Research->Music & Fluctuation->What is Fluctuation?
This page provides overview of what fluctuation is.
Fluctuation in Nature
Faithfully repeating the specific pattern does not fit the definition of fluctuation. Rather, it indicates the phenomenon that the repeated pattern changes according to the progression of time. Therefore, it means that quality and quantity  are not fixed all the time and  can become bigger and smaller or stronger and weaker according to the progression of time.
However,  when the word "fluctuation" is used,  it implies that you can see something like regularity in the change. It doesn't  mean that the change occurs at random like the numerical value generated with random numbers.
In other words, there are movements with regularity behind the fluctuation. So, you can interpret fluctuation as "regularity to be destroyed".
In brief, when you say  "there is fluctuation",  it  means that the sound  is not monotone and includes both "order" and "disorder".
We are surrounded by a lot of fluctuations that are produced by nature. If we examine the strength of wave by each frequency,  the degree of the fluctuation transformation can be grasped. It is said that when so-called 1/f Fluctuation is included in the sound , it would be pleasant to human ear. 1/f Fluctuation indicates both order and disorder. For instance, human pulse, sound of stream, changes of temperature, and annual rings of a tree possess the characteristic of 1/f Fluctuation. 
Fluctuation in Music
As you may know, you cannot pick every sound that makes up the music freely. The way you select the sound would be restricted by music scale, harmony, and what you put back and forth of each sound. The simplest rule can be said that the same sound does not continue too much and the sound does not jump too much. Therefore,  the basic condition is that the music includes repetition and change moderately.
For instance, the melody that has conjunct motion and slow tempo would not bring discomfort but makes you bored. Because you can expect what the next sound would be. If you really want to attract your listeners, you need to give moderate changes by including  some sounds that jump by 3 degrees, 4 degrees, and 5 degrees. As a conclusion,  if there is a balance between predicted sounds and unexpected sounds, it can be said that it fills the condition of a good melody. Therefore, conjunct motion and disjunct motion are the key to relaxing and ear-pleasing melody.
It is said that Mozart's tunes have characteristic of 1/f Fluctuation. If you listen to music of 1/f Fluctuation, you will be relaxed and alpha wave will be detected  in your brain.