Harmonics

The simplest signal is the tone, which can be represented in the frequency domain by a very narrow line. On the other hand, more complex but still periodic signals can be expressed as a sum of many tones or harmonics, which can be represented in either the time or frequency domain. Both representations fully describe the signals, since each can be mathematically transformed into the other. In other words, each representation contains all the characteristics of the sound being considered. Apart from that, the response of the human ear is approximately logarithmic in both amplitude and frequency.