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Chapter 15 : WAVES - Sound - LCHS
Physical Science |
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Objectives: The learner will...
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and solve problems including: frequency, period, wavelength, velocity,
resonant tubes, strings and the Doppler effect |
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Vocabulary:
wavelength : the length of
a single wave measured between repeating points
frequency : the number of waves that past a fixed point in
1 second
period : the time for a single wave to pass a fixed point
amplitude : the strength (volume) of the wave
transverse : perpendicular to the direction of the wave like
cracking a whip
longitudinal : parallel to the direction of the waves like
reversible dominos |
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WAVES in General
There are three types of waves: Mechanical,
Electromagnetic and Matter waves. Mechanical
waves require a material medium to travel (air, water,
ropes). Electromagnetic waves do not require a medium
to travel (light, radio). Matter waves are produced
by electrons and particles.
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Waves are again divided into three different sub-types based on
their style of movement. Transverse
waves cause the medium to move perpendicular to the direction of
the wave like cracking a whip. Longitudinal
waves cause the medium to move parallel to the direction of the
waves like reversible dominos. Sound waves are longitudinal
waves! Surface waves
are both transverse waves and longitudinal waves mixed in one medium
like water waves.
The length of a wave ( ),
or wavelength is measured in meters from crest to crest or trough
to trough. The number of waves that pass a fixed point in
a second is called the frequency ( f ) of a wave and is measured
in Hertz ( Hz, waves/cycles per second ). The period
( T ) of a wave is the time (seconds) for one wave to pass a fixed
point, or the inverse of the frequency (f
= 1 / T). The velocity of the wave ( v ) is a product
of the frequency and the wavelength 
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SOUND WAVES
| Sound energy travels through matter in the form of longitudinal/compression
waves and if it radiates equally in all directions from its
source then it diminishes in strength with distance according
to the inverse square law. |
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Think of sound as travelling in spherical concentric wavefronts
away from the center of a sphere with the source of the sound at
the center. The surface area of a sphere is equal to 4 R^2,
so if the radius doubles the surface area quadruples! The sound
energy is then dispersed over 4 times the area, meaning that the
Intensity is 1/4 of its original value.
In air, sound travels at 331.5 m/s (at 0 degree Celsius)
+ .6 m/s per degree Celsius (vs =
331.5 + .6C ). In water, sound travels at 1497 m/s regardless
of temperature.
TUBES & STRINGS
A tube or string will vibrate with a given frequency depending on
its physical characteristics. An open tube has to be twice as long
as a closed tube (trumpet, flute...) to vibrate at the same frequency
(tone). Width of the tube does have an effect but will not be discussed
here (research project?). When a string is plucked on a guitar,
the frequency can be changed (f ' ) if the length
(finger on fret), tension (tuning pegs), or density (nylon, steel...)
is changed.
DOPPLER EFFECT
The DOPPLER EFFECT is a change in frequency due to the source
of the wave moving with a rapid velocity causing a scrunching
or stretching of the wave. The sudden increase and then decrease
of pitch in the sound of a passing race car is an example.
For light waves a star travelling toward us in the universe
causes a spectral shift toward the blue while away causes
a red shift.
f = fs (v +
vr)/(v - vs ) ...this equation for sound waves
fs = source frequency, vr
= receiver velocity (might be zero), vs = source velocity |
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Nifty Sonic Doppler Effect Demo Click
Here
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Scientist Spotlight:
Chladni,
Ernst (1756-1827) known as the father of acoustics. He set
plates covered with a thin layer of sand vibrating and observed nodal
lines. He calculated the velocity of sound by filling an organ pipe
with different gases and listening to the frequency.
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Resources (Study Links/Study Tips/Reading Lists)
Nifty Sonic Doppler
Effect Demo Click Here
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Chapter 15 : WAVES - Sound - LCHS
Physical Science |
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