Spectral density

Spectral density is like a measure of how much frequency is in a signal. Imagine you have a bowl full of different-colored coins. Spectral density is like counting how many coins of each color are in the bowl. If there are more of one color coin than another, that color has a higher spectral density. It can tell us what kind of sound is in a signal. For example, a signal with a high spectral density of lower frequencies will sound 'boomy', while a signal with a high spectral density of higher frequencies will sound 'tinny'.