Resonator

An acoustic element which is used to attenuate the sound at narrow band frequencies in ducts and tubes. A simple resonator comprises a cavity enclosing a mass of air, with a narrow opening to the outside. The air mass effectively acts as a 'spring' at the resonant frequency of the cavity and under those conditions absorb appreciable sound energy exciting the resonance. The use of resonators dates back to Roman times when amphora (vases - helmholtz resonators) were used to improve the acoustics of ampitheatres.

See also: Helmholtz Resonator, Quarter Wave Tube.