Download Sound Intensity

The Doppler Effect
A source emits a sound of constant frequency. If the
apparent frequency of the source is increased which of
the 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 B
D. neither A nor B
The Doppler Effect
A source emits a sound of constant frequency. If the
apparent frequency of the source is increased which of
the 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 B
D. neither A nor B
Sound Intensity: Learning Goals
The student will be able to explain how the
intensity of a sound wave is expressed and varies
with distance from a source. (E2.1, E3.6)
Sound Intensity
3U Physics
Energy and Amplitude
The amplitude of a wave depends upon the
amplitude of the vibration at the source:
the more work done to displace the medium at
the source, the more energy the wave will carry.
Intensity
The rate at which energy is transported past a
given area of the medium is known as the
intensity of the sound wave.
Energy
Intensity 
Area  Time
Intensity
And since energy/time = power:
Energy
Power
Intensity 

Area  Time
Area
Intensity
And since energy/time = power:
Energy
Power
Intensity 

Area  Time
Area
The units of intensity are W/m2.
Intensity and Area
As a wave carries its energy through a 3D medium, the
intensity of the wave decreases with increasing distance
because energy is being distributed over a greater
surface area.
(Energy, remember, is conserved.)
Intensity and Area
The mathematical relationship between
intensity 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 a
factor of 22, or 4.
Intensity and Hearing
Humans are equipped with very sensitive ears capable of
detecting sound waves of extremely low intensity, as
faint as 1×10-12 W/m2.
Intensity and Hearing
Humans are equipped with very sensitive ears capable of
detecting sound waves of extremely low intensity, as
faint as 1×10-12 W/m2.
(This intensity corresponds to a pressure wave in which a
compression 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.)
Intensity and Hearing
Humans are equipped with very sensitive ears capable of
detecting sound waves of extremely low intensity, as
faint as 1×10-12 W/m2.
(This intensity corresponds to a pressure wave in which a
compression 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).
Intensity Scale
The most intense sound which
the ear can safely detect
without suffering any
physical damage is more
than one billion times more
intense.
Intensity Scale
The most intense sound which
the ear can safely detect
without suffering any
physical damage is more
than one billion times more
intense.
Physicists therefore often use a
logarithmic scale (based on
powers of 10) for intensity:
Intensity Scale
The most intense sound which
the ear can safely detect
without suffering any
physical damage is more
than one billion times more
intense.
Physicists therefore often use a
logarithmic scale (based on
powers of 10) for intensity:
the decibel (dB) scale.
Decibels
The threshold of hearing (1×10-12 W/m2) is assigned a
sound level of 0 dB.
Decibels
The threshold of hearing (1×10-12 W/m2) is assigned a
sound 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 sound
level of 10 dB.
Decibels
The threshold of hearing (1×10-12 W/m2) is assigned a
sound 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 sound
level of 10 dB.
A sound (e.g. a whisper) which is 10×10 or 100 times
more intense ( 1×10-10 W/m2) is assigned a sound level
of __ db.
Decibels
The threshold of hearing (1×10-12 W/m2) is assigned a
sound 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 sound
level of 10 dB.
A sound (e.g. a whisper) which is 10×10 or 100 times
more intense ( 1×10-10 W/m2) is assigned a sound level
of 20 db.
Decibels
The threshold of hearing (1×10-12 W/m2) is assigned a
sound 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 sound
level of 10 dB.
A sound (e.g. a whisper) which is 10×10 or 100 times
more intense ( 1×10-10 W/m2) is assigned a sound level
of 20 db.
A sound which is 10×10×10 or 1000 times more intense
(1×10-9 W/m2) is assigned a sound level of __ db.
Decibels
The threshold of hearing (1×10-12 W/m2) is assigned a
sound 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 sound
level of 10 dB.
A sound (e.g. a whisper) which is 10×10 or 100 times
more intense ( 1×10-10 W/m2) is assigned a sound level
of 20 db.
A sound which is 10×10×10 or 1000 times more intense
(1×10-9 W/m2) is assigned a sound level of 30 db.
Etc.
Decibels
If one sound is 10x times more intense than another
sound, then it has a sound level which is 10x more
decibels than the less intense sound.
Decibels
If one sound is 10x times more intense than another
sound, then it has a sound level which is 10x more
decibels than the less intense sound.
Example: 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?
Decibels
If one sound is 10x times more intense than another
sound, then it has a sound level which is 10x more
decibels than the less intense sound.
Example: 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?
Answer: 100 times.
Decibels
Another Example: Someone standing 1 m away
from a busy street measures the decibel level of
the traffic to be 70 dB. What level would they
measure if they were standing 10 m away?
Decibels
Another Example: Someone standing 1 m away
from a busy street measures the decibel level of
the traffic to be 70 dB. What level would they
measure if they were standing 10 m away?
Answer: 50 dB. (Ten times further away, the
effective area over which the sound must travel
is increased by 100, thus decreasing the intensity
by a factor of 100, or 20 dB.)
Loudness
Note that sound intensity is an objective quantity, but
loudness is a subjective response which will vary with,
for example, frequency.
Intensity
High intensity sound waves
that have large
amplitudes will result in
large-amplitude
vibrations in the ear . . .
which can damage the
ear, especially the
sensitive hair cells of the
cochlea.
How Loud is Too Loud?
Regular exposure to 110 dB for
more than a minute -- or to 100
dB for more than 15 minutes -risks permanent hearing loss.
Prolonged exposure to any noise
above 85 dB can cause gradual
hearing loss.
Most MP3 players have a
maximum volume setting equal
to about 105 dB.
Just Say No to Earbuds
Earbud headphones can be the most destructive
to hearing since
(a) they do not filter out external sounds, causing
the listener to increase the volume, and
(b) they are positioned very close to the eardrum.
Remember
Power
Intensity 
Area
More Practice
Decibel Lab Activity
Homework: Sound Intensity