The 'Undertone Series' (by José A. Sotorrio)

Previously regarded as only a theoretical inversion of the 'overtone' series', this short video presentation explains how the 'undertone series' (or 'sub-harmonic spectrum') can actually be generated spontaneously using a tuning-fork and a piece of paper. (Subharmonics are also generated by lamellophone (thumb-piano) instruments when the soundbox vibrates against a surface and when objects are placed on the keys or body. It is also possible to produce undertones on violin instruments through a skillfull bowing technique; the bow reacts against the string in a similar ways as a tuning fork against paper.)