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ResonatorA Resonator is a natural acoustic amplifier found on stringed instruments and developed for playing ensemble with other instruments in the days before Electronic Amplifiers.

While many stringed instruments have a closed box construction forming a Soundbox (e.g. Lute, Mandolin, Guitar etc), others have an open back (e.g. Banjo) and therefore do not have a resonant cavity to amplify the sound. A Resonator fixes on to the back of the instrument to complete the cavity and boost the volume. The Resonator is designed to reflect the air vibrations produced by the instrument's Soundboard or head and amplify the sound. The degree of success is mostly a function of the mass of air inside and the size. Both types of resonant cavity are known as Helmholtz Resonators after the 19th Century physicist Hermann von Helmholt.

The material can be almost anything. Wood is traditional but Aluminium, Brass, Copper and Plastic Composites have all been used. Resonators can be plain or highly decorated and wooden ones are often inlaid with different woods and Mother of Pearl. Metal ones tend to be engraved.

There is another type of Resonator using metal cones, developed for the Guitar - the Dobro.