The second requirement is that there must be at least four harmonic sounds between 300Hz and 4kHz. Harmonic sounds are technically defined as a multiple of the fundamental frequency. In this graph, the fundamental frequency (also known as the first harmonic) is 900Hz. The second harmonic is at 1800Hz, the third harmonic is at 2700Hz, and the fourth is at 3600Hz. Therefore, the sound shown does have four harmonics between 300Hz and 4kHz. The other frequencies that make up this sound are musically known as overtones.