Kerry Haps and 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.
