Download 12.4 Sources of Sound

S-90
A pendulum bob with a mass of 30 kg is
placed at the end of a string that is 1.2 m long.
A) What is the period of oscillation of the
pendulum?
B) If the same mass is placed on a spring,
what must the spring constant be to
produced the same period of oscillation?
Electromagnetic Waves
AP Physics
Chapter 12
Vibration and Waves
12.1 Characteristics of Sound
12.1 Characteristic of Sound
Sound is a longitudinal wave
Caused by the vibration of a medium
The speed of sound depends on the medium
it is in, and the temperature
For air, it is calculated as
TK
vs  331.5
273.15
12.1
12.1 Characteristic of Sound
Loudness – sensation of intensity
Pitch – sensation of frequency
Range of human hearing – 20Hz to 20,000 Hz
ultrasonic – higher than human hearing
dogs hear to 50,000 Hz,
bats to 100,000 Hz
infrasonic – lower than human hearing
12.1
12.1 Characteristic of Sound
Often called pressure waves
Vibration produces areas of higher pressure
These changes in pressure are recorded by
the ear drum
12.1
Vibration and Waves
12.2 Intensity of Sound: Decibels
12.2 Intensity of Sound: Decibels
Loudness – sensation
Relative to surrounding and intensity
Intensity – power per unit area
Humans can detect intensities
as low as 10-12 W/m2
The threshold of pain
is 1 W/m2
P
I
A
12.2
12.2 Intensity of Sound: Decibels
Sound intensity is usually
in
Source ofmeasured
Sound
Sound Level
(dB)
decibels (dB)
Jet Plane at 30 m
140
Sound level is given Threshold
as
of Pain
120
Loud Rock Concert
120
I
  10
logat 30 m
Siren
100
I
Auto Interior at 090 km/h
75
Busy Street Traffic
I – intensity of the sound
Conversation at-12
0.50 m
I0 – threshold of hearing (10 W/m2)
Quiet Radio
– sound level in dB Whisper
Rustle of Leaves
Threshold
of Hearing
Some common relative
intensities
70
65
40
20
10
0
12.2
Vibration and Waves
12.3 The Ear
S-89
A cat screams at the top of his tiny lungs. If a
listener 3.2 m away measures the relative
intensity at 115 dB.
A. What is the power output
of the cats lungs?
B. What is the relative
intensity for a listener only
1.1 m away from the cat?
12.3 The Ear
Steps in sound transmission
12.3
Vibration and Waves
12.4 Sources of Sound: Strings and Air Columns
12.4 Sources of Sound: Strings and Air Columns
Vibrations in strings
Fundamental frequency
  2L
v
f1 
2L
L
v f 2  2 f1
f2 
L
Next Harmonic
12.4
12.4 Sources of Sound: Strings and Air Columns
Vibrations in strings
Next Harmonic
  23 L
v f 3  3 f1
f3  2
3 L
Strings produce all harmonics – all whole
number multiples of the fundamental
frequency
12.4
12.4 Sources of Sound: Strings and Air Columns
Vibrations in an open ended tube (both ends)
Fundamental frequency
  2L
v
f1 
2L
L
v f 2  2 f1
f2 
L
Next Harmonic
12.4
12.4 Sources of Sound: Strings and Air Columns
Vibrations in open ended tubes
Next Harmonic
  23 L
v f 3  3 f1
f3  2
3 L
Open ended tubes produce all harmonics – all
whole number multiples of the fundamental
frequency
12.4
12.4 Sources of Sound: Strings and Air Columns
Vibrations in an closed end tube (one end)
Fundamental frequency
  4L
v
f1 
4L
  43 L
v f 3  3 f1
f3  4
3 L
Next Harmonic
12.4
12.4 Sources of Sound: Strings and Air Columns
Vibrations in open ended tubes
Next Harmonic
  54 L
v f 5  5 f1
f5  4
5 L
Closed end tubes produce only odd
harmonics
12.4
Vibration and Waves
12.6 Interference of Sound Waves; Beats
12.6 Inteference of Sound Waves; Beats
If waves are produced by two identical
sources
A pattern of constructive and destructive
interference forms
Applet
12.6
S-91
A cat plays a guitar. The
speed of sound through
the string is 525 m/s. If
the string is 45 cm long
A. What is the
fundamental
wavelength of the string?
B. What is the fundamental frequency of the
string.
C. What would be the next two harmonics
produced on the string?
Vibration and Waves
12.7 The Doppler Effect
12.7 The Doppler Effect
Doppler Effect – the change in pitch due to the
relative motion between a source of sound
and the receiver
Doppler Effect
Applies to all wave phenomena
Light Doppler
Objects moving toward you have a higher
apparent frequency
Objects moving away have a lower apparent
frequency
12.7
12.7 The Doppler Effect
If an object is stationary the equation for the
wave velocity is
v  f
Sound waves travel outward evenly in all
directions
Doppler Applet
If the object moves toward the observed, the
waves travel at the same velocity, but each
new vibration is created closer to the observer
12.7
12.7 The Doppler Effect
The general equation is
 V  V0 

f  f s 
 V  Vs 
The values of Vo (speed of observer) and Vs
(speed of source) is positive when they
approach each other
Radar Gun
12.7