EDUF 1017 Physical Phenomena - Lecture 6
Waves
Properties of Waves
A mechanical wave is a disturbance that travels through a
material . As the wave travels, the particles of the medium
undergo some sort of displacement.
Transverse wave: displacement perpendicular to the
direction of travel of the wave
Longitudinal wave: displacement in the same direction as
the direction of travel of the wave
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The wave motion transports energy, not matter.
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As the block oscillates up
and down, it produces a
transverse wave travelling
to the right on the string
The wave pattern travels at
a constant speed v and
advances a distance of one
wavelength in a time
interval if one Period T
(equal to 1/Frequency or 1/f )
compression
rarefaction
Longitudinal waves
such as sound are
perhaps a bit harder to
understand although
the concept is the same
Audible range is about
20 to 20,000 Hz.
Above 20,000 HZ is
ultrasonic
Below 20 Hz is
sometimes called
infrasonic.
T
These quantities are related by
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Sound Waves
Periodic Waves
Wavelength
Waves have a wave speed - determined by the mechanical
properties of the medium.
The particles of the medium oscillate back and forth but
don’t travel far - its the wave disturbance that travels.
We have to put energy in by doing work.
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Ultrasound
Sound
Waves
The speed of
sound is
about 330
m/s,
depending on
temperature
etc.
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Dolphins emit high frequency sound waves at ~100,000 Hz for
guidance and hunting. The speed of sound in water is ~1500 m/s,
so the wavelength is
This allows them to sense object roughly as small as the
wavelength.
Ultrasound imaging uses high frequency (short wavelength)
sound to generate ‘echoes’ from internal organs to create an
image.
With an ultrasound frequency of 5 MHz, the wavelength in water
(the main constituent of the body) is ~0.3 mm and feature as
small
can be
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EDUF 1017 Physical Phenomena - Lecture 6
The Ear
Perception of Sound
Sound sets the
eardrum oscillating,
in turn causing the 3
small bones in the
inner ear to oscillate,
transmitting the
motion to the fluidfilled inner ear. Hair
cells in the inner ear
transmit nerves
impulses to the brain.
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Harmonic content
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Harmonic content
Musical sounds are more complex than a simple wave. The
pressure fluctuations illustrated below are complex because
the column of air in the wind instruments vibrates at a
fundamental frequency and many harmonics (multiples of
the fundamental frequency) at once.
Clarinet at
fundamental
frequency of
233 Hz.
Alto Recorder
at fundamental
frequency
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523 Hz.
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Sound intensity - Loudness
For a given frequency, the greater the pressure amplitude,
the greater the loudness - the ear is not equally sensitive to
all frequencies.
high frequency sensitivity decreases with age and exposure.
Pitch (high or low) is primarily determined by frequency,
although amplitude also has a small affect (louder sounds
are usually perceived as slightly lower pitch).
Tones produced by different instruments may have the same
fundamental frequency (hence pitch) but sound different
because of different amounts of various harmonics - a
difference called tone colour, quality or timbre.
Tone quality is also determined by the behaviour at the
beginning (attack) and end (decay) of a tone - exactly when
and how fast each frequency grows and dies away.
Unlike musical tones, noise is a combination of all
frequencies, not just harmonics of a fundamental.
White noise is equal amounts of all audible frequencies.
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Hearing range
The ear is sensitive over a broad range of intensities, so a
logarithmic intensity scale is usually used - the sound
intensity level in decibels (dB). 1 dB is a factor of 10 in
intensity.
Military jet 30 m away
Threshold of pain
Busy street traffic
Ordinary conversation
Quiet car
Average whisper
Threshold of hearing at 1000 Hz
140 dB
120 dB
70 dB
65 dB
50 dB
20 dB
0 dB
100 W/m2
1 W/m2
10-5 W/m2
3.2 x 10-6 W/m2
10-7 W/m2
10-10 W/m2
10-12 W/m2
1% of people
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50% of people
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