Office of Technology Transfer – University of Michigan

Trapped-Fluid Micromachined Capacitive Acoustic Sensor

Technology #3233


The typical human cochlea operates over a two and a half decade frequency band, from 20 Hz-20 kHz, covers 120 dB of dynamic range, and can distinguish tones which differ by less than 0.5%. Sounds as quiet as 0 dB SPL (20 .mu.Pa RMS) can be heard. Humans are also able to discriminate sounds temporally with spacing as small as 10-20 .mu.s. The human cochlea is small, occupying a volume of about 1 cm.sup.3. The cochlea uses a mechanical process to separate audio signals into ap-proximately 3000 channels of frequency information; it is a sensitive real-time fre-quency analyzer. Marine mammals such as whales hear over an even broader band than humans, utilizing acoustic signals for communication (at low frequencies) and navigation (high frequency “SONAR”). One difference between submerged and in-air operation is that the middle ear impedance matching functions may be inactive or modified in marine mammals due to the different characteristic impedance of the environment.


Researchers at the University of Michigan have invented silicon and glass micro-machined (MEMS) acoustic sensors incorporating trapped-liquid architectures. The trapped liquid serves as an acoustic transmission medium allowing the input port to the system to be physically separated from the sensing location. The trapped liquid interacts with a conductive, flexible sensing membrane. Sound pressure waves enter the trapped liquid through an input membrane, travel to the sensing membrane, and excite vibrations of the sensing membrane. The vibrations of the sensing membrane are measured using on-chip capacitive sensing. The capacitive sensing structure is formed by the conductive sensing membrane and a fixed conducting top electrode. As the gap between the conductive sensing membrane and the fixed top electrode varies, the capacitance varies, leading to an electrical signal which is the electrical output of the system.

Applications and Advantages


  • Acoustic sensors for commercial applications-nl-such as low power military systems such as-nl-unattended sensors, handheld SONAR, or-nl-autonomous vehicles)
  • Acoustic sensors for medical applications such-nl-as cochlear implant front-end
  • Environmental monitoring using a low-power-nl-unattended sensor


  • Small size
  • Low cost
  • High sensitivity