In the world of acoustics, there are discussions of "harmonics", and this has application to amplifiers and other components in sound reproduction. Sounds of nature, as well as musical instruments, are distinguished from one another not simply on the basis of whether they are high or low in pitch, but their harmonic content. A sound has what is called a "fundamental" frequency. Humans can hear, by normal air conduction through the ear, frequencies between 20 Hz and 20 kHz. This is only an average, and as we age, we tend to lose the ability to hear high frequencies above about 12 kHz. Secondly, the sensitivity throughout the range is not smooth. We are most sensitive to the range of about 500 Hz to 5 kHz. The fundamental frequency of a sound is the lowest frequency component in that sound. The sound also has harmonics, which are multiples of the fundamental frequency. For example, a sound might have a fundamental frequency of 500 Hz. The second harmonic (there is no first harmonic unless you want to call the fundamental frequency as such) would be twice the frequency of the fundamental, or 1 kHz, the third harmonic three times the fundamental, or 1.5 kHz, and so on.