HPP Activity 53v2
Do muscle, fat, and bone sound the same?
Exploration
You might expect that sound echoes in the body might be a little different than sound echoes
from a brick wall in a parking lot. We will explore how some of those differences facilitate
creating images of tissue.
Let's consider first the question of why an echo occurs. Consider a person standing in a parking
lot in front of a brick wall. He claps and hears an echo. The echo is a result of sound reflecting
from the wall. A fancy word for the air-brick surface is an interface, which is an abrupt change
in the medium carrying the sound wave.
Let's explore how to create an interface.
GE 1.
1. What changes when a sound wave goes from air to brick? Think about the
properties of the media through which the sound travels and the properties of
the sound itself.
2. How does the number of atoms in a cubic centimeter change as you go from
air into brick? Is this a relatively small or large change?
3. What physical property of a material is related to the number atoms per unit
volume?
Obtain a soft sponge. It should be slightly damp for the following activity.
4. What happens to the volume of the sponge as you press on it while it is
laying on the table?
5. Is a large pressure change necessary to reduce the volume by half?
Now consider the wooden block laying on your table.
6. Can you change the volume of the block by pressing on it? What if you
could press very hard, maybe by using a machine?
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 53v2
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7. Compare the pressures required to reduce the volume by half for the sponge
and for the wooden block.
8. What if the wooden block were replaced by a piece of Styrofoam or brick
the same size? How much pressure would be required to change the volume of
the Styrofoam? How much pressure would be required to change the volume
of the brick, even just a little?
9. In your own words, describe the property of a material that resists a change
in its volume when pressure is applied.
Invention
We can imaging creating an interface by making an abrupt change in density of the material, by
changing the "springiness" of the material, or by doing both.
In fluids or soft tissue the "springiness" of the material is described by the bulk modulus B,
defined as
P
(1)
V V 
where P is a change in pressure and V/V is the corresponding proportional change in volume.
B
Application
GE 2.
1. If you increase the pressure on the sponge, what happens to the volume of
the sponge?
2. Comparing the change in pressure to the corresponding change in volume,
should the bulk modulus be positive or negative? (Remember, in physics we
look at any change as “final – initial” so the sign of the change is important.)
Explain.
3. Compare the relative sizes of the bulk modulus for the sponge, the
Styrofoam, the wooden block and the brick. Again, think about the
relationship between a change in pressure and the corresponding change in
volume.
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 2010 The Humanized Physics Project
HPP Activity 53v2
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Exploration
In order to help explain how an interface might affect the speed of sound let’s first examine
waves moving through a uniform material. We will focus on what happens when the springiness
of the material changes, since that usually plays a bigger role in biological tissues than density
changes. Since mechanical waves in a spring are easier to visualize than sound waves, we will
use a simple spring as a model for what happens when we change the springiness of a material.
Obtain a long spring. With a partner, hold the ends of the spring so that it is extended. Create a
pulse on the spring. Now step back so that there is more tension in the spring. Again, create a
pulse.
GE 3.
1. Compare the pulse speeds between the low- and high-tension cases?
2. Does the linear mass density (m/L) of the spring change as you change the
tension? Explain.
To insure that we see primarily the effect of change in springiness, let's repeat
the experiment with a rubber hose. The change in length that occurs as the
tension increases is very small, so we will clearly see the effect of changing
only the tension.
Observe the video clip 0.5N250fps.mov.
2. Estimate the speed of the pulse.
Now observe the video clip 2.0N250fps.mov.
3. Estimate the speed of this pulse.
4. Compare the pulse speeds in the two cases. Remember that the tension was
four times greater for the second movie (2.0 N) than for the first movie (0.5 N)
while the linear mass density remained essentially constant.
5. If the tension in the spring or hose plays a role similar to bulk modulus in a
fluid, what should happen to sound speed as bulk modulus increases?
Activity Guide
 2010 The Humanized Physics Project
HPP Activity 53v2
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Invention
The speed of mechanical waves in a medium depends on two major factors, as seen in the
explorations above. It depends on an inertial property of the medium (related to mass), and on an
elastic property or "springiness" of the medium. Typically, the relationship is
v
elastic property
inertial property
(1)
For fluids or soft tissue, the inertial property that determines sound speed is density. So, the
speed of sound in a fluid or soft tissue is given by
v
B

(3)
A significant change in bulk modulus, creating a change in sound speed, will create an interface,
as we will investigate soon.
The two medium properties, density and sound speed, are combined to define the characteristic
acoustic impedance, Z, of the tissue.
Z = v
(4)
So, another way to define an interface for sound is to say that there is an abrupt change in the
characteristic acoustic impedance.
The following table shows some densities and sound speeds of tissue in the human body.
Tissue
v [m/s]
 [kg/m3]
muscle
1576
1.058 x 103
(along fibers)
muscle
1592
1.058 x 103
(across fibers)
liver
1570
1.055 x 103
kidney
1560
1.055 x 103
brain
1520
1.032 x 103
fat
1476
0.928 x 103
bone
3360
2.32 x 103
air
343
1.205
Table 1. from Diagnostic Ultrasound, Matthew Hussey, Blackie & Son Limited (London, 1975).
Application
Activity Guide
 2010 The Humanized Physics Project
HPP Activity 53v2
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GE 4.
1. Calculate the characteristic acoustic impendence for the materials in Table
Tissue
Z [kg/m2s]
muscle (along fibers)
muscle (across fibers)
liver
kidney
brain
fat
bone
air
2. Which tissue junctions could be considered well-defined acoustic interfaces
(where you might expect large echoes to arise from)?
Watch the movie Twosprings.mov. Pay careful attention to the pulse after the
point where the two springs are connected.
3. Describe the change in the height (or amplitude) and the width of the pulse
on each side of the connecting point.
4. Is the speed of the pulse the same in both springs? If you’re not sure, go
back and make some measurements of the pulse.
5. Based on your answer to question 4, do each of the springs have a different
impedance? Explain.
6. Is the connection point between the two springs an interface? Explain.
7. How much of the pulse bounces off (reflects from) the interface? How
much of the pulse is transmitted through the interface?
8. Recall the parking lot echo activity. Does any of the sound energy from the
clap pass through the brick wall? Does any bounce off? How do you know?
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 2010 The Humanized Physics Project
HPP Activity 53v2
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9. Compare your observations of the pulse on the two springs to the clap and
the parking lot echoes. How are they similar? How are they different?
We will revisit reflection and transmission in the next activity.
Activity Guide
 2010 The Humanized Physics Project