Properties of Sound – Chapter 14
Sound Frequency
•
•
•
•
Infrasonic Below 20 Hz
Audible Range 20 Hz to 20 000 Hz
Ultrasonic Above 20 kHz
Bats can detect frequencies as high as 120 000
Hz.
• Dolphins can detect frequencies as high as
200000 Hz.
• Elephants possesses the unusual ability to detect
infrasound, having an audible range from
approximately 5 Hz to approximately 10 000 Hz.
Pitch and Frequency
• The sensation of a
frequencies is commonly
referred to as the pitch
of a sound.
• A high pitch sound
corresponds to a high
frequency sound wave
• A low pitch sound
corresponds to a low
frequency sound wave.
Example 1
Two notes which have a frequency ratio of 2:1 are
said to be separated by an octave. A frequency
which is separated by an octave from middle C
(256 Hz) is
a. 128 Hz
b. 254 Hz
c. 258 Hz
d. 345 Hz
e. none of these
Example 1
Two notes which have a frequency ratio of 2:1
are said to be separated by an octave. A
frequency which is separated by an octave
from middle C (256 Hz) is
Two notes separated by an
a. 128 Hz
octave have a frequency
b. 254 Hz
ration of 2:1. If a note is one
c. 258 Hz
octave below 256 Hz, then it
must have one-half the
d. 345 Hz
frequency.
e. none of these
Sound Intensity
• The amount of energy which is transported past a
given area of the medium per unit of time is
known as the intensity of the sound wave.
• The greater the amplitude of vibrations of the
particles of the medium, the greater the rate at
which energy is transported through it, and the
more intense that the sound wave is
• Intensity of a point source is inversely proportional
to the square of the distance from the source.
Intensity of Sound
Intensity of a point source
is inversely proportional
to the square of the
distance from the source.
Threshold of Sound
• The faintest sound which the typical human ear
can detect has an intensity of 1*10-12 W/m2.
• The faintest sound which a human ear can detect
is known as the threshold of hearing, TOH.
• The most intense sound which the ear can safely
detect without suffering any physical damage is
more than one billion times more intense than
4
2
the threshold of hearing. 1*10 W/m or 160 dB
Sound intensity
• The scale for measuring intensity is the decibel
scale.
• The threshold of hearing is assigned a sound level of
0 decibels (abbreviated 0 dB); this sound
corresponds to an intensity of 1*10-12 W/m2.
# of Times
Greater Than
TOH
Source
Intensity
Intensity
Level
Threshold of Hearing (TOH)
Whisper
1*10-12 W/m2
0 dB
100
1*10-10 W/m2
20 dB
102
Normal Conversation
Busy Street Traffic
Vacuum Cleaner
Large Orchestra
1*10-6 W/m2
60 dB
106
1*10-5 W/m2
70 dB
107
1*10-4 W/m2
80 dB
108
6.3*10-3 W/m2
98 dB
109.8
1*10-2 W/m2
100 dB
1010
1*10-1 W/m2
110 dB
1011
1*101 W/m2
130 dB
1013
1*102 W/m2
140 dB
1014
1*104 W/m2
160 dB
1016
Walkman at Maximum
Level
Front Rows of Rock Concert
Threshold of Pain
Military Jet Takeoff
Instant Perforation of
Eardrum
Hearing Loss Is Permanent
Example 2
A mosquito's buzz is often rated with a decibel
rating of 40 dB. Normal conversation is often
rated at 60 dB. How many times more intense is
normal conversation compared to a mosquito's
buzz?
a. 2
b. 20
c. 100
d. 200
e. 400
Example 3
On a good night, the front row of the Twisted
Sister concert would surely result in a 120 dB
sound level. An IPod produces 100 dB. How
many IPods would be needed to produce the
same intensity as the front row of the Twisted
Sister concert?
• The speed of the sound wave depends of
the properties of the medium it travels
through
– IMFs and elasticity (called elastic modulus)
– Inertial properties and Density
• In general, solids have the strongest
interactions between particles, followed by
liquids and then gases
• A highly elastic material causes vibrational
disturbances to propagate faster because
the restoring forces are higher
• vsolids > vliquids > vgases
SPEED OF SOUND IN AIR
• The speed of a sound wave in air depends
upon the properties of the air
– temperature and pressure
• What is the speed of sound in air at room
temperature (20 oC)?
USING THE SPEED OF SOUND TO
MEASURE DISTANCES
• At normal atmospheric pressure and a
temperature of 20 degrees Celsius, a sound wave
will travel at approximately 343 m/s
• Light travels through air at a speed of
approximately 300 000 000 m/s
• The arrival of the light wave from the location of
the lightning strike occurs in so little time that it is
essentially negligible. Yet the arrival of the sound
wave from the location of the lightning strike
occurs much later.
EXAMPLE FOUR:
THE LIGHTENING STRIKE
• During a storm, the air temperature
is 21 C. A lightening strike is observed
and only 3 seconds later, a clap of
thunder can be heard.
• How far away did the lightening
strike occur (in meters and in miles)?
ECHOLOCATION
• Echo: perceived time delay between the
production of a sound and it’s reflection
from a boundary.
• EXAMPLE 5: If a person on one side of a
canyon hollers and the echo is heard 1.40s
later, how far away is the other canyon wall
(at 21o C)?
v = f
WAVE SPEED CAN BE CALCULATED BY
FREQUENCY AND WAVELENGTH
• the wave speed is not dependent upon these
quantities.
• An alteration in wavelength DOES NOT affect wave
speed. Rather, an alteration in wavelength affects
the frequency in an inverse manner.
• The speed of a sound wave depends on the
properties of the medium through which it moves
and the only way to change the speed is to change
the properties of the medium.
Example Six
An automatic focus camera is able to focus on
objects by use of an ultrasonic sound wave.
The camera sends out sound waves which
reflect off distant objects and return to the
camera. A sensor detects the time it takes for
the waves to return and then determines the
distance an object is from the camera.
If a sound wave (speed = 340 m/s) returns to
the camera 0.150 seconds after leaving the
camera, how far away is the object?
Example Seven
On a hot summer day, a pesky little mosquito
produced its warning sound near your ear. The
sound is produced by the beating of its wings
at a rate of about 600 wing beats per second.
A. What is the frequency in Hertz of the sound
wave?
B. Assuming the sound wave moves with a
velocity of 350 m/s, what is the wavelength
of the wave?
The Doppler Effect
• The Doppler effect is a phenomenon observed
whenever the source of waves is moving with
respect to an observer.
• There is an apparent upward shift in
frequency for the observer when the source of
sound is approaching and …
• an apparent downward shift in frequency
when the observer and the source is receding
• As the car approached with its siren blasting,
the pitch of the siren sound (a measure of the
siren's frequency) was high; and then
suddenly after the car passed by, the pitch of
the siren sound was low
During target practice, a man shoots a 7 g bullet
with a horizontal velocity of 150 m/s directly at a 3
kg pumpkin sitting on a post.
– If the bullet embeds in the pumpkin, how fast will the
pumpkin be knocked off the post?
– If the post is 1 meter tall, how much time will it take the
pumpkin to strike the ground?
– How far from the base of the post will the pumpkin
strike the ground?
– What will be the pumpkin's resultant impact velocity
just as it strikes the ground?
– If the dirt where the pumpkin strikes the ground offers
an average resisting force of 750 N, how deep of a
"dent" will the pumpkin make upon impact as it is being
brought to a rest?
– The shooter heard the bullet strike the pumpkin 0.9s
after releasing it. If the air temperature on that day was
78 oF, how far away was the pumpkin on the post?
Independent Practice
• P. 498-504; 1, 2, 3, 29, 30, 32, 34, 41, 45