An important feature of the source are its harmonics. One of the most important ideas in understanding the filter is resonance.
Harmonics Consider again the waves produced by the bass clarinet and the flute.
The lines in these spectra look suspiciously evenly-spaced. This is a typical property of naturally occurring waves. Consider a guitar string. Each of the higher-frequency simple waves is called a harmonic . The wave produced by the vibration of the vocal cords also has this kind of structure.