Sound waves can also travel
trough liquids and solids. The velocity of a sound
wave depends on the temperature of the medium and its
elasticity (more elasticity means that molecules will
move easily). Through air, sound waves travel
at 343 m/s. Actually, sound waves move
faster through liquids and solids than through gases.
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The human ear is able to feel frequencies between
20 Hz to 15 000 Hz, depending on the age of the
person. Sound waves with a frequency above 20 000 Hz
are called ultrasonic waves.
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Scientists measure the amplitude in atmospheres.
Humans can detect from less than a billionth of an
atmosphere to values one million times higher.
However, it is hard to deal with this huge range of
different values. Instead, the pressure is measured
by the intensity of the sound. The quietest sound
corresponds to a value of zero decibels
(unit of sound intensity) and a value above a
hundred corresponds to annoying sounds.
A sound wave
with a wavelength of 1.24 m is moving at 343 m/s.
What is the frequency of this wave?
wl = wavelength
wl = 1.24 m
v = 343 m/s
f = v/wl = (343 m/s)/(1.24 m) = 276.61 Hz
two or more waves with the same frequency reach the
ear, the ear interprets these waves as one wave with
amplitude as big as the sum or difference of the
On the other hand, an unpleasant sound or noise
will be produced if the frequencies have ratios
others than small whole numbers. This is called dissonance.
The sound that
reaches John's ear has a higher pitch than
the sound that reaches Bob's ear. This effect
is called Doppler effect.
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Try chapter 9 quiz!
[Ch8] - [Ch9]