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Sound Intensity Level
Doppler Effect
Mrs. Coyle
Sound is a longitudinal mechanical wave.
Compressions(High Pressure)
 Rarefactions(Low Pressure)
Speed of Sound
 Depends
on the medium.
 The
more elastic the medium the
faster sound will travel through it.
 Speed
in metals>speed in
water>speed in air
 Sound
can’t travel through vacuum.
Speed of Sound in Air
v = 331 + 0.6 T
( in meters/sec)
T is the temperature in 0C.
 In higher humidity, sound will travel faster.
Properties of Sound
 Reflection
 Refraction
 Interference
 Diffraction
Pitch: the frequency of a
sound wave.
Musical notes have a given pitch.
The note C has a frequency of 327Hz.
When two notes differ by a ratio of 2:1
they are one octave apart.
What would be the next higher C?
Bow (Shock) Waves
When the speed of a moving sound
source is greater than the speed of the
wave, a pressure ridge builds similar to
the wave created by the bow of a ship.
physlet animation
Sonic Boom
When the pressure ridge of a bow wave
of a jet passes over an observer on the
ground, the observer experiences a sonic
Doppler Effect
The change in a wave's perceived frequency
due to the motion of either the sound source or
the observer.
It is applicable to any type of wave.
Austrian physicist Christian Doppler (1803-1853).
train sound clip
 physlet animation
The Doppler Effect
Doppler Effect
Detected Frequency, (fD)
fD = f ( v±vo )
f : frequency of source
v : speed of sound
vo : speed of observer
(+)when observer moving towards source
vs : speed of source
(-)when source is moving towards observer
Example 1
An ambulance is approaching a
stationary observer.
The siren of the ambulance emits a
frequency of 480Hz and the speed of
the ambulance is 50km/h(=13.88m/s).
What frequency will the stationary
observer hear when the ambulance is
approaching? Assume T=20oC.
Answer: fd = 500Hz
Sound Intensity (I)
I = (Power transmitted by the Wave)/Area
Threshold of human hearing:
Io =1 x 10-12 Watts/m2
Threshold of Pain:
IP = 1 Watts/m2
Sound (Intensity) Level
A measure of our perception of the
loudness of the sound.
Unit: decibel(dB)
Sound (Intensity) Level, (decibel)
b= 10 log I
, unit for b is dB (decibel)
The decibel compares the sound intensity (I), to
Io (1 x 10-12 Watts/m2) ,
the threshold of human hearing.
Remember: log(10x )= x
Examples of Sound Intensity Levels
jet plane taking off
air raid siren
threshold of pain
loud rock music
ear damage starts
busy traffic
normal conversation
quiet library
soft whisper
threshold of human hearing
140 dB
125 dB
120 dB
115 dB
85 dB
70 dB
60 dB
40 dB
20 dB
0 dB
Example 2
What is the sound level (decibel) of
a sound of 10-4 W/m2 intensity ?
Answer: 80dB
Example 3
How many times louder is a quiet library
sound (40 dB) compared to a soft whisper
(20 dB)?
Hint 1: Property of logs:
log(A/B) = logA –logB
Hint 2: Take the difference in dB and then find
(I2 /I1)
 Answer: (I2 /I1 )= 100/1