In the most basic sense, a resonator is a natural AMPLIFIER. It is usually a cavity or hollow body which will vibrate sympathetically with another vibrating system, such as the air through which the sound waves are travelling.
See: EIGENTON, HELMHOLTZ RESONATOR, RESONANCE, STANDING WAVES, SYMPATHETIC VIBRATION. Compare: ACOUSTIC RADIATION.
The vocal tract, for instance, amplifies certain frequency ranges called FORMANTs, and hence colours the sound produced by the vibration of the vocal cords. See: VOWEL.
The strings of a violin are connected to the hollow wooden body of the instrument by a bridge and the air between them. Sound waves from the strings will pass through the bridge and the air into the body causing the system to vibrate, thus amplifying the initial sound. See diagram under RESONANCE CURVE.
The resonant frequency of a tube or string depends on its length. For instance, the length of a tube open at both ends (or a string fixed at both ends) corresponds to a half WAVELENGTH of the resonant frequency, whereas if the tube is closed at one end, its length corresponds to a quarter wavelength.
Sound Example: Knocking on the resonant body of a cello which acts as a resonator for the sound of the strings.