Audio
Engineering Society
Chicago SectionMeeting Recap - December 19, 2012
AES
Chicago Section Meeting Recap
Meeting Date: December 19, 2012
Written By: Jeff Segota
Topic:
Application of Microelectromechanical
(MEMs) Microphones to Hearing Aids
Presented
by: Wade
Conklin
On
December 19th, the Chicago section enjoyed a presentation on the
application of microelectromechanical (MEMs) microphones to
Hearing aids by Wade Conklin of Knowles Electronics. About 30
members and non-member guests were in attendance. Mr. Conklin
holds a Master’s Degree in Acoustics from Penn State University,
and has been involved in electret and MEMs microphone development,
modeling, and validation at Knowles for the past 6 years. Most
recently he acted as the lead engineer for the first MEMs
microphone used in a hearing aid and his current work focuses on
developing more MEMs designs for hearing aid use.
Electret
microphones have been the staple of the hearing aid industry since
1962 but still have issues with stability, manufacturing
automation, matching for multi-microphone applications, and
resistance to reflow temperatures. MEMs microphones have addressed
these issues, but suffer from low sensitivity and high noise which
has prevented their use in hearing aids. Knowles has developed
multi-element MEMs microphones that significantly “narrow the
gap” in sensitivity and noise performance compared to electret
microphones.
Mr.
Conklin reviewed condenser microphone theory of operation,
described how it is implemented in both electret and MEMs
configurations, and briefly described the manufacture of both. He
then covered the “wish list” of microphone characteristics for
hearing aid use, and how both types fare in this context.
MEMs
microphones are inherently more consistent within a single wafer
and are much more stable with time and environmental conditions,
all of which are critical for multi-microphone applications. MEMs
microphones are also less expensive because their manufacture is
highly automated, and they are more easily built into hearing aids
because they can withstand reflow temperatures.
Currently
available single –element MEMS microphones have lower
sensitivity and higher noise than electret microphones, mainly
because they cannot achieve as high of a bias voltage for the
transducer. MEMs microphones must step up the 0.9 V hearing aid
battery voltage to provide this bias, and there is a practical
limit. Electret microphones do not rely on the battery voltage,
but rather contain a layer of insulating material that can be
electrostatically charged to a very high voltage. MEMs microphones
also have complex “charge pump” circuits for the bias that
presents challenges. And the MEMs microphones consume more power
than electrets, resulting in shorter battery life.
But
by using multiple, parallel MEMs elements in a single package, the
sensitivity and noise performance is improved. Mr. Conklin
described their four- element design that has a 6 dB
signal-to-noise improvement over a single element MEMs microphone.
And the power consumption issue has been addressed.
However,
the “Quad” design still doesn’t quite match the electrets
for signal-to-noise
performance, so the MEMs microphones have not penetratedthe
hearing aid market… yet.
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