudest. Write down the ordering here.
3. Choose a second test subject, who should listen to the 6 sounds. Can you
hear them all?
4. Have the second test subject order the sounds.
5. How do the two test subjects compare?
6. Do you think sound waves carry energy?
7. What is the evidence for your answer?
8. How do you think the energy in the different sound waves compares?
Invention
We need a way to describe sound intensities quantitatively. Since sound intensity is related to
the energy transported by the wave let's review some energy-related terms.
GE 2.
Activity Guide
 2010 The Humanized Physics Project
Supported in part by NSF-CCLI Program under grants DUE #00-88712 and DUE #00-88780
HPP Activity A50v1
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Suppose you lift a 1.0 [kg] block from the floor to a height of 2.0 [m] in 0.50
[s] at constant speed.
1. How much work must you do?
2. How much power are you using in this process?
Observe the two video clips, SquareBarrier1.mov and SquareBarrier2.mov.
3. How do you think the total power in the waves compares for the two clips?
4. How do you think the power deposited into the barriers compares between
the two clips?
We can make a quantitative description of the power being deposited onto a surface by defining
intensity as
I
P
A
(1)
where P is the power in the wave and A is the area of the surface perpendicular to the wave. The
threshold of normal hearing is usually taken to be an intensity of 1.0x10-12 [W/m2]. The
threshold of experiencing pain is an intensity of 1.0 [W/m2].
GE 3.
1. Why might the normal SI unit of intensity, watts per meter squared, be an
inconvenient unit when discussing human hearing?
To eliminate problems with the wide range in numbers when dealing with intensity
measurements in the context of human hearing, we will often use a relative measure of intensity
called the decibel level, . It is defined from
  10 log
I
I0
(2)
In this definition, I is the intensity measured in watts per meter squared, I0, is the intensity
associated with the threshold of human hearing, 1.0x10-12 [W/m2], and the logarithm is the base
10 logarithm.
Activity Guide
 2010 The Humanized Physics Project
HPP Activity A50v1
Application
GE 4.
1. What is the decibel level for a sound at the threshold of human hearing?
2. What is the decibel level for a sound at the threshold of pain for human
hearing?
3. Suppose a speaker produces a sound with decibel level of 10 dB. A second
speaker also produces a sound with intensity of 10 dB. The two speakers are
placed next to each other, effectively doubling the intensity of the sound you
can hear. What is the decibel level of the combination? Make sure you
explain how you arrive at the answer.
4. Develop a general rule for how the decibel level changes when the intensity
of the source doubles.
5. Suppose a speaker produces a sound with decibel level of 10 dB. Now the
intensity is increased to 20 dB. By how much did the intensity increase in
Watts per meter squared?
Exploration
How does the intensity of a sound source vary as you move away from it? Of course, it is
reasonable to say the intensity gets smaller, but physicists are more interested in a quantitative
prediction.
Obtain a sound intensity meter.
Choose a person to be the sound source.
GE 5.
1. Before carrying out an exploratory experiment, consider this problem.
Mary produces a sound of constant intensity. Tomoko stands 2 meters away
and measures the intensity to be 45 [dB]. If she moves away so that the
Activity Guide
 2010 The Humanized Physics Project
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HPP Activity A50v1
intensity, measured in [Watts/m2] is half the intensity at the first position,
what will the decibel level be?
2. Get the sound producer to stand at a fixed location. Let the initial position
of the sound observer be two meters away. How far from the source do you
think the observer must be to observe a sound intensity, measured in
Watts/m2, that is half the value at two meters?
3. Now make the actual observations. How far away did the observer have to
move?
4. How was your prediction?
Invention/Application
We will develop a mathematical model for how sound intensity varies with distance from the
source. To make things a little easier, assume that the source is a point that produces spherical
waves moving away in all directions. We will use a computer simulation to collect data, since
obtaining a real source of spherical waves is a little difficult. The computer simulation can be
accessed from the "Intensity and Distance" web page.
GE 6.
1. eXplore: Using the computer simulation, describe how the sound intensity
varies as you move the microphone away from the source.
2. Record: Identify the relevant variables that you must record. Make a
diagram that helps define variables. Collect necessary data systematically.
Description of data to be collected:
Diagram:
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HPP Activity A50v1
Data:
3. Apply: Calculate the actual physical intensity (in Watts/m2 ) for each
measurement. Develop a mathematical model of I versus r for your data.
Numerical:
Graphical:
Verbal:
Equation:
4. Scrutinize: Is your model consistent with the qualitative observations you
made at the beginning?
5. How does your model compare with ones obtained by other groups?
Activity Guide
 2010 The Humanized Physics Project
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