Chapter 20
Sound
Slide - A tree in the forest.
1. ORIGIN OF SOUND
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The frequency of a sound wave is
the same as the frequency of
the source of the sound wave.
Demo - Oscillator and speaker
Video – You Tube - Hearing Test
The human hearing range is about
20 Hz - 20,000 Hz.
Frequencies below 20 Hz are
infrasonic.
Frequencies above 20,000 Hz are
ultrasonic.
2. NATURE OF SOUND IN AIR
Video - Tuning Fork
Demo - Strobed Tuning Fork
Sound in air is longitudinal with
Compressions (Condensations)
and
Rarefactions
Demo - Vortex Box
3. MEDIA THAT TRANSMIT SOUND
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Air is most common but is a poor
conductor.
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Solids and liquids are good
conductors.
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A medium is required.
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Video - Vacuum Pump
4. SPEED OF SOUND IN AIR
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Much slower than light
Video – Lumberjack
Thunder and Lightning
Explosion
Video – Sound Delay
Depends on wind, temperature,
humidity
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Slide - Wind and Snoopy
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Does not depend on frequency
At 0o C (dry air)
v = 1090 ft/s = 750 mi /hr= 330 m /s
How v varies:
increases with humidity
increases with
temperature
is faster in liquids and
solids
5. REFLECTION OF SOUND
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Occurs anytime waves change media.
Example: Echoes
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Reverberation - prolonging of sound
by reflection
Example:
Rumble of distant thunder
(several reflections from
different distances)
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What is not reflected is
transmitted and absorbed.
 The
study of sound properties is
acoustics.
Examples:
San Francisco Symphony Hall
Back of lecture room
6.
REFRACTION OF SOUND
When different parts of a wave front move at
different speeds, the wave front will bend.
This bending is known as refraction.
It occurs when different parts of a wave front
are traveling in different media.
Slide - Sounds and Sleeping Dog
Other Examples
Thunder and lightning
Sometimes distant lightning is not
heard well.
Other times it is.
Submarines and Sonar
Refraction due to thermal gradients can
“hide” submarines.
Ultrasound in medicine
Ultrasound echo and dolphins
7.
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ENERGY IN SOUND WAVES
Energy in sound is weak when compared
to the energy in light.
The human ear is a remarkable detector.
10 million people speaking at the same
time produce approximately enough
energy to light one flashlight.
High frequencies of sound in air
more easily lose their energies to
thermal energy than do low
frequencies.
That is why low frequencies can be
heard farther away.
8. FORCED VIBRATIONS
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Demo - Tuning Fork Touching a Table
Sound is intensified because of the larger
surface area that can vibrate the air.
The surface is forced to vibrate at the
frequency of the tuning fork. (It is not a
resonance phenomenon.)
Demo – Call Mobile Phone on Table
Examples: Musical sounding boards
9. NATURAL FREQUENCY
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Demo - Drop Different Sounding
Objects
Objects have natural frequencies at
which they vibrate.
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The natural frequency depends on
elasticity and shape.
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Demo - Gravity Chimes
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Demo – Boomwhackers
Yankee Doodle
(arranged by Anthony T.)
Beethoven’s Ode
to joy
(arranged by Anthony T.)
Christmas Boom Whackers
Can Can BoomWhackers
10. RESONANCE
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Resonance occurs when successive
impulses are applied to a vibrating
object in time with its natural
frequency.
Result - increased amplitude
Examples: Swinging
Marching on a bridge
(rout step or break step)
Video - Matched Tuning Forks
Tuning Fork on Guitar
Breaking Glass
Demo - Matched Tuning Forks
Demo - Singing Glass
Glass armonica by Ben Franklin
Demo - Vibrating Rod
Demo - Vibrating Plate
Demo - Fog Horn
Note: Submultiples of the natural
frequency also produce
resonance.
Demo - 256 & 512 Hz Tuning Forks
Video - Tacoma Narrows Bridge (url)
11. INTERFERENCE
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Defined in Chapter 19
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Demo - Oscillator & Two Speakers
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Slide - Oscillator & Two Speakers
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Examples:
Dead spots in theaters and music
halls
Anti-noise technology
12. Beats
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Fluctuating loudness due to two tones
of slightly different frequencies that
are sounded together (tremolo)
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It is an interference effect.
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Beats/s = difference in frequencies
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Musical instruments are tuned using
beats.
Dolphins use beats and Doppler effect.
Slide - Beats
Consider two waves of slightly different
frequencies traveling together.
Constructive Interference
Destructive Interference
C
D
URL - Beats
Demo - Vibrating Rods
Demo - Two Oscillators & Two
Speakers
Demo - Guitar
RADIO BROADCASTS
Slide - Radio Broadcasts
AM - 535 kHz to 1605 kHz
FM - 88 MHz to 108 MHz