  Most microphones operate by detecting the sound-pressure induced motion of a
membrane. In contrast, here we introduce a microphone that operates by
monitoring the sound-pressure-induced modulation of the compressibility of air.
By driving a graphene membrane at its resonance frequency, the gas, that is
trapped in a squeeze-film beneath it, is compressed at high frequency. Since
the stiffness of the gas film depend on the air pressure, the resonance
frequency of the graphene is modulated by variations in sound pressure. We
demonstrate that this squeeze-film microphone principle can be used to detect
sound and music by tracking the membrane's resonance frequency using a
phase-locked loop (PLL). Since the sound detection principle is different from
conventional devices, the squeeze-film microphone potentially offers advantages
like increased dynamic range, and a lower susceptibility to pressure-induced
failure and vibration-induced noise. Moreover, it might be made much smaller,
as demonstrated by the microphone in this work that operates using a circular
graphene membrane with an area that is more than a factor 1000 smaller than
that of MEMS microphones.
