sound intensity nov 2012
TRANSCRIPT
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The Doppler Effect
A source emits a sound of constant frequency. If theapparent frequency of the source is increased which ofthe following is true?
A. the source is moving toward you and you are stationary
B. the source is stationary and you are moving toward it
C. either A or BD. neither A nor B
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The Doppler Effect
A source emits a sound of constant frequency. If theapparent frequency of the source is increased which ofthe following is true?
A. the source is moving toward you and you are stationary
B. the source is stationary and you are moving toward it
C. either A or BD. neither A nor B
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Sound Intensity: Learning Goals
The student will be able to explain how theintensity of a sound wave is expressed and varieswith distance from a source. (E2.1, E3.6)
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Sound Intensity
3U Physics
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Energy and Amplitude
The amplitude of a wave depends upon theamplitude of the vibration at the source:
the more work done to displace the medium atthe source, the more energy the wave will carry.
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Intensity
The rate at which energy is transported past agiven area of the medium is known as the
intensity of the sound wave.
IntensityEnergy
Area Time
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Intensity
And since energy/time = power:
Intensity
Energy
Area Time
Power
Area
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Intensity
And since energy/time = power:
The units of intensity are W/m2.
Intensity
Energy
Area Time
Power
Area
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Intensity and Area
As a wave carries its energy through a 3D medium, theintensity of the wave decreases with increasing distancebecause energy is being distributed over a greater
surface area.(Energy, remember, is conserved.)
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Intensity and Area
The mathematical relationship betweenintensity and distance is an:
inverse square relationship.
i.e., if the distance from the source is
doubled (increased by a factor of 2),then the intensity is decreased by afactor of 22, or 4.
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Intensity and Hearing
Humans are equipped with very sensitive ears capable ofdetecting sound waves of extremely low intensity, as
faint as 110-12 W/m2.
http://en.wikipedia.org/wiki/Image:Studearring.JPG -
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Intensity and Hearing
Humans are equipped with very sensitive ears capable ofdetecting sound waves of extremely low intensity, as
faint as 110-12 W/m2.
(This intensity corresponds to a pressure wave in which acompression increases the air pressure by a mere 0.3 billionths of
an atmosphere, or (in terms of amplitude) a wave in which the
particle displacement is a mere one-billionth of a centimetre.)
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Intensity and Hearing
Humans are equipped with very sensitive ears capable ofdetecting sound waves of extremely low intensity, as
faint as 110-12 W/m2.
(This intensity corresponds to a pressure wave in which acompression increases the air pressure by a mere 0.3 billionths of
an atmosphere, or (in terms of amplitude) a wave in which the
particle displacement is a mere one-billionth of a centimetre.)
This is known as the threshold of hearing (TOH).
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Intensity Scale
The most intense sound whichthe ear can safely detect
without suffering any
physical damage is morethan one billion times moreintense.
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Intensity Scale
The most intense sound whichthe ear can safely detect
without suffering any
physical damage is morethan one billion times moreintense.
Physicists therefore often use a
logarithmic scale (based onpowers of 10) for intensity:
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Intensity Scale
The most intense sound whichthe ear can safely detect
without suffering any
physical damage is morethan one billion times moreintense.
Physicists therefore often use a
logarithmic scale (based onpowers of 10) for intensity:the decibel (dB) scale.
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Decibels
The threshold of hearing (110-12 W/m2) is assigned asound level of 0 dB.
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Decibels
The threshold of hearing (110-12 W/m2) is assigned asound level of 0 dB.
A sound (e.g. the sound of rustling leaves) which is 10
times more intense (1
10-11
W/m2
) is assigned a soundlevel of 10 dB.
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Decibels
The threshold of hearing (110-12 W/m2) is assigned asound level of 0 dB.
A sound (e.g. the sound of rustling leaves) which is 10
times more intense (1
10-11
W/m2
) is assigned a soundlevel of 10 dB.
A sound (e.g. a whisper) which is 1010 or 100 timesmore intense ( 110-10 W/m2) is assigned a sound level
of __ db.
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Decibels
The threshold of hearing (110-12 W/m2) is assigned asound level of 0 dB.
A sound (e.g. the sound of rustling leaves) which is 10
times more intense (1
10
-11
W/m
2
) is assigned a soundlevel of 10 dB.
A sound (e.g. a whisper) which is 1010 or 100 timesmore intense ( 110-10 W/m2) is assigned a sound level
of 20 db.
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Decibels
The threshold of hearing (110-12 W/m2) is assigned asound level of 0 dB.
A sound (e.g. the sound of rustling leaves) which is 10
times more intense (1
10
-11
W/m
2
) is assigned a soundlevel of 10 dB.
A sound (e.g. a whisper) which is 1010 or 100 timesmore intense ( 110-10 W/m2) is assigned a sound level
of 20 db.A sound which is 101010 or 1000 times more intense(110-9 W/m2) is assigned a sound level of __ db.
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Decibels
The threshold of hearing (110-12 W/m2) is assigned asound level of 0 dB.
A sound (e.g. the sound of rustling leaves) which is 10
times more intense (1
10
-11
W/m
2
) is assigned a soundlevel of 10 dB.
A sound (e.g. a whisper) which is 1010 or 100 timesmore intense ( 110-10 W/m2) is assigned a sound level
of 20 db.A sound which is 101010 or 1000 times more intense(110-9 W/m2) is assigned a sound level of 30 db.
Etc.
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Decibels
If one sound is 10xtimes more intense than anothersound, then it has a sound level which is 10xmoredecibels than the less intense sound.
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Decibels
If one sound is 10xtimes more intense than anothersound, then it has a sound level which is 10xmoredecibels than the less intense sound.
Example: A mosquito's buzz is often rated with a decibelrating of 40 dB. Normal conversation is often rated at60 dB. How many times more intense is normalconversation compared to a mosquito's buzz?
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Decibels
If one sound is 10xtimes more intense than anothersound, then it has a sound level which is 10xmoredecibels than the less intense sound.
Example: A mosquito's buzz is often rated with a decibelrating of 40 dB. Normal conversation is often rated at60 dB. How many times more intense is normalconversation compared to a mosquito's buzz?
Answer: 100 times.
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Decibels
Another Example: Someone standing 1 m awayfrom a busy street measures the decibel level ofthe traffic to be 70 dB. What level would they
measure if they were standing 10 m away?
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Decibels
Another Example: Someone standing 1 m awayfrom a busy street measures the decibel level ofthe traffic to be 70 dB. What level would they
measure if they were standing 10 m away?
Answer: 50 dB. (Ten times further away, theeffective area over which the sound must travel
is increased by 100, thus decreasing the intensityby a factor of 100, or 20 dB.)
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Loudness
Note that sound intensity is an objective quantity, butloudness is a subjective response which will vary with,for example, frequency.
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Intensity
High intensity sound wavesthat have largeamplitudes will result in
large-amplitudevibrations in the ear . . .which can damage theear, especially the
sensitive hair cells of thecochlea.
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How Loud is Too Loud?
Regular exposure to 110 dB formore than a minute -- or to 100dB for more than 15 minutes --
risks permanent hearing loss.Prolonged exposure to any noise
above 85 dB can cause gradualhearing loss.
Most MP3 players have amaximum volume setting equalto about 105 dB.
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Just Say No to Earbuds
Earbud headphones can be the most destructiveto hearing since
(a) they do not filter out external sounds, causingthe listener to increase the volume, and
(b) they are positioned very close to the eardrum.
Remember
IntensityPower
Area
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More Practice
Decibel Lab Activity
Homework: Sound Intensity