Human ear - The inner ear contains minute hair (cilia) of graduated sizes that respond to different speeds of sound vibrations transmitted by the eardrum. The network of cilia receptors is directly connected to the nervous system where the signals are processed.
Hertz (Hz) are named after Heinrich Hertz (1857-1894) who generated and detected electromagnetic waves across the length of his laboratory on a wavelength of approximately one metre. To detect electromagnetic waves Hertz used a simple form of oscillator which he called a 'resonator'. Cycles per second (cps) are now expressed as Hz, i.e. 800 Hz, rather than 800 cps.
Harmonics are a combination of notes whose frequencies are related by simple whole number ratios. A harmonic series is a set of frequencies which are successive multiples of the fundamental (or 1st harmonic). For example, the set of frequencies 100, 200, 300, 400, 500 Hz is a harmonic series whose fundamental is 100 Hz and whose 5th harmonic is 500 Hz. In general, the nth harmonic of a series has a frequency which is n times the fundamental frequency.
Node - Point of no displacement. Because the ends of the string are attached and fixed in place to the guitar's structure - the bridge at one end and the frets at the other - the ends of the string are unable to move.