I should first mention that this whole section is based on my educated conjecture. I have never read on this topic, and really don't know how many people, if any, have written on it. Feel free to do research and fill me in. In any case here is some food for thought.[br][br]It is almost not worth mentioning that different people like different music. What some find soothing and harmonic, others find boring. What some find dissonant and noisy, others find pleasing. In spite of these listening differences, and different preferences regarding the genre of music, there are certainly some fundamental aspects of sound that all will agree upon.[br][br]The idea of the sound of smooth harmony is universal. Everyone without amusia (an inability to distinguish between tones) will agree that two notes an octave apart sound smooth. Add a perfect 5th as when one plays C, G, C and it still sounds great. In fact everything from heavy metal power chords to J.S. Bach employs such intervals. So what, if anything, can be said about sounds that humans in general tend to enjoy?

When one plots the intensity of a sound versus the frequency, it is called a Fourier spectrum. A numerical means of doing such calculations is called the fast Fourier transform, and so the abbreviation "FFT" is often used. If you take the FFT of a sound signal you end up with the Fourier spectrum.[br][br]The simplest possible spectrum is that of a pure sine wave of fixed frequency. This is like the tone of a hearing test. The spectrum will have one peak and be flat everywhere else. The single peak's height is proportional to the loudness of the sound and the x-component is the sound's frequency. The spectrum of a tuning fork is nearly as simple, but usually there are a few harmonics or overtones, albeit weak ones.[br][br]Recall that when a single note is played on a musical instrument of any sort, that a fundamental tone (first harmonic) is produced, along with a multitude of harmonics. In this sense the Fourier spectrum of a single note is far less void than that of a pure sine wave.[br][br]Add a second note to the sound, and many more peaks in the spectrum emerge. Fewer new peaks emerge when the two tones have small integer ratios of their fundamental frequencies, such as 3/2 or 4/3. In the first case, every third harmonic of one note will coincide with every second tone of the other note.[br][br]With this idea of the spectrum in mind, we can consider the density of the spectrum. The density of the sine wave's spectrum is as low as it can be. What's the opposite in this sense of a sine wave? It happens to be noise. Noise is the simultaneous sounding of a continuum of frequencies. Neither one of these extremes is pleasing to the ear. It seems like we like sounds that have a moderate spectral density. Too sparse and it sounds boring, too full and it sounds noisy. Just how much spectral density we like seems to be where we differ most in taste.[br][br]When a guitarist uses distortion on an electric guitar, what it really does is it alters the spectral density. It increases it. With too much distortion the music literally has the same spectrum as noise. In a very similar way, when jazz musicians started using very dissonant intervals like 13[sup]th[/sup]s and minor 2[sup]nd[/sup]s, since those notes had very few common overtones with the other notes in a chord, they basically served to fill in the spectral gaps and increase the density of the spectrum. These two examples may both really be solutions to the same "problem" - the problem of being bored with too little spectral density.[br][br]So it seems that we all tend to have a preference for a certain amount of spectral density just as in the story of Goldilocks and the Three Little Bears wherein Goldilocks liked her porridge just right.