Download Chapter 15: Sound

Chapter 15
The Nature of Sound
What is Sound???
Sound is a Longitudinal Wave
traveling through matter.
Longitudinal Waves
Longitudinal Waves
Matter vibrates in the same
direction as the wave travels.
Longitudinal Waves
Compression
Rarefaction
λ
Sound from a Tuning Fork
Speed of Sound
Sound is transmitted
through matter
.
The Velocity of Sound
depends on the
matter that carries it.
Sound travels at a velocity of
332m/s in air at 0C.
•Sound travels faster through warm air
than through cold air.
•The velocity of sound increases about
0.6m/s for each degree in temperature.
•At 20C sound travels at 344m/s.
•Sound travels much faster through liquids
and solids than through gases.
Comparing Media
Media
Speed of Sound
Air at 0°C
Air at 20°C
Water at 25°C
Sea Water at 25°C
Iron at 25°C
Rubber at 25°C
331m/s
343m/s
1493m/s
1533m/s
5130m/s
1550m/s
Human Hearing
Frequency of Sound
20 Hz to 20,000 Hz.
Sound above 20,000 Hz - Ultrasonic
Sound less than 20 Hz – Subsonic
(Infrasonic)
Frequency
is
Pitch
Detection of Pressure Waves
Detection of Pressure Waves
Ear
Drum
Intensity and Loudness
Intensity of Sound
Depends on the amplitude
of the wave.
Loudness
Describes a person’s response
to sound intensity.
Loudness is measured in
Decibels(dB)
For every 10dB change
the sound doubles!!
70dB is twice 60dB
80dB is four times 60dB
Faintest Sound Heard
Whisper
Rustling Leaves
Purring Cat
Average Home
Vacuum Cleaner
Noisy Restaurant
Power Mower
Chain Saw
------Painful ------Jet Plane Taking Off
0dB
15dB
20dB
25dB
50dB
75dB
80dB
100dB
115dB
120dB
150dB
Interference
Constructive Interference
Occurs when the compressions
and rarefactions of two or
more waves come together.
Louder
Sound
Interference
Destructive Interference
Occurs when a compression of one
wave arrives at the same time as a
rarefaction of another wave.
Quieter
Sound
Interference
Beats
The result of compressions and
rarefactions of two slightly
different frequencies reaching
your ears together.
Beats
Beats
f1 = 512Hz
f2 = 514Hz
Beats = f1 - f2
= 514Hz - 512Hz
Beats = 2Hz (beats/s)
The Doppler Effect
The change in wave frequency
caused by the motion of the
sound source or the motion
of the observer.
The Doppler Effect
Shorter Wavelength
Higher Frequency
The Doppler Effect
Longer Wavelength
Lower Frequency
Speed of Sound
Greater than the
Speed of Sound
Resonance
A resonant frequency is a
natural frequency of
vibration determined by
the physical parameters of
the vibrating object.
Harmonics
Vibrations which occur
at a particular
frequency is known as
a harmonic.
First Harmonic
The lowest possible frequency
at which a string could vibrate
to form a standing wave
pattern is known as the
fundamental frequency or the
first harmonic.
First Harmonic
Second Harmonic
Third Harmonic
Resonance in Air Columns
Closed Air Column
L
λ = 4L
λ=
4/ L
3
λ=
4/ L
5
Resonance in Air Columns
Open Air Column
L
λ = 2L
λ=L
λ=
2/ L
3
Example
A tuning fork is placed above an
open-pipe resonator in which the
length can be changed. The loudest
sound is heard at a length of 67cm
and the next loudest was heard at
100.5cm. If the temperature of the
air is 20°C what is the frequency of
the tuning fork?
Example
33.5cm = ½λ
2•33.5cm = λ
67cm = λ
67cm
100.5cm
(100.5 - 67)= 33.5cm
Example
λ = 67cm = 0.67m
v@20°C = 343m/s
v = λf
f = v/λ
f = 343m/s
0.67m
f = 512Hz
Music to Your Ears
A back and forth motion is set up in a
string, resulting in a regular vibration.
The vibration is called a standing wave
the location of the crests and troughs
are always in the same place.
In a wind instrument, holes are
opened and closed, changing the
length of the vibrating column of air.
This changes the size
of the standing wave.
Noise
Sound with no regular pattern or
definite pitch.
Tone Quality
The differences among sounds of
the same pitch and loudness.
Music
Musical Sounds
Based on a series of notes
called a musical scale.
The Sound Spectrum:
Fundamental and Harmonics
Open Air Column
L
λ = 2L
λ=L
f1 = v/λ
f2 = v/L
f1 = v/2L
f2 = 2f1
λ=
2/ L
3
f3 = v/2/3L
f3 = 3f1
Fundamental
Frequency
262Hz
First
Overtone
524Hz
Second
Overtone
786Hz
Third
Overtone
1048Hz
Closed Air Column
L
λ = 4L
f1 = v/4L
λ=
f2 = v/4/3L
f2 = 3f1
4/ L
3
4
λ = /5L
f3 = v/4/5L
f3 = 5f1
Fundamental
Frequency
256Hz
First
Overtone
768Hz
Second
Overtone
1280Hz
Third
Overtone
1792Hz
Harmony
Notes that sound pleasing together.
The ratio of the frequencies of
tones that are in harmony
are small whole numbers.


Notes that are one octave apart.
Middle C and C 524/262 = 2/1
Notes E and C
330/262 = 5/4
Dissonance and Consonance
• Dissonance combination of
pitches that sound unpleasant.
• Consonance combination of
pitches that sound pleasant.
Musical Intervals
Octave: Two notes that have a
ratio of 1:2.
Example: 440Hz
880Hz one octave higher.
220Hz one octave lower.
Interference
Constructive Interference
Occurs when the compressions
and rarefactions of two or
more waves come together.
Louder
Sound
Interference
Destructive Interference
Occurs when a compression of one
wave arrives at the same time as a
rarefaction of another wave.
Quieter
Sound
Interference
Beats
The result of compressions and
rarefactions of two slightly
different frequencies reaching
your ears together.
Beats
Beats
f1 = 512Hz
f2 = 514Hz
Beats = f1 - f2
= 514Hz - 512Hz
Beats = 2Hz (beats/s)
Homework #15-3
Practice Problem:10
Section Review
Page: 367
Due: 3/18/03
Homework #15-4
Study Guide
Due: 3/19/03
Test: 3/20/03