Transcript Sound - Ms. Lisa Cole-

Sound
Chapter 15
Topics for Sound
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Sound wave properties
Speed of sound
Echoes
Beats
Doppler shift
Resonance
Anatomy of Ear
Sound Wave
Properties
Sound Waves are
Longitudinal Waves
The air molecules shown below are either
compressed together, or spread apart. This
creates alternating high and low pressure.
Frequency
• The frequency of a sound wave (or any
wave) is the number of complete vibrations
per second.
• The frequency of sound determines its
pitch.
The higher the frequency,
the higher the pitch
Wavelength
• Wavelength is the distance between two high pressures, or
two low pressures. This property is dependent on the
velocity of the sound and it’s frequency.
• Wavelength and frequency are inversely related.
• Short wavelength (high frequency) results in a high pitch.
Frequency and the human ear
• A young person can hear pitches with frequencies from
about 20 Hz to 20000 Hz. (most sensitive to frequencies
between 1000 and 5000 Hz).
• As we grow older, our hearing range shrinks, especially at
the high frequency end.
• By age 60, most people can hear nothing above 8000 Hz.
• Sound waves with frequencies below 20 Hz are called
infrasonic.
• Sound waves with frequencies above 20000 Hz are called
ultrasonic.
The Amplitude of a Sound
Wave Determines its
loudness or softness
Velocity of Sound
The velocity of sound depends on
• the medium it travels through
• the temperature of the medium
• Sound travels faster in liquids than
in air (4 times faster in water than in
air)
• Sound travels faster in solids than in
liquids (11 times faster in iron than
in air)
• Sound does not travel through a
vacuum (there is no air in a vacuum
so sound has no medium to travel
through)
• The speed depends on the elasticity
and density of the medium.
Effects of Temperature
• In air at room temperature, sound
travels at 343m/s (~766 mph)
• v = 331 m/s + (0.6)T
– v: velocity of sound in air
– T: temperature of air in oC
• As temperature increases, the velocity
of sound increases
Relationship between
velocity, frequency, and
wavelength
• V = f
• V = velocity of sound
•  = wavelength of sound
• f = frequency of sound
Echoes:
REFLECTION
Echoes are the result of
the reflection of sound
Sound waves leave a source,
travel a distance, and bounce
back to the origin.
Things that use echoes...
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Bats
Dolphins/ Whales
Submarines
Ultra sound
Sonar
REFRACTION OF
WAVES
Refraction of Sound
• as the sound wave
transmits into the
warmer air at lower
levels, they change
direction, much like
light passing through
a prism
DIFFRACTION:
THE BENDING OF WAVES
THROUGH A SMALL OPENING
BENDING OF A WAVE
Sound waves move out like this:
•http://www.kettering.edu/~drussell/Demos/doppler/doppler.html
But when they move, the front of the wave
gets bunched up (smaller wavelength) and
the back of the wave starts to expand
(larger wavelength):
•http://www.kettering.edu/~drussell/Demos/doppler/doppler.html
Observer C hears a high pitch (high frequency)
Observer B hears the correct pitch (no change in frequency)
Observer A hears a low pitch (lower frequency)
•http://www.kettering.edu/~drussell/Demos/doppler/doppler.html
When the source goes faster, the wave
fronts in the front of the source start to
bunch up closer and closer together,
until...
The object actually starts to go faster
than the speed of sound. A sonic boom
is then created.
•http://www.kettering.edu/~drussell/Demos/doppler/doppler.html
Doppler Effect
• The doppler effect is a change in the
apparent frequency due to the motion of the
source or the receiver.
• Example: As an ambulance with sirens
approaches, the pitch seems higher. As the
object moves by the pitch drops.
Police use the Doppler Shift when
measuring your speed with radar
• A frequency is sent out of the radar gun
• The sound wave hits the speeding car
• The frequency is changed by the car moving
away from the radar and bouncing back
• The amount the frequency changes
determines how fast you are going
• The faster you are going, the more the
frequency is changed.
Equation that describes the doppler
effect.
fd = fs (v + vd)
(v - vs)
fs is the actual frequency being emitted
fd is the perceived frequency as the source approaches or
recedes
vd is (+) if the observer moves toward the source
vd is (-) if the observer moves away from the source
vs is (+) if the source moves toward the observer
vs is (-) if the source moves away from the observer
Example
• Sitting at Six Flags one afternoon, Mark
finds himself beneath the path of the
airplanes leaving Hartsfield International
Airport. What frequency will Mark hear as
a jet, whose engines emit sounds at a
frequency of 1000 Hz, flies toward him at a
speed of 100 m/sec? (temp is 10oC)
Solution
• v = 331 + (0.6)T
v = 331 + (0.6)(10)
v = 337 m/s
• fd = fs(v + vd)
(v – vs)
f=?
fs = 1000 Hz
vd = 0 m/s
vs = 100 m/s
Solution
f= 1000 (331 + 0)
(331 – 100)
f = 1430 m/s
SOUND INTENSITY:
THE LOUDNESS OF SOUND
Sound Intensity
• The intensity of a sound is the amount of energy
transported past a given area in a unit of time.
• Intensity = power/area
• The greater the amplitude, the greater the rate at
which energy is transported-the more intense the
sound
• Intensity is inversely related to the square of the
distance. As distance increases, the intensity
decreases.
Threshold of Hearing
• The human ear is sensitive to variations in
pressure waves, that is, the amplitude of sound
waves.
• The ear can detect wave amplitudes of 2x10-5 Pa
up to 20 Pa.
• The amplitudes of these waves are measured on a
logarithmic scale called sound level.
• Sound level is measured in decibels (dB).
DECIBEL
• MEASURES THE
LOUDNESS OF
SOUND
• RELATES TO THE
AMPLITUDE OF
THE WAVE
• EVERY INCREASE
OF 10dB HAS 10x
GREATER
AMPLITUDE
Source of Sound
Level (dB)
Increase over
Threshold
Threshold
0 dB
0
Normal Breathing
10 dB
10
Whisper
20 dB
100
Normal Conversation
60 dB
106
Busy street traffic
70 dB
107
Vacuum cleaner
80 dB
108
Average factory
90 dB
109
IPod at maximum level 100 dB
1010
Threshold of pain
120 dB
1012
Jet engine at 30 m
140 dB
1014
Perforation of eardrum
160 dB
1016
A SOUND 10 TIMES AS
INTENSE IS PERCEIVED
AS BEING ONLY TWICE AS
LOUD
NOISE POLLUTION
· Prolonged exposure to noise greater than
85-90 dB may cause hearing loss
· Brief exposures to noise sources of 100130 dB can cause hearing loss
· A single exposure to a level of 140 dB or
higher can cause hearing loss
EXPOSURE TO LOUD NOISE
Hours Per Day
8
4
2*
1
0.5
Noise Level (dB)
90
95
100*
105
110
Reducing Sound Intensity
• Cotton earplugs reduce sound intensity by
approximately 10 dB.
• Special earplugs reduce intensity by 25 to 45 dB.
• Sound proof materials weakens the pressure
fluctuations either by absorbing or reflecting the
sound waves.
• When the sound waves are absorbed by soft
materials, the energy is converted into thermal
energy.
Resonance
Natural Frequency
• Nearly all objects when hit or disturbed will
vibrate.
• Each object vibrates at a particular frequency or
set of frequencies.
• This frequency is called the natural frequency.
• If the amplitude is large enough and if the natural
frequency is within the range of 20-20000 Hz,
then the object will produce an audible sound.
Timbre
• Timbre is the quality of the sound that is
produced.
• If a single frequency is produced, the tone is pure
(example: a flute)
• If a set of frequencies is produced, but related
mathematically by whole-number ratios, it
produces a richer tone (example: a tuba)
• If multiple frequencies are produced that are not
related mathematically, the sound produced is
described as noise (example: a pencil)
Factors Affecting Natural Frequency
• Properties of the medium
• Modification in the wavelength that is
produced (length of string, column of air in
instrument, etc.)
• Temperature of the air
Resonance
• Resonance occurs when one object vibrates at the
same natural frequency of a second object, forcing
that second object into vibrational motion.
• Example: pushing a swing
• Resonance is the cause of sound production in
musical instruments.
• Energy is transferred thereby increasing the
amplitude (volume) of the sound.
•
http://www.pbs.org/wgbh/nova/bridge/meetsusp.html
Types of Resonance
• Resonance takes place in both closed pipe
resonators and open pipe resonators.
• Resonance is achieved when there is a standing
wave produced in the tube.
• Closed pipe resonators
– open end of tube is anti-node
– closed end of tube is node
• Open pipe resonators
– both ends are open
– both ends are anti-nodes
Closed pipe resonator
Harmonics of Closed Pipe
Resonance
• The shortest column of air that can have a pressure
anti-node at the closed end and a pressure node at
the open end is ¼ wavelength long. This is called
the fundamental frequency or first harmonic.
• As the frequency is increased, additional
resonance lengths are found at ½ wavelength
intervals.
• The frequency that corresponds to ¾ wavelength
is called the 3rd harmonic, 5/4 wavelength is called
the 5th harmonic, etc.
Open pipe resonator
Harmonics of Open Pipe Resonance
• The shortest column of air that can have nodes (or
antinodes) at both ends is ½ wavelength long. This
is called the fundamental frequency or first
harmonic.
• As the frequency is increased, additional
resonance lengths are found at ½ wavelength
intervals.
• The frequency that corresponds to a full
wavelength is the second harmonic, 3/2
wavelength is the third harmonic, etc.
Problems
1. Matt is playing a toy flute, causing
resonating waves in a open-end air column.
The speed of sound through the air column
is 336 m/s. The length of the air column is
30.0 cm. Calculate the frequency of the
first, second, and third harmonics.
Solution
1. L = λ/2
2xL=λ
2 x .30 = .60 m
v=fλ
336 = f (.60)
f = 560 Hz. (first harmonic)
2nd harmonic = 560 + 560 = 1120 Hz.
3rd harmonic = 1120 + 560 = 1680 Hz
Problem
2. Tommy and the Test Tubes have a concert
this weekend. The lead instrumentalist uses
a test tube (closed end air column) with a
17.2 cm air column. The speed of sound in
the test tube is 340 m/s. Find the frequency
of the first harmonic played by this
instrument.
Solution
2. L = λ/4
4xL=λ
4 x .172 = .688 m
v=fλ
340 = f (.688)
f = 494 Hz
Beats
A beat occurs when sound waves of two
different (but very much alike) frequencies are
played next to each other. The result is
constructive and destructive interference at
regular intervals.
•This oscillation of wave
amplitude is called a beat.
•The frequency of a beat is the
magnitude of difference between
the frequencies of the two waves,
f= fA – fB
•See example problem 10 on p.
367.
Anatomy of the Ear
Sound starts at the Pinna
Then goes through the
auditory canal
The sound waves will then vibrate the
Tympanic Membrane (eardrum) which
is made of a thin layer of skin.
The tympanic membrane will then vibrate
three tiny bones: the Malleus (hammer),
the Incus (anvil), and the Stapes (stirrup)
The stapes will then vibrate the
Cochlea
Inside look of the Cochlea
• The stapes vibrates the
cochlea
• The frequency of the
vibrations will stimulate
particular hairs inside
the cochlea
• The intensity at which
these little hairs are
vibrated will determine
how loud the sound is.
• The auditory nerve will
then send this signal to
the brain.
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