Electrostatic Acoustic Sensor with an Impedance-Matched Diaphragm Characterized for Body Sound Monitoring

Acoustic sensors are able to capture more incident energy
if their
acoustic impedance closely matches the acoustic impedance of the medium
being probed, such as skin or wood. Controlling the acoustic impedance
of polymers can be achieved by selecting materials with appropriate
densities and stiffnesses as well as adding ceramic nanoparticles.
This study follows a statistical methodology to examine the impact
of polymer type and nanoparticle addition on the fabrication of acoustic
sensors with desired acoustic impedances in the range of 1–2.2
MRayls. The proposed method using a design of experiments approach
measures sensors with diaphragms of varying impedances when excited
with acoustic vibrations traveling through wood, gelatin, and plastic.
The sensor diaphragm is subsequently optimized for body sound monitoring,
and the sensor’s improved body sound coherence and airborne
noise rejection are evaluated on an acoustic phantom in simulated
noise environments and compared to electronic stethoscopes with onboard
noise cancellation. The impedance-matched sensor demonstrates high
sensitivity to body sounds, low sensitivity to airborne sound, a frequency
response comparable to two state-of-the-art electronic stethoscopes,
and the ability to capture lung and heart sounds from a real subject.
Due to its small size, use of flexible materials, and rejection of
airborne noise, the sensor provides an improved solution for wearable
body sound monitoring, as well as sensing from other mediums with
acoustic impedances in the range of 1–2.2 MRayls, such as water
and wood.