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What is sound
Sounds starts with the vibration of a
medium. This vibration can come from
striking a surface such as a drum or the
plucking of a string. Air molecules around
the medium are normally evenly distributed
in a state of equilibrium. When the
medium vibrates the air molecules create
areas of compression and rarefaction.
Air molecules are closer together during
compression and farther apart during
rarefaction
The amount of pressure that is created by air molecules during this process is called the sound
pressure level or SPL for short. The above graph shows the sound pressure level in a wave
form. The top of the wave is called the crest and the bottom is the trough. The line through the
center is the equilibrium and represents the steady state of air molecules before they are
excited. The amplitude is the distance from the crest or trough from the equilibrium and the
wavelength is the distance between two crests or troughs. The time that it takes for a wave to
make one full cycle is called a period and the amount of periods or oscillations per second is
called the frequency and is measured in herts (hz)
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How do we hear sound
The ear is made of three parts; the outer ear, middle ear, and the inner ear. When sound comes
through the outer ear (the part that you can see on your head) it is channel through the ear canal
and comes in contact with the tympanum or eardrum. The tympanum vibrates with the areas of
compression and rarefaction causing the three bones in the middle ear (hammer, anvil, and
stirrup) to vibrate. The inner ear is filled with fluid and sound travels at a different speed in fluid
than it does in air. The bones in the middle ear make up for this change of speed by enacting a
mechanical ratio between the two speeds.
The cochlea is contained in the inner ear. It is shaped like a
shell, filled with fluid, and has thousands of tiny hairs in it called
cilia. Each hair vibrates at a certain frequency. When the hair
vibrates, a message is sent through the auditory nerve to the
brain and it registers as a sound.
Frequency and pitch
Pitch is the brain’s interpretation of a certain frequency. When the frequency is higher, the pitch
goes up and when the frequency is lower the pitch goes down. If you double the frequency of a
pitch, the resulting pitch will be one octave above the other.
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