Model railroad type example Sn3.5 is as follows, S (scale) being the prefix to n (narrow gauge) followed by 3.5 (prototype track gauge in feet), 3 and 1/2 feet, or 42 inches, which in Sn3.5 equates to actual modeling track width of .649 - .672 thousandths, 16.5 - 17.1 mm.
Definition of harmonic. 1  1a : overtone; especially : one whose vibration frequency is an integral multiple of that of the fundamentalb : a flutelike tone produced on a stringed instrument by touching a vibrating string at a nodal point.
In physics, resonance is a phenomenon in which a vibrating system or external force drives another system to oscillate with greater amplitude at specific frequencies. Frequencies at which the response amplitude is a relative maximum are known as the system's resonant frequencies or resonance frequencies. At resonant frequencies, small periodic driving forces have the ability to produce large amplitude oscillations, due to the storage of vibrational energy.