Apparatus and methods for upshifting the frequency of acoustic energy

An apparatus comprising a frequency converter arranged to be coupled to an acoustic source to receive at least a portion of an unshifted acoustic signal emitted by the acoustic source in a first frequency range, wherein:

• the frequency converter is configured to convert at least a portion of acoustic energy in the unshifted acoustic signal into acoustic energy in an upshifted acoustic signal in a second frequency range;
• the frequency converter is arranged to emit at least a portion of the upshifted acoustic signal into an ambient acoustic environment;
• and
• the second frequency range is higher than the first frequency range, and the unshifted acoustic signal comprises one or more unwanted acoustic noise signals,
• wherein:
• the frequency converter comprises two or more resonant members arranged to be connected to the acoustic source and a coupling member connecting the resonant members together;
• each resonant member exhibits at least one corresponding acoustic resonance frequency in the first frequency range;
• each resonant member is arranged so that, with the resonant member connected to the acoustic source, the unshifted acoustic signal drives each resonant member at its acoustic resonance frequency;
• the coupling member exhibits a nonlinear acoustic response so as to generate in the second frequency range sum-frequency acoustic signals from the resonant frequencies of the driven resonant members;
• and
• the upshifted acoustic signal includes the sum-frequency acoustic signals.