Sound is moving energy that travels as a pattern of changing pressure. Sound can travel through gases, liquids and solids. As it travels, its energy is slowly lost as heat, through friction. Because a gas, such as the air, presents less friction to a soundwave than a solid, it can travel a longer distance before all its energy is lost as heat. A solid, on the other hand, will quickly absorb the soundwave, turning its energy into heat, and preventing it from travelling as far. Of course if the solid object is not too wide, such as a wall, a sound may travel through it, but its overall energy (amplitude) will be diminished.

Because most of the sound we hear arrives at our ears through the air, it is most useful to study the theory of air pressure soundwaves.

Air molecules

The air that surrounds us is comprised of particles known as air molecules. It is not important to understand what these molecules are made of.

Air pressure

Air pressure is a measure of how tightly packed together the air molecules are, in other words, the air density. When a sound is created, the surrounding air pressure is temporarily disturbed.

"Normal atmospheric pressure" depends on altitude. The higher you move through the atmosphere the less air molecules, the lower the density and therefore the lower the "normal" air pressure.

In sound theory, changes to air pressure are measured in units called decibels. This is the amplitude, volume, or loudness, of the sound.

Source

Sound is produced when an object vibrates and disturbs the pressure of the surrounding air. Such an object is called the source. The source may be a vocal chord, a guitar string, car engine or anything that makes a sound.

Cycle of source (a sphere) vibration - compression and rarefaction

Vibration

As a model to aid understanding, it is helpful to define vibration as the repetitive expansion and contraction of the source. A cycle of vibration works like this ...

Source starts at original size
Source expands (gets bigger)
Source reaches maximum size (bigger than normal)
Source contracts (gets smaller)
Source returns to original size
Source contracts (gets smaller)
Source reaches minimum size (smaller than normal)
Source expands (gets bigger)
Source returns to original size

Edited screenshots from a the Logic sample editor window (amplitude and time
axis have been added)

1) A single snare hit

2) A vocal recording (2 bars)

Vocal take waveform

Compression

As the source expands it pushes the surrounding air molecules away causing them to "bunch" together and the air pressure to increase.

Rarefaction

As the source contracts the surrounding air molecules spreads out to fill the increasing space, causing a decrease in air pressure.

Cycle of a loudspeaker

Another good model for understanding the cycle of a sound wave is provided by the movement of a loudspeaker cone. Click below for a useful diagram.

Loudspeakers, the inner ear and waveform diagrams

Sound waves

As the source expands and contracts a pattern of changing air pressure energy moves away from the source. This pattern of energy is called a sound wave. Sound radiates out in all directions from a sound source.

It is important to understand that individual air molecules do not move from the source to the listeners ear but move (or vibrate) back and forth around a fixed point knocking against each other thereby transferring the changing air pressure energy.

Imagine yourself standing in the middle of a dense crowd. You push the people next to you away. They push the people next to them before rocking back towards you. The people they push push other people and so on. A wave of energy can be said to have passed through all the people surrounding you, started by you. This process is not unlike a Mexican wave.

Human perception

When this pattern of changing air pressure reaches the ear it moves through a series of "chambers" before reaching the inner ear. Here it encounters a line of tiny hair receptors. Put simply, these hairs are pushed one way by air pressure above normal atmospheric pressure (compression) and the opposite way by air pressure below normal atmospheric pressure (rarefaction). When there is no pressure change they remain stationary. The movement of these hairs causes an "electrical" signal to be sent to the brain which we perceive as sound.

If you haven't done so already look at this diagram ...
Loudspeakers, the inner ear and waveform diagrams

Speed of sound

Soundwaves travels relatively slowly through air at around 340 metres per second (around 1 foot or 30cm per millisecond) compared to a solid such as steel at 5100 metres per second. Some simple equations relate the speed of sound to frequency and wavelength ...