The sympathetic or induced vibration of a system (a solid, a membrane, or an air space) in response to the presence of vibration in the air; the tendency of a mechanical or electrical system to vibrate at a certain frequency when excited by an external force and to keep vibrating after the excitation is removed, e.g., a bell. When a vibrating object (such as a guitar body) is stimulated by a second oscillator (such as a vibrating string), its pattern of vibration may be altered. If the two vibrate at the same or a harmonically related frequency, they tend to phase-lock together, reinforcing the sympathetic vibration at this common resonant frequency. Oscillations at non-harmonic frequencies have far less effect due to phase cancellation. (1) A function of a filter in which a narrow band of frequencies, the resonant peak, becomes relatively more prominent. If the resonant peak is high enough, the filter will begin to oscillate, producing an audio output even in the absence of input. Filter resonance is also known as emphasis and Q. It is also referred to in some older instruments as regeneration or feedback, because feedback was used in the circuit to produce a resonant peak. (2) The tendency of a speaker to vibrate most at a particular frequency; sometimes referred to as the natural frequency.