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Origin of SoundNature of Sound in Air
Media that Transmit SoundSpeed of Sound in Air
RefractionEnergy in Sound WavesForced VibrationsNatural FrequencyResonanceInterferenceBeatsRadio Broadcasts
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Sound...
...a longitudinal wave in air caused by a vibrating object.
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Origin of Sound
infrasonic
frequencies < 20 Hz
ultrasonic
frequencies > 20,000 Hz
human hearing range
frequencies between 20 Hz and 20,000 Hz
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Nature of Sound in Air
Sound requires a medium.
solid, liquid or gas
Will a bell ring in a evacuated Bell Jar?
Sound waves have compression and rarefaction regions.
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Speed of Sound in Air
340 meters/second
760 miles/hour
Mach 1
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increases with humidityincreases with temperatureincreases with density
How it varies:
SPEED OF SOUND
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What is the approximate distance of a thunderstorm when you note a 3 second delay between the flash of the lightning and the sound of the thunder?
Lightning and Thunder
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What is the approximate distance of a thunderstorm when you note
a 3 second delay between the flash of the lightning and the
sound of the thunder?
Answer: 3 seconds 340 meters/second
= 1020 meters
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Acoustics...
...the study of sound properties.
When a sound wave strikes a surface it can be.
(a) reflected.
(b) transmitted.
(c) absorbed.
(d) all of these.
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Acoustics...
...the study of sound properties.
When a sound wave strikes a surface it can be.
(a) reflected.
(b) transmitted.
(c) absorbed.
(d) all of these.
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Reflection of Sound
e.g. an echo
Reverberation - re-echoed sound, multiple reflections ofsound waves from walls
Compare reflections from a hard wall with that from a carpet wall.
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Refraction of Sound
Refraction - the bending of a wave
Sound travels faster in warm air than in cool air.
Sound waves bend toward cooler air.
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Subsonic - slower than the speed of sound
Supersonic faster than the speed of sound
Mach Number =speed of sound
speed of object
Speed of Sound
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Forced Vibrations/Resonance
the setting up of vibrations in an object by a vibrating force.
Examples of Forced Vibration:
A tuning fork touching a wood surfaceSounding boards for stringed instruments
Matching tuning fork boxes
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During forced vibration sound is intensified because a larger su rface
area is available to vibrate air molecules.
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Resonances
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Sound Interference
Overlapping compressions of a sound wave will result in
constructive interference.
and a louder sound.
Overlapping a compression and a rarefaction results in...destructive interference.
and a softer sound.
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Constructive Interference Destructive Interference
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Sound Interference
Noise reduction headphones
Echo cancellation
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Beats - the periodic variation in loudness of two sounds played together
The beat frequency is equal to the difference in the frequency of the
two sounds.
What is the beat frequency when a 262 Hz and a 266 Hz tuning forkare sounded together?
Beats
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Electromagnetic Radiation
Radio Transmission and Reception
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Electromagnetic Radiation
Interrelated electric and magnetic fields traveling through space
All electromagnetic radiation travels at c = 3 108 m/s in vacuum the cosmicspeed limit!
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Electromagnetic Waves
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Examples of Electromagnetic Radiation
AM and FM radio waves (including TV signals)
Cell phone communication links
Microwaves
Infrared radiation
Light
X-rays
Gamma rays
What distinguishes these from one another?
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Uses of Electromagnetic Waves
Communication systems
One-way and two-way
Radar
Cooking (with microwaves)
Medical Imaging (X rays)Night Vision (infrared)
Astronomy (radio, wave, IR, visible, UV, gamma)
All that we experience through our eyes is conveyed byelectromagnetic radiation And much, much more!
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Encoding Information on Radio Waves
What quantities characterize a radio wave?
Two common ways to carry analog information with radio waves
Amplitude Modulation (AM)
Frequency Modulation (FM): static free
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Radio Broadcasts
Modulation - an impression of the sound wave on a higher frequency radiowave
AM
Amplitude Modulation
535 kHz to 1605 kHz
FM
Frequency Modulation
88 MHz to 108 MHz
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AM Radio
Amplitude Modulation (AM) uses changes in the signal strength to conveyinformation
pressure modulation(sound)
electromagneticwave modulation
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FM Radio
Frequency Modulation (FM) uses changes in the wave s frequency toconvey information
pressure modulation(sound)
electromagnetic
wave modulation
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What affects propagation of radio waves?
Absorption
by ground, terrain
Loss of energy density with distance
weak far away
Scatteringreflections: especially off metals
leads to multi-path effects
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Class Problem
Suppose at a concert a singer'svoice is radio broadcast all theway around the world beforereaching the radio you hold to
your ear. This takes 1/8 second. If
you're close you hear her voice inair before you hear it from the
radio. But if you are far enoughaway, both signals will reach youat the same time. How manymeters distant must you be for this
to occur?
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Class Problem
If you sit 42.5 meters away fromthe singer, both the sound fromthe radio that is broadcast all theway around the world and that
through the air will reach you in
the same 1/8 second. Distance inair = speed of sound x time in air =
340m/s x 1/8s = 42.5m.If you sitfarther back, you'll hear the radiosignal before you hear the soundsignal!
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Class Problem
Does the wind affect the pitch ofthe factory whistle you hear on awindy day?
If so, why?
If not, why?
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Class Problem
No, the wind does not affect the pitch.The wind does affect the speed ofsound because the medium thatcarries the sound moves. But thewavelength of the sound changesaccordingly, which results in no
change in frequency or pitch. This canbe seen by analogy:
Suppose a friend is placing packageson a conveyor belt, say at a"frequency" of one each second. Thenyou, at the other end of the belt, takeoff one package each second.
Suppose the speed of the beltincreases while your friend still placesone package per second on the belt.Can you see that the packages(farther apart now) will still arrive toyou at the rate of one per second?
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Class Problem
A train whistle at rest has a frequency of 3000 Hertz. If you are standingstill and observe the frequency to be 3010 Hertz, then you can concludethat...
a) the train is moving away from you.
b) the train is moving toward you
c) the sound from the whistle has echoed
d) not enough information is given
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Class Problem
A train whistle at rest has a frequency of 3000 Hertz. If you are standingstill and observe the frequency to be 3010 Hertz, then you can concludethat...
a) the train is moving away from you.
b) the train is moving toward you
c) the sound from the whistle has echoed
d) not enough information is given
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What dictates the frequency of a sound wave?
(a) wavelength
(b) medium
(c) source of the sound
(d) speed
(e) amplitude
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What dictates the frequency of a sound wave?
(a) wavelength
(b) medium
(c) source of the sound
(d) speed
(e) amplitude
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When you move away from a fixed source of sound, the frequency ofthe sound you hear
(a) is greater than what the source emits
(b) is less than what the source emits
(c) is the same as what the source emits
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When you move away from a fixed source ofsound, the frequency of the sound you hear
(a) is greater than what the source emits
(b) is less than what the source emits
(c) is the same as what the source emits
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Sonic booms from a plane are produced
(a) because the plane breaks through the sound barrier
(b) when the plane reaches the speed of sound
(c) by the plane traveling faster than the speed of sound
(d) by the plane traveling slower than the speed of sound
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Sonic booms from a plane are produced
(a) because the plane breaks through the sound barrier
(b) when the plane reaches the speed of sound
(c) by the plane traveling faster than the speed of sound
(d) by the plane traveling slower than the speed of sound
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For the same temperature for air, does sound travel faster in humidGalveston or in dry El Paso?
(a) Galveston
(b) El Paso
(c) same speed in either city
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For the same temperature for air, does sound travel faster in humidGalveston or in dry El Paso?
(a) Galveston
(b) El Paso
(c) same speed in either city
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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
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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
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Lightning is seen, then ten seconds later thunder is heard.Approximately , how far away in meters is the thundercloud?
(a) 10,500 m
(b) 5280 m
(c) 1050 m
(d) 3400 m
(e) 680 m
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Lightning is seen, then ten seconds later thunder is heard.Approximately , how far away in meters is the thundercloud?
(a) 10,500 m
(b) 5280 m
(c) 1050 m
(d) 3400 m
(e) 680 m
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A 250 Hz tuning fork and a 260 Hz tuning fork are vibrating neareach other. How many beats per second are heard?
(a) 255
(b) 510
(c) 10
(d) 250
(e) 260
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A 250 Hz tuning fork and a 260 Hz tuning fork are vibrating neareach other. How many beats per second are heard?
(a) 255
(b) 510
(c) 10
(d) 250
(e) 260
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Noise Versus Music
What is the difference between noise and music?
Answer: The appearance of the waveform.
Mac Mic
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NOISE VERSUS MUSIC
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Pitch...
is the "highness" or "lowness" of a tone.
Pitch corresponds to frequency.
Concert A on the Musical Scale has a frequency of 440 Hertz.
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Same Note - Different Instrument
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Harmonica partial tone that is an integer multiple of the fundamentalfrequency
Fundamental Frequency
the lowest frequency of vibrationa.k.a. the first harmonic
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BackBack
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Sound Intensity and Loudness
Intensity of Sound
refers to the amplitude of the pressure variations in the sound wave
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Loudness
the physiological sensation directly related to the sound intensity
measured in bels
1bels = 10 decibels
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Loudness
A sound of 10 decibels is or 101 or 10 times as intense as 0decibels.
20 decibels is 102 or 100 times the intensity 0 decibels.
How much more intense is sound at 100 dB than sound at 50 dB?
Answer: 100000 times
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Source of Sound Loudness (db)
Threshold of Hearing 0
Conversation 60
Ear Damage Begins 85
Amplified Music 110Jet Airplane at 30 meters 140
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Common Sound Intensities
Source of Sound Intensity (W/m2) Sound Level (db)
Jet 30 m away 102
140
Air-raid siren, nearby 1 120
Disco music, amplified 10-1 110
Riveter 10-3 90
Busy street traffic 10-5 70
Conversation in home 10-6 60
Quiet radio in home 10-8 40
Whisper 10-10 20
Rustle of leaves 10-11 10
Threshold of hearing 10-12 0
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