Unidirectional mechanical amplification as a design principle for an   active microphone
  Amplification underlies the operation of many biological and engineering systems. Simple electrical, optical, and mechanical amplifiers are reciprocal: the backward coupling of the output to the input equals the forward coupling of the input to the output. Unidirectional amplifiers that occur often in electrical and optical systems are special non-reciprocal devices in which the output does not couple back to the input even though the forward coupling persists. Here we propose a scheme for unidirectional mechanical amplification that we utilize to construct an active microphone. We show that amplification improves the microphone's threshold for detecting weak signals and that unidirectionality prevents distortion. 
Tobias Reichenbach and A. J. Hudspeth
physics.ins-det physics.bio-ph q-bio.QM