PHY2464 - The Physical Basis of Music
PHY
-2464
PHY-2464
Physical Basis of Music
Presentation
Presentation 22
Basics
Basics of
of Sound
Sound Waves
Waves –– Part
Part A
A
An
An edited
edited Version
Version of
of Sam
Sam Matteson’s
Matteson’s Unit
Unit 11 Session
Session 22
Sam
Sam Trickey
Trickey
Jan.
Jan. 10,
10, 2005
2005
PHYPHY-2464
Pres. 2 Sound Waves - A
So far we’ve seen that:
• Sound is a longitudinal mechanical wave that
originates from a vibration or vibrations and
propagates in a (nearly elastic) material
medium.
•
“Wave” = disturbance in material that
doesn’t transfer mass; “Elastic” = springs
back when deformed
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PHY2464 - The Physical Basis of Music
PHYPHY-2464
Pres. 2 Sound Waves - A
Longitudinal waves
→↔→
Transverse waves
→↕→
Sound is a longitudinal mechanical wave in
a material medium. (Stadium wave with
people’s arms is transverse.)
PHYPHY-2464
Pres. 2 Sound Waves - A
Fact: Sound does not propagate (move from
one place to another) in a vacuum.
Why?
True vacuum is empty space → NO matter → NO
sound (Practical vacuum has so little matter that
there also is no sound.)
Sound is a mechanical wave: such waves require a
material medium.
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PHY2464 - The Physical Basis of Music
PHYPHY-2464
Pres. 2 Sound Waves - A
Aside: Radio (and TV)
Radio waves are Electro-magnetic waves,
not sound. EM waves are “a disturbance in the force”
Sound information is coded in the amplitude
modulation (AM) or frequency modulation (FM) of
the radio waves.
Radio waves are NOT sound.
PHYPHY-2464
Pres. 2 Sound Waves - A
• Lightning is a high energy
electrical discharge
• The superheated air
expands explosively
• A “shock wave” travels to
your ear
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PHY2464 - The Physical Basis of Music
PHYPHY-2464
Pres. 2 Sound Waves - A
Velocity (speed) of a wave = the distance a point
on the wave travels
divided by the time it takes to cover that distance
v = d /t
(clearly also d = v x t and t = d /v )
PHYPHY-2464
Pres. 2 Sound Waves - A
Fact - Sound is non-dispersive: its velocity
is independent of wavelength.
Fact - The velocity of sound in air
at room temperature (20 C) and normal
atmospheric pressure is
343. m/s
4
PHY2464 - The Physical Basis of Music
PHYPHY-2464
Pres. 2 Sound Waves - A
FLASH!
Rrrrumble
PHYPHY-2464
Pres. 2 Sound Waves - A
Recall Fact - The velocity of sound
in air at room temperature (20 C) is
343. m/s
d = v x t = (343m/s)x (3 seconds)
= 1029 m ≈ 1 km
5
PHY2464 - The Physical Basis of Music
PHYPHY-2464
Pres. 2 Sound Waves - A
Summary:
•
•
•
•
Sound is a longitudinal mechanical wave
that originates from a mechanical vibration
and propagates in an elastic material
medium.
Sound is non-dispersive.
The velocity of sound in air is 343 m/s
at 20 C.
Radio waves are E-M waves, not sound.
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