Sound is a vibration that propagates as a typically audible mechanical wave of pressure and displacement,
through a medium such as air or water. It can propagate through compressible media such as air, water and
solids as longitudinal waves and also as a transverse waves in solids
(It is longitudinal when the particles vibrate parallel to the direction of the wave’s velocity and transversal when
the vibration is perpendicular to the direction of the wave’s velocity).
The sound waves are generated by a sound source, such as the vibrating diaphragm of a stereo speaker. The
sound source creates vibrations in the surrounding medium. As the source continues to vibrate the medium,
the vibrations propagate away from the source at the speed of sound, thus forming the sound wave. At a fixed
distance from the source, the pressure, velocity, and displacement of the medium vary in time. At an instant in
time, the pressure, velocity, and displacement vary in space. Note that the particles of the medium do not travel
with the sound wave.
This periodic nature can be graphed as sinusoidal disturbance, provided the amplitudes are measured in terms
of the changes in the density or pressure in the medium. Compressions (also called condensations) are
regions of high pressure and would be represented as crests; while, rarefactions, or low pressure, would be
represented as troughs. The frequency of sound, as interpreted by our ears, is called the sound’s pitch. If only
a single disturbance creates a sound wave, such as a ball crashing into a bat, or a tree falling in a forest, that
disturbance is not repetitive and does not result in a periodic wave, it would instead be called a pulse.
Sound waves travel through the outer ear, are modulated by the middle ear, and are transmitted to the
vestibulocochlear nerve in the inner ear. This nerve transmits information to the temporal lobe of the brain,
where it is registered as sound.
The human ear can generally hear sounds with frequencies between 20 Hz and 20 kHz (the audio range).
Although hearing requires an intact and functioning auditory portion of the central nervous system as well as a
working ear, human deafness (extreme insensitivity to sound) most commonly occurs because of abnormalities
of the inner ear, rather than in the nerves or tracts of the central auditory system. Sound below 20 Hz is
considered infrasound, which the ear cannot process.
Part A: Visualizing sound waves
Discussion
The oscilloscope is a powerful piece of diagnostic equipment because it automatically makes a graph of
voltage versus time. In other words, it lets you see on a screen how the voltage in a circuit is changing with
time. It also lets you change the scale of the graph both in the voltage magnitude and in the time scale.
Oscilloscopes at first look complicated, but with practice they become very useful tools. Just keep in mind that
this is a tool for letting you see how things change with time
