A sound wave is produced by a vibrating object. As a guitar string
vibrates, it sets surrounding air molecules into vibrational motion. The
frequency at which these air molecules vibrate is equal to the
frequency of vibration of the guitar string. The back and forth
vibrations of the surrounding air molecules creates a pressure wave
which travels outward from its source. This pressure wave consists of
compressions and rarefactions. The compressions are regions of high
pressure, where the air molecules are compressed into a small region of
space. The rarefactions are regions of low pressure, where the air
molecules are spread apart. This alternating pattern of compressions and
rarefactions is known as a sound wave.
In solids, sound can exist as either a longitudinal or a transverse
wave. But in mediums which are fluid (e.g., gases and liquids), sound
waves can only be longitudinal. The animation above depicts a sound wave
as a longitudinal wave. In a longitudinal wave, particles of the medium
vibrate back and forth in a direction which is parallel (and
anti-parallel) to the direction of energy transport. In the animation
above, the energy is shown traveling outward from the guitar string -
from left to right. A careful inspection of the particles of the medium
(represented by lines) in the animation above reveal that the particles
of the medium are displaced rightward and then move back leftward to
their original position. There is no net displacement of the air
molecules. The molecules of air are only temporarily disturbed from
their rest position; they always return to their original position. In
this sense, a sound wave (like any wave) is a phenomenon which
transports energy from one location to another without transporting
matter.
A guitar string vibrating by itself does not produce a very loud sound. The string itself disturbs very little air since its small surface area makes very little contact with surrounding air molecules. However, if the guitar string is attached to a larger object, such as a wooden sound box, then more air is disturbed. The guitar string forces the sound box to begin vibrating at the same frequency as the string. The sound box in turn forces surrounding air molecules into vibrational motion. Because of the large surface area of the sound box, more air molecules are set into vibrational motion. This produces a more audible sound.
A guitar string vibrating by itself does not produce a very loud sound. The string itself disturbs very little air since its small surface area makes very little contact with surrounding air molecules. However, if the guitar string is attached to a larger object, such as a wooden sound box, then more air is disturbed. The guitar string forces the sound box to begin vibrating at the same frequency as the string. The sound box in turn forces surrounding air molecules into vibrational motion. Because of the large surface area of the sound box, more air molecules are set into vibrational motion. This produces a more audible sound.
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