Sound
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Origin of Sound
Nature of Sound in Air
Media that Transmit Sound
Speed of Sound in Air
Refraction
Energy in Sound Waves
Forced Vibrations
Natural Frequency
Resonance
Interference
Beats
Sound...
• ...a longitudinal wave in air caused by a vibrating object.
• Sound requires a medium.
– solid, liquid or gas
– Will a bell ring in a evacuated Bell Jar?
Nature of Sound in Air
• Sound waves have compression and
rarefaction regions.
Origin of Sound
•
infrasonic
– frequencies < 20 Hz
•
ultrasonic
– frequencies > 20,000 Hz
•
human hearing range
– frequencies between 20
Hz and 20,000 Hz
Speed of Sound in Air
• 340 meters/second
• 760 miles/hour
• Mach 1
SPEED OF SOUND
How it varies:
increases with humidity
increases with temperature
increases with density
Lightning and Thunder
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?
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
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.
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.
Reflection of Sound
• e.g. an echo
• Reverberation - re-echoed
sound, multiple reflections of
sound waves from walls
• Compare reflections from a
hard wall with that from a
carpet wall.
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.
Speed of Sound
• Subsonic - slower than the speed of sound
• Supersonic – faster than the speed of sound
Mach Number
=
speed of object
speed of sound
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 surface
– Sounding boards for stringed instruments
– Matching tuning fork boxes
• During forced vibration sound is intensified because a larger surface
area is available to vibrate air molecules.
Resonances
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.
Constructive Interference
Destructive Interference
Sound Interference
• Noise reduction headphones
• Echo cancellation
Beats
•
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.
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
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
The bending of sound through air of uneven temperature is called
(a) reflection
(b) refraction
(c) interference
(d) reverberation
(e) resonance
The bending of sound through air of uneven temperature is called

(a) reflection
(b) refraction
(c) interference
(d) reverberation
(e) resonance
Lightning is seen, then ten seconds later thunder is heard.
Approximately, how far away in meters is the thunder
cloud?
(a) 10,500 m
(b) 5280 m
(c) 1050 m
(d) 3400 m
(e) 680 m
Lightning is seen, then ten seconds later thunder is heard.
Approximately, how far away in meters is the thunder
cloud?

(a) 10,500 m
(b) 5280 m
(c) 1050 m
(d) 3400 m
(e) 680 m
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
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
Musical Sounds
Noise Versus Music
• What is the difference between noise and music?
– Answer: The appearance of the waveform.
Mac Mic
NOISE VERSUS MUSIC
Same Note - Different Instrument
• Harmonic
– a partial tone that is an integer multiple of the fundamental
frequency
• Fundamental Frequency
– the lowest frequency of vibration
– a.k.a. the first harmonic
Back
Sound Intensity and Loudness
• Intensity of Sound
– refers to the amplitude of the pressure variations in the sound wave
• Loudness
– the physiological sensation directly related to the sound intensity
– measured in bels
– 1bels = 10 decibels
• Loudness
– A sound of 10 decibels is or 101 or 10 times as intense as 0
decibels.
– 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
Source of Sound
Threshold of Hearing
Conversation
Ear Damage Begins
Amplified Music
Jet Airplane at 30 meters
Loudness (db)
0
60
85
110
140
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