(This post is based on an answer I wrote to a question in Quora)
What is the difference between music and noise? There are multiple ways to answer this question.
From an aesthetics point of view, the difference is hard to tell. In fact, it could be said that there is no difference—it depends on the taste of the listener. Any sound could be considered music or noise, as long as the listener draws pleasure from the listening activity.
According to the online Encyclopædia Britannica, music is the:
“art concerned with combining vocal or instrumental sounds for beauty of form or emotional expression, usually according to cultural standards of rhythm, melody, and, in most Western music, harmony.”
If we are willing to accept this more strict definition, the biggest difference between music and noise is periodicity. Music tends to have repetitive structures that allow us to know, to a certain degree, what to expect. This hapens at several levels. For instance, a single instrument note sounds like, let’s say, an A4 note, because it has a repetitive pattern that occurs 440 times per second. If such pattern wouldn’t exist, we would perceive it as noise.
The same happens at rhythmic level. A monkey bashing a drum kit sounds like noise because it doesn’t have a minimum degree of repetition that could be perceived as rhythm. Rhythm is by definition a repetition of a sound (or a set of sounds) at periodic intervals. Without such repetition, there is no rhythm.
An explanation of why repetition is so essential to music might be in its origin as a social activity. For a group of people to engage in such activity, i.e., for the participants to develop a connection based on sound and silence, it is necessary for them to be synchronized. And the best (or only) way to be synchronized is to have a certain degree of expectation of what is about to come. If a participant of a social music session is playing a single drumming sound, she needs to know when to hit it. If he is “in-sync” with the activity, he can predict when is the right time to do so. By having certain repetitive patterns, this can be accomplished.
All this does not mean that music should be predictable. At all the aforementioned levels (melody, rhythm), changes are introduced to avoid making the listening (and performing) experience boring. It is precisely the delicate balance between what we expect and what surprises us what makes good music so valuable.