4/2 do now
• [explain your answer] The wavelength of a
wave doubles as it travels from medium A into
medium B. Compared to the wave in medium
A, the wave in medium B has
a. half the speed
b. twice the speed
c. half the frequency
d. twice the frequency
13-1 sound waves
1.
2.
3.
4.
Explain how sound waves are produced.
Relate frequency to pitch.
Compare the speed of sound in various media.
Recognize the Doppler effect, and determine the
direction of a frequency shift when there is relative
motion between a source and an observer.
Homework
Castle learning
Reading 13.2
Quarter ends Friday, No post Friday
The production of sound waves
• Sound waves, like all waves, are produced by VIBRATING
objects, such as tuning fork, vocal cord, string on guitar, etc.
• Sound is a MECHANICAL wave that propagates through a
MEDIUM from one location to another.
• Sound is a LONGITUDINAL wave. The vibrations of medium is
PARALLEL to the direction of wave motion.
•
The frequency of a sound wave, like all waves, is the same as its originator.
•
Sound cannot travel through vacuum
• High pressure regions are known as compressions and low
pressure regions rarefactions.
•
•
The wavelength is commonly measured as the distance from one compression to
the next adjacent compression.
Since a sound wave consists of a repeating pattern of high pressure and low
pressure regions moving through a medium, it is sometimes referred to as a
pressure wave.
Exmaple #1
1. A sound wave is different than a light wave in that a
sound wave is
a. produced by an oscillating object and a light wave is
not.
b. not capable of traveling through a vacuum.
c. not capable of diffracting and a light wave is.
d. capable of existing with a variety of frequencies and
a light wave has a single frequency.
Example #2
•
1.
2.
3.
4.
5.
A sound wave is a pressure wave; regions of high
(compressions) and low pressure (rarefactions) are
established as the result of the vibrations of the sound
source. These compressions and rarefactions result because
sound
is more dense than air and thus has more inertia, causing
the bunching up of sound.
waves have a speed which is dependent only upon the
properties of the medium.
is like all waves; it is able to bend into the regions of space
behind obstacles.
is able to reflect off fixed ends and interfere with incident
waves
vibrates longitudinally; the longitudinal movement of air
produces pressure fluctuations.
Example #3
•
1.
2.
3.
4.
As a sound wave travels through air, there is
a net transfer of
energy, only
mass, only
both mass and energy
neither mass nor energy
Characteristics of sound waves
• Frequency determines the pitch.
– The ears of a human (and other animals) are sensitive
detectors of the fluctuations in air pressure which impinge
upon the eardrum.
The sensation of a frequency of sound is commonly referred to as
the pitch. A high pitch sound corresponds to a high frequency
sound wave and a low pitch sound corresponds to a low frequency
sound wave.
• Infrasound and ultrasound
– The human ear is capable of detecting sound waves
with a wide range of frequencies, ranging between
approximately 20 Hz to 20 000 Hz. Any sound with
a frequency below the audible range of hearing (i.e.,
less than 20 Hz) is known as an infrasound and any
sound with a frequency above the audible range of
hearing (i.e., more than 20 000 Hz) is known as an
ultrasound.
– Ultrasound waves can produce images.
• Speed of sound depends on the medium
– Sound is a mechanical wave, it depends on the
medium interaction to travel. The speed of sound
waves depends on how fast the particles can
interact with each other. In general, sound wave
travels faster in denser materials.
vsolids > vliquids > vgases
• Sound in air
– The speed of a sound wave in air depends upon the properties
of the air, namely the temperature and the pressure.
– The speed of sound wave at STP is 331 m/s
– At normal atmospheric pressure, the temperature dependence of
the speed of a sound wave through air is approximated by the
following equation:
v = 331 m/s + (0.6 m/s/oC)•T
• where T is the temperature of the air in degrees Celsius.
– Using this equation, we can determine the speed of a sound
wave in air at a temperature of 20 degrees Celsius:
v = 331 m/s + (0.6 m/s/oC)•(20oC)
v = 331 m/s + 12 m/s
v = 343 m/s
• Using Wave Speed to Determine Distances
– At normal atmospheric pressure and a temperature of 20oC, a
sound wave will travel at approximately 343 m/s;
– The speed of a sound wave is slow in comparison to the
speed of a light wave. Light travels through air at a speed of
approximately 3 x 108 m/s;
– The time delay between the arrival of the light wave
(lightning) and the arrival of the sound wave (thunder) allows
a person to approximate his/her distance from the storm
location.
– For instance if the thunder is heard 5 seconds after the
lightning is seen, then sound has traveled a distance of
d = v • t = 345 m/s • 5 s = 1715 m, which means the storm is
about one mile away.
Every 5 seconds is about a mile.
• Another phenomenon related to the perception of time
delays between two events is an echo.
• For instance if an echo is heard 1.40 seconds after
making the holler, then the distance to the canyon wall
can be found as follows:
distance = v • t = 345 m/s • 0.70 s = 242 m
• Echolocation is a physiological
process for locating distant or
invisible objects (as prey) by
sound waves reflected back to
the emitter (as a bat) from the
objects
• Sound wave propagate in three dimensions
The Wave Equation Revisited
• Like any wave, a sound wave has a speed which is
mathematically related to the frequency and the wavelength of
the wave.
• Speed = Wavelength • Frequency
v=f•λ
• Even though wave speed is calculated using the frequency and
the wavelength, the wave speed is not dependent upon these
quantities.
– An alteration in wavelength does not affect (i.e., change)
wave speed. Rather, an alteration in wavelength affects the
frequency in an inverse manner.
– A doubling of the wavelength results in a halving of the
frequency; yet the wave speed is not changed.
– 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.
Class work – today’s date
1. What occurs when sound passes from air into
water?
a. Its speed decreases, its wavelength becomes
smaller, and its frequency remains the same.
b. Its speed decreases, its wavelength becomes
smaller, and its frequency increases.
c. Its speed increases, its wavelength becomes larger,
and its frequency remains the same.
d. Its speed increases, its wavelength becomes larger,
and its frequency decreases.
2. A sound wave is produced by a musical instrument for 0.50
second. If the frequency of the wave is 360 hertz, how many
complete waves are produced in that time period?
3. An automatic focus camera is able to focus on objects by use
of an ultrasonic sound wave. The camera sends out sound
waves that 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?
4. 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.
b.
What is the frequency in Hertz of the sound wave?
Assuming the sound wave moves with a velocity of 350 m/s, what is
the wavelength of the wave?
5. True or False: Doubling the frequency of a wave source
doubles the speed of the waves.
6. Playing middle C on the piano keyboard produces a sound
with a frequency of 256 Hz. Assuming the speed of sound in
air is 345 m/s, determine the wavelength of the sound
corresponding to the note of middle C.
7. Most people can detect frequencies as high as 20 000 Hz.
Assuming the speed of sound in air is 345 m/s, determine the
wavelength of the sound corresponding to this upper range of
audible hearing.
8. An elephant produces a 10 Hz sound wave. Assuming the
speed of sound in air is 345 m/s, determine the wavelength of
this infrasonic sound wave.
9. The diagram shows a tuning fork vibrating in air. The dots
represent air molecules as the sound wave moves towards
the right. Which diagram below best represents the
direction of motion of the air molecules?
A
B
C
D
10. An electric bell connected to a battery is sealed inside a large
jar. What happens as the air is removed from the jar?
a. The electric circuit stops working because electromagnetic
radiation cannot travel through a vacuum.
b. The bell's pitch decreases because the frequency of the
sound waves is lower in a vacuum than in air.
c. The bell's loudness increases because of decreased air
resistance.
d. The bell's loudness decreases because sound waves
cannot travel through a vacuum.
11. A student plucks a guitar string and the
vibrations produce a sound wave with a
frequency of 650 hertz. The sound wave
produced can best be described as a
a. transverse wave of constant amplitude
b. longitudinal wave of constant frequency
c. mechanical wave of varying frequency
d. electromagnetic wave of varying
wavelengths
Doppler Effect
Stationary
source
Wave fronts: A wave front is the
curve of all adjacent points on a
wave that are in phase.
To observer A and observer B, the
frequency is the same everywhere.
Lower frequency,
longer λ
Higher frequency, short λ
Moving source
The Doppler effect can be described as the effect produced by a
moving source of waves, the observer, or both –
an apparent upward shift in frequency if the observers and the
source is approaching each other
an apparent downward shift in frequency if the observers and
the source is moving away from each other.
Relative motion creates an apparent change in frequency.
Explaining the Doppler Effect
• The Doppler effect is observed because the distance between the
source of sound and the observer is changing.
• If the source and the observer are approaching each other, then
the distance is decreasing and the waves is compressed into the
smaller distance. The observer perceives sound waves reaching
him or her at a more frequent rate (_______
pitch).
high
• If the source and the observer are moving apart, then the distance
is increasing. the waves can be spread apart; the observer
perceives sound waves reaching him or her at a less frequent rate
low
(____pitch).
• It is important to note that both the speed and the frequency of the
source does not change.
• The Doppler effect can be observed for any type of wave - water
wave, sound wave, light wave, etc.
• car horn - coming and going
• 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. That was the Doppler effect - an apparent shift in frequency
for a sound wave produced by a moving source.
Shock Waves and Sonic Booms
• If a moving source of sound moves at the same speed as
sound or faster than sound, then shock waves will be
produced.
• http://edweb.sdsu.edu/doppler/elab/elab1.htm
..\..\RealPlayer Downloads\Plane break sound barrier - sonic boom.flv
Blue shift and red shift
The human eye perceives light waves of different frequencies as
differences in color. The lowest frequency we can see is red and the
highest frequency we can see is blue-violet.
Due to Doppler effect, the apparent color of an approaching light
source is shifted toward the blue end of the spectrum, while that of
a receding source is shifted toward the red end.
Applications of the Doppler Effect
• Police work – the speed of a car is determined by a radar system.
– when a car is at rest, the sent out frequency is the same as
received frequency.
– If the car is moving toward the source of radar, the reflected
waves have higher frequency, the greater the car’s speed, the
greater the Doppler shift in frequency.
– If the car is moving away from the source of radar, the reflected
waves have lower frequency.
• Weather stations – Doppler radars are used to determine the
location and intensity of precipitation as well as directions and speed
of the winds blowing around rain drops.
Class work – today’s date
1. A police officer's stationary radar device
indicates that the frequency of the radar
wave reflected from an automobile is less
than the frequency emitted by the radar
device. This indicates that the automobile is
a. moving toward the police officer
b. moving away from the police officer
c. not moving
example
2. The diagram shows radar waves being emitted from a
stationary police car and reflected by a moving car back to
the police car. The difference in apparent frequency
between the incident and reflected rays is an example of
a. constructive interference
b. refraction
c. the Doppler effect
d. total internal reflection
3.
As observed from the Earth, the light from a star is shifted
toward lower frequencies. This is an indication that the
distance between the Earth and the star is
a. decreasing
b. increasing
c. Constant
4. Suppose you are standing on the passenger-loading platform
of the commuter railway line. As the commuter train
approaches the station, what pitch or changes in pitch will
you perceive as the train approaches you on the loading
platform?
5. A stationary research ship uses sonar to send a 1.18 × 103hertz sound wave down through the ocean water. The
reflected sound wave from the flat ocean bottom 324 meters
below the ship is detected 0.425 second after it was sent
from the ship. Calculate the wavelength of the sound wave in
the ocean water.
6. What is the frequency of a sound wave with a wavelength of
0.04 meter in air? What type of wave is this, transverse or
longitudinal?
7. A bat is using sound waves to locate an insect. The bat
produces sounds with a frequency of 120 kilo-hertz and
notes that the sound it transmits echo’s back in 0.02 second.
a. What is the bat’s distance to its prey?
b. What is the wavelength of the bat’s radar?
8. A radar station is tracking a dense section of cloud cover. If
the radar station transmits energy at a frequency of 300
megahertz and receives reflected energy from the clouds at a
frequency of 150 megahertz, what can the forecasters say
about the motion of the clouds?
9. If a ship sends a sonar signal through the water to the sea
floor and it bounces back to arrive back at the ship in 2.0
seconds, how deep is the water under the ship if sound
travels through water at 1500 meters per second?
10. If it takes a sound wave 2.5 seconds to travel from the spot
where a lightning strike occurs to an observer, how far away
from the observer did the strike occur?
11. Why would an astronaut not be able to communicate with
another astronaut by talking in the vacuum of space?
12. Circle the terms that properly complete the sentences below.
a.
b.
c.
Sound is transmitted as a TRANSVERSE / LONGITUDINAL wave.
Sound is produced as a(n) MECHANICAL / ELECTROMAGNETIC wave.
Sound travels more QUICKLY / SLOWLY in water than it does in air.
Explain why this is true!
13. The diagram below shows a source of sound waves moving with a
constant speed near an observer. The source produces sound waves
with a frequency of 100 hertz.
1)
Is the source getting closer to the observer or farther away?
2)
Which frequencies could the observer be hearing as the source
approaches?
(a) 80 Hz
3)
(b) 100 Hz
(c) 110 Hz
(d) 120 Hz
As the source approaches, will the frequency heard by the observer
be constant, increasing, or decreasing?
14. A train is moving at a constant 35 meters per second away from
an observer. As the train is moving it blasts its horn which
produces a sound with a frequency of 1000 hertz. The observer
will perceive that the horn’s frequency is
(1) less than 1000 hertz and constant
(2) less than 1000 hertz and decreasing
(3) greater than 1000 hertz and constant
(4) greater than 1000 hertz and increasing
15. A police car is accelerating toward an observer. The police car’s
siren produces a sound with a frequency of 1200 hertz. The
observer will perceive that the siren’s frequency is
(1) less than 1200 hertz and constant
(2) less than 1200 hertz and decreasing
(3) greater than 1200 hertz and constant
(4) greater than 1200 hertz and increasing
Lab – determine the speed of sound
Purpose (5 pt): To determine the speed of sound
Material (5 pt): computer, Vernier computer interface, Logger Pro, microphone,
tube, thermometer, meter stick
Procedure (10 pt):
–
Briefly describe how the lab is going to be done. Someone who was not
present during the lab should be able to understand how the experiment
was perforem and be able to reporduce the results by reading your
procedure.
Data section (20 pt):
–
The Data Section should include two tables of data with labeled column
headings (and units) to demonstrate a systematic study of the effect of
amplitude and length upon the period of the pendulum.
Data analysis / conclusion (20 pt):
–
Calculate the speed of sound. Remember that your time interval
represents the time for sound to travel down the tube and back.
–
The accepted speed of sound at atmospheric pressure and 0oC is 331.5
m/s. The speed of sound increaseds 0.607 m/s for every oC. Calculate the
speed of sound at the temperature of your room and compare your
measured value to the accepted value.
PROCEDURE
1. Connect the Vernier Microphone to Channel 1 of the interface.
2. Use a thermometer or temperature probe to measure the air temperature of the classroom
and record the value in the data table.
3. Open the file “Speed of Sound” in the Physics with Vernier folder. A graph of sound level vs.
time will be displayed.
4. Close the end of the tube. Measure and record the length of the tube in your data table.
5. Place the Microphone as close to the end of the long tube as possible, so that it can detect
the initial sound and the echo coming back down the tube.
6. Click
to begin data collection. Snap your fingers near the opening of the tube.
7. If you are successful, the graph will resemble the one below. Repeat your run if necessary.
The second set of vibrations with appreciable amplitude marks the echo. Click the Examine
button, . Move the mouse and determine the time interval between the start of the first
vibration and the start of the echo vibration. Record this time interval in the data table.
8.
Repeat the measurement for a total of five trials and determine the average time interval.
Length of tube
Temperature of room
DATA TABLES
Trial
1
2
3
4
5
Average
Average speed =
Total travel time
m
°C
(s)