  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.
