What is Music?  Music is sound waves organized by people on purpose, to dance  to, to tell a story, to make people feel a certain way or just to be entertaining.  Music is organized on many levels, melodies, harmonies, rhythms , textures, phrases, beats, measures, cadences and forms.  These attributes keep music organized and understandable.  But the most basic way music is organized is by arranging the actual sound waves a certain way so that it is pleasing to the ear.

Crashes, thuds and bangs are called noise when they are random but when arranges rhythmically they become a drum solo.  However to get the melodic sounds more commonly associated with music, the sound waves themselves must be organized and regular, not random mixtures.  Most sounds we hear are brought to our ears through the air.  A movement of an object causes a disturbance of the normal motion of air molecules near the object.  These molecules in turn disturb other nearby molecules out of their normal pattern so that the disturbance itself becomes a thing that moves through the air - a Sound Wave.  If the movement of the object is a fast regular vibration, then the sound waves are are also regular.  This is what we hear as tones, sound with a particular pitch.  This is what we associate with music and what most musical instruments are designed to do.

Sound Wave

Musicians have terms they use to describe tones but so do scientists and engineers.  As we talk about where music theory comes from is is useful to know both the musical and scientific terminology.  For example the closer together the evenly spaced waves are the higher the note sounds.  Musicians talk about the pitch of the sound or name certain notes or talk about tuning.  Scientists and engineers on the other hand will talk about frequency and wavelength of the sound.  They are basically talking about the same thing but in different ways.

Longitudinal and Transverse Waves.  So what are we talking about when we speak of sound waves?  Waves are disturbances, changes in something - the ocean surface, air, electromagnetic fields are a few.  Normally these waves are moving away from what created it causing a domino effect.  Most kinds of waves are transverse waves.  In a transverse wave, as the wave is moving in one direction it is causing a disturbance in another direction. Similar to waves on the ocean. 

Sound waves are not transverse waves, sound waves are Longitudinal Waves.  Because longitudinal waves are a little difficult to imagine and there are few examples in everyday life most diagrams will show sound waves as if they are transverse waves.

Wave Amplitude and Loudness: In a sound wave there is no displacement whenever the air molecules are at normal density. The most displacement occurs wherever the molecules are most crowded or least crowded.

Amplitude is the measure of displacement:  how big is the change from no displacement to the peak of the wave.  Amplitude is Loudness.