chapter 20 sound 1. origin of sound the frequency of a sound wave is the same as the frequency of...
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Chapter 20
Sound
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1. ORIGIN OF SOUND
•The frequency of a sound wave is the same as the frequency of the source of the sound wave.
Demo - Oscillator and speaker Demo - Oscillator and speaker
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The human hearing range is about
20 Hz - 20,000 Hz.
infrasonic.
Frequencies below 20 Hz are
Frequencies above 20,000 Hz are
ultrasonic.
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2. NATURE OF SOUND IN AIR
Video - Tuning ForkVideo - Tuning Fork
Demo - Strobed Tuning ForkDemo - Strobed Tuning Fork
Sound in air is longitudinal with
Compressions (Condensations)
and
Rarefactions
Demo - Vortex BoxDemo - Vortex Box
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3. MEDIA THAT TRANSMIT SOUND
• Air is most common but is a poor conductor.
• Solids and liquids are good conductors.
• A medium is required.
• Video - Vacuum PumpVideo - Vacuum Pump
(plus next 3 videos)(plus next 3 videos)
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4. SPEED OF SOUND IN AIR
•Much slower than light
• Video - LumberjackVideo - Lumberjack
• Video - Thunder and Lightning Video - Thunder and Lightning
• Video - ExplosionVideo - Explosion
•Depends on wind, temperature, humidity
•Does not depend on frequency
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At 0o C (dry air)
v = 1090 ft/s = 750 mi/hr = 330 m/s
increases with humidity
increases with
temperature
is faster in liquids and
solids
How v varies:
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prolonging of sound by reflection
Rumble of distant thunder
(several reflections from
different distances)
5. REFLECTION OF SOUND
• Occurs anytime waves change media.
Example: Echoes
Example:
• Reverberation -
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•What is not reflected is transmitted
and absorbed.
Examples: San Francisco Symphony Hall
Back of lecture room
•The study of sound properties is
acoustics.
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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.
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Warm Air
Cool Air
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Other Examples
Thunder and lightning
Sometimes distant lightning is not heard well.
Other times it is.
Submarines and SonarRefraction due to thermal gradients can
“hide” submarines.
Ultrasound in medicineUltrasound echo and dolphins
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7. 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.
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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.
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8. FORCED VIBRATIONS
• Demo - Tuning Fork Touching a TableDemo - 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.)
• Examples: Musical sounding boards
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9. NATURAL FREQUENCY• Demo - Drop Different Sounding Demo - Drop Different Sounding
ObjectsObjects
• Objects have natural frequencies at which they
vibrate.
• The natural frequency depends on elasticity
and shape.
• Demo - Gravity ChimesDemo - Gravity Chimes
• Demo – Boomwhackers – Boomwhackers
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Yankee doodleYankee doodle
(arranged by Anthony T.)
Yankee doodleYankee doodle
(arranged by Anthony T.)
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10. RESONANCE• 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)
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Video - Matched Tuning ForksVideo - Matched Tuning Forks
Video - Tuning Fork on GuitarVideo - Tuning Fork on Guitar
Video - Breaking GlassVideo - Breaking Glass
Demo - Matched Tuning ForksDemo - Matched Tuning Forks
Demo - Singing GlassDemo - Singing Glass
Glass armonica by Ben Franklin by Ben Franklin
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Demo - Vibrating RodDemo - Vibrating Rod
Demo - Vibrating PlateDemo - Vibrating Plate
Demo - Fog HornDemo - Fog Horn
Submultiples of the naturalfrequency also produce resonance.
Demo - 256 & 512 Hz Tuning ForksDemo - 256 & 512 Hz Tuning Forks
Video - Tacoma Narrows BridgeVideo - Tacoma Narrows Bridge
Note:
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11.INTERFERENCE
• Defined in Chapter 19
• Demo - Oscillator & Two SpeakersDemo - Oscillator & Two Speakers
• Slide - Oscillator & Two Speakers
• Examples:
Dead spots in theaters and music halls
Anti-noise technology
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12. Beats
• Fluctuating loudness due to two tones of slightly different frequencies that are sounded together (tremolo)
• It is an interference effect.
• Beats/s = difference in frequencies
• Musical instruments are tuned using beats.
• Dolphins use beats and Doppler effect.
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Demo - Vibrating RodsDemo - Vibrating Rods
Demo - Two Combs on OverheadDemo - Two Combs on Overhead
Demo - Two Oscillators & Two Demo - Two Oscillators & Two
SpeakersSpeakers
Demo - GuitarDemo - Guitar
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Constructive Interference
Destructive Interference
Consider two waves of slightly different frequencies traveling together.
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RADIO BROADCASTS
AM - 535 kHz to 1605 kHz
FM - 88 MHz to 108 MHz
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Chapter 20 Review Questions
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Frequencies of sound that are too high for the human ear to hear are called
(a) faster than the speed of sound(b) supersonic(c) infrasonic(d) ultrasonic(e) subsonic
(d) ultrasonic
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For the same temperature for air, does sound travel faster in humid Galveston or in dry El Paso?
(a) Galveston(b) El Paso(c) same speed in either city
(a) Galveston
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The bending of sound through air of uneven temperature is called
(a) reflection(b) refraction(c) interference(d) reverberation(e) resonance
(b) refraction
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Lightning is seen, then ten seconds later thunder is heard. Approximately, how far away in feet is the thunder cloud?
(a) 10,500(b) 5280(c) 1050(d) 5250(e) 105
(a) 10,500
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A 250 Hz tuning fork and a 260 Hz tuning fork are vibrating near each other. How many beats per second are heard?
(a) 255(b) 510(c) 10(d) 250(e) 260
(c) 10
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(a) the lowest frequency had a node in the middle
(b) the vibration was longitudinal(c) there were antinodes at the ends(d) the lowest frequency had a wavelength
(in the rod) equal to the length of the rod
(d) the lowest frequency had a wavelength (in the rod) equal to the length of the rod
Which of the following isn’t true concerning the metal rod that I vibrated in class by stroking it along its length?