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Acoustics, the science of sound
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Examples of sound sources
- A guitar vibrating
- A saxophone vibrating
- Loudspeaker vibrating
because of feeding oscillating electric currents
- Vocal cords vibrating e.g.
a cat meowing
Frequency and pitch
If an object oscillates, the
surrounding air will also oscillate. For example, if you hold a ruler firmly
against a table while half the ruler is outside the table edge, and the
overhanging ruler part is bent down and released, a sound is heard.
Pull down the ruler and then release it. The ruler oscillates up
and down and a specific tone sounds. Estimate the frequency?
If two metal objects are
banged against one another, they also begin vibrating. This is felt if
holding the metal objects. It can feel uncomfortable holding the metal
objects that one may want to drop them. The metal objects set the
surrounding air into vibration. The oscillating air reaches our ears and we
hear a tone. The reason we can hear the sound is to be found inside our
ears. Inside each ear there is an eardrum. The eardrum is a small diaphragm
oscillating in pace with the sound condensations and dilutions. Nerve
impulses are sent to the hearing center in the brain.
If we instead let the ruler in the previous example extent further from the
table and struck, a slightly deeper tone is heard. A deeper tone occurs when
the ruler oscillates fewer times per second. The ears are thus hit by fewer
cycles per second. We say the tone is lower, or we have a low tone. If
instead, only a small part of the ruler extends from the table edge and
struck, a higher tone is heard. This tone oscillates more times per second.
This makes the eardrums oscillating more cycles per second.
Hz (Hertz) is the unit of sound. It means
number of cycles per second.
When talking about high or low tones the term pitch is also used, i.e.
high-pitched and low-pitched tones.
Decibel scale (audibility threshold, pain threshold, and harm threshold)
The decibel scale is a way to measure of the intensity (power) of a sound.
The more intense a sound, the more ache in the ears. In everyday life we say
that we increase the volume e.g. of a stereo equipment.
The decibel scale is structured so that 20 dB is experienced twice as loud
as 10 dB. 40 dB is experienced 4 times more than 20 dB.
Figure 2: Decibel scale
generation and its transfer through the air - use the particle
Figure 3: Sound waves consist
of condensations and dilutions. The curved lines above are
condensations and the spaces in between are dilutions. The air particles are
crowded and alternately diluted.
The condensations and dilutions in air travel with a speed of 340 m/s.
Figure 4: In the
wave-formation above each condensation is showed as a wave-crest and each
dilution as a wave trough. This wave-formation is called a sine curve. The
amplitude decides the intensity (power) of the sound. High amplitude means
that the sound is intense. The number of wave-crests per second (cycles per
second) decides whether the tone is high (many cycles per second) or low
(few cycles per second).
speed of sound
The speed of sound in air is 340 m/s. Mainly, the denser a substance is, the
faster the sound can travel through it. The sound speed in water is 1500 m/s
and in steel 5100 m/s.
Noise, echo and resonance
As seen above on the sounding speaker there is equal distances between the
sound waves. So we have a uniform tone. With noise, the lengths between the
sound waves are not the same. The sound varies in an irregular manner.
Furthermore, the amplitude varies in an irregular way. If experiencing the
sound as unpleasant, it is called noise.
If shouting close to a rock wall, it seems the rock wall shouts back. This
phenomenon is called echo. Echo means that the sound waves bounce towards
and off the rock wall and further back to the person shouting.
If a tuning fork is struck it produces a uniform tone. When this frequency
is transferred onto another object, it too begins oscillating at the same
rate. The object resonates (is in resonance with) with the tuning fork.
Figure 5: The tuning fork to
the left is struck. Its sound waves make the other identical tuning fork to
oscillate with the same frequency.
Try striking a tuning fork inserted in an acoustic chamber while having an
identical tuning fork with an acoustic chamber close. Close with your hand
the oscillations in the first tuning fork and see what happens with the
second tuning fork. The second tuning fork still sounds. This phenomenon is
Human audibility frequency range
Humans can hear frequencies between 20 and 20.000 Hz. One Hertz (1 Hz) is
one cycle per second. 20 Hz means 20 cycles per second.
Ultrasound and infrasound
All sounds below 20 Hz are called infrasound. These sounds cannot be heard.
Research has shown that some infrasound can make us feel bad, sick or tired.
Ultrasounds have frequencies above 20.000 Hz. These sounds as well we cannot
hear. Ultrasound is used at fetal diagnostics. The ultrasound device is hold
against the mother’s stomach. The sound waves bounce back from various fetal
tissues inside the womb. The device does this continuously and we can see
the baby moving on a TV screen.
Bats also send out ultrasounds. The ultrasounds bounce off objects located
by the bat. Some of these objects are possible preys. The echo comes back to
the bat. Also dogs can hear ultrasounds. Dog Whistles emit ultrasound. When
blowing into dog whistles we humans hear nothing at all. However, for the
dog, it sounds a lot.
by Lars Helge Swahn, Last revision: 2012-02-15
(translation of our Swedish version)
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