Transcript Sound Bites - The Lesson Locker

Sound Bites
Basics
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Sound is a mechanical, longitudinal wave.
The medium usually associated with sound is air,
but sound can travel through both liquids and gases
as well.
As sound travels through a medium, it alternately
compresses and expands the medium the same
way a slinky behaves when you create a longitudinal
wave in it.
compressed areas - high pressure area
expanded areas- low pressure areas
Drawing a sound wave
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It is somewhat difficult to
draw a longitudinal wave in
terms of high and low
pressure areas. Instead you
will draw a graph of the
pressure changes
themselves. Once you have
done that, you will have a
picture that looks like a
transverse wave, which is
easier to describe.
Speed of Sound
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The speed of sound depends on the medium that it is passing
through.
Generally speaking, the denser the medium, the greater the
speed of sound. As a result of this, sound travels much faster in
solids than it does in gases.
Material
Air
Water
Wood
Glass
Steel
Speed (m/s)
343
1440
4000
4500
5000 explains the ear to the train track!
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The speed of sound also depends on the
temperature.
For sound travelling through air, the relationship
between the temperature and the speed is given by:
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v ≈ (331 + 0.60 T) m/s
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v = speed of sound in air
T = temperature in degrees Celsius
Give it a go
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What is the speed of sound at room
temperature (20° C)?
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Use this answer if the temp. is not given
Example 2
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The speed of sound in air is 350 m/s. To the
nearest degree, what is the air temperature?
Echoes
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All the properties of waves apply to sound waves.
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reflection,
superposition,
refraction
diffraction
Reflected sound waves are given a special name,
echoes.
The wave equation also applies to sound.
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v= f
Example
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Some cameras use very high frequency sound to determine
the distance from the object being photographed. If an
object is 2.0 m from such a camera, how long will it take for
the sound to return to the camera?
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You yell into a mine shaft and hear your echo 3.00 s later.
How deep is the well?
Pitch
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With sound we use specialized terms for the general
wave properties.
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ultrasonic – sound with freq above 20000 Hz
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pitch - frequency of sound.
The human ear responds to sounds in the range of 20 Hz
to 20000 Hz.
Bats can hear sounds with frequencies as large as 100000
Hz.
Infrasonic - sound with freq below 20 Hz
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created by earthquakes and machinery
Interference with Sound
Waves
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Sound waves can interfere just like other
waves.
An interesting phenomenon occurs if the two
sound waves that are interfering are fairly
close in frequency.
Beats
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The top two green waves have
slightly different frequencies. When
they interfere with one another there
will be times when they are nearly in
phase and a loud sound will be
heard and at other times they will be
nearly out of phase and very little
sound will be heard.
The red wave represents the result
of the interference. Notice that the
change in the amplitude has a
frequency of its own.
The frequency at which the
amplitude changes is called the
beat frequency. The beat
frequency is simply the difference
between the frequencies of the
original two waves.
Example
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tuning fork #1 = 256 Hz.
tuning fork #2 = ? Hz
Beat freq = 4 Hz
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The two possible answers for the second fork are 260 Hz
AND 252 HZ.
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How can this property be used to tune musical
instruments?
Doppler Shift
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You have probably noticed the frequency (or pitch)
of a siren change as it approached and passed you.
If either the source of a sound, the observer of a
sound or both are moving, the "observed" frequency
of the sound will change.
Of course, the actual frequency of the sound source
remains the same. When the source and observer
are moving towards one another, the observed
frequency will be higher and when they are moving
away the frequency will be lower.
Doppler Shift
This is a stationary sound
source emitting waves at
a constant frequency.
Notice that the
wavelengths are equal.
Now the sound source is moving to the
right at a constant velocity. As the
source emits sound waves, it "catches
up" to the waves in front of it and pulls
away from the waves behind. This has
the effect of decreasing the wavelength
of the waves in front and decreasing the
ones behind.
Doppler Shift
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As sound moves away form you
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↑ then f ↑ (low pitch)
As sound moves towards you
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 then f ↑ (high pitch)
Calculating Doppler Shift
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f' = observed frequency
(Hz)
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f = frequency of the sound
source (Hz)
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v = speed of sound (usually
343 m/s)
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vo = velocity of the observer
(m/s)
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vs = velocity of the sound
source (m/s)
Example
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An ambulance carrying Ms. Ryan (coffee overdose) is
approaching you at 30 m/s. If the ambulance's siren emits
sound at a frequency of 1500 Hz, what frequency will you
hear as you stand in shock at possibly losing your beloved
physics teacher?
Give it a go
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A tornado siren goes off so you jump on your bicycle and
drive away from the siren at a speed of 20.0 m/s. If the
siren has a freq of 500 Hz, what freq will you hear?
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(471 Hz)
Sonic Boom
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An interesting thing happens when an object (such
as a plane) travels with a velocity equal to the speed
of sound. The sound waves in front of the plane
can't move away and build up in one very large
pressure wave. It actually takes a burst of extra
energy to break through this barrier. Once the plane
has broken through, it will be outrunning its own
sound waves and is said to be supersonic.
The sound waves will all line up to constructively
interfere and to form what is known as a shock
wave. This shock wave is commonly called a sonic
boom.