What is Sound?

Sound is a wave of energy, generally traveling through air, but sound waves can also travel in liquids and solids.  In air, sound is a compression wave, that causes the air molecules to be pushed together, and then apart.  How rapidly the compression cycle occurs is the Frequency (or pitch) of the sound, and how close together and far apart the molecules are moving determines the intensity (or loudness).

The Sound Wave 

Figure 1: A Typical Sine Wave
Figure 1: A typical sine wave is shown here, with its period and amplitude
Figure 1 illustrates how a simple sound can be visualized.  The air pressure changes over time in a cycle.  The wave shown is a simple wave representing a "pure tone" where there is only one frequency pattern in the sound.

Frequency of Sound 

As shown in Figure 1, the time between the cycles in the wave is called the period.  The inverse of the period (1/period) is the frequency.  The frequency is measured in Hz (hertz or cycles per second).  Healthy humans can hear sounds between about 20 Hz and 20,000 Hz.

Loudness of Sound 

The loudness of the sound is determined by the amplitude of the wave.  Figure 1 shows how the amplitute is the measure of how much the pressure changes from the average air pressure.  The greater the change in air pressure, the louder the sound.

Movement of Sound

Figure 2: Animation of Sound Wave Propagation
Figure 2: Animation of Sound Wave Propagation. The amplitude of the wave decreases as it travels from the source.
Sound moves through gasses (air), liquids, and solids.  In air sound moves as a compression wave, where the air molecules are pushed together and apart (bumping each other in an expanding pattern).  Figure 2 is a demonstration of how a sound wave propagates outward from a sound source.  The air molecules bump into each other causing the energy of the wave to more outward in a circular (or spherical) pattern.

As a sound wave moves away from the source, the amplitude of the wave decreases, because the same "energy" that was supplied at the sound source spreads out over a greater area.  Hence, the farther away a noise is, the softer is sounds.


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