Audio Engineering Society

Chicago Section

Meeting Review, September 13, 2007

9/13/07 Meeting Highlights
by Nick Kettman

Thursday September 13th, the Chicago Section of AES met on-site at Knowles Electronics in Itasca, Illinois, for an insightful meeting and tour showcasing many of Knowles� capabilities in design, testing and manufacturing of subminiature acoustic transducers. The meeting featured presentations by two top technical staff members from Knowles, Evan Llamas-Young, Director of Receiver Development and Dr. Daniel Warren, Director of Research. Their presentations focused on how balanced armature receivers (micro-speakers) have evolved to serve the growing industry segment of high-fidelity in-ear applications. To begin, Dr. Warren gave the group some background on the problem and the acoustic environment being dealt with. He began by pointing out that, in noisy or otherwise acoustically poor environments, one way to improve the transfer function from the signal source to the listener, is to �go direct�, by cutting out most of the acoustical path and placing the acoustic transducer immediately into the user�s ear. This clearly has the potential to improve signal-to noise ratio, as well as having other advantages like bypassing oddly reflective or echoing rooms, eliminating some or all environmental noise (depending on whether a sealing eartip is used), and significantly reducing the power and size of the transducers used.

However, Dr. Warren was quick to point out that as people more familiar with traditional electrodynamic transducers encounter this application, it is important to understand the desired acoustic response curve is not the traditional �flat� line targeted by many loudspeaker designers. The reason for this is that in free-air, sounds travelling past the soft portions of the outer ear, and the horn-like cavity at the entrance to the inner ear, have an �ear transfer function� applied to them, and this response has a sizeable peak in the midrange frequencies, plus a dip in the higher frequencies.

At this point, Mr. Llamas-Young took over, and pointed out some specifics of the Knowles� balanced armature transducers, including some of the electroacoustic models, and how various component parameters may be varied to affect the acoustic response of the finished transducer. In particular, Mr. Llamas-Young highlighted one of the newest and smallest Knowles units, the FK receiver which has been used very successfully in the in-ear application, due to an ability to combine and filter multiple FK�s into woofer/tweeter-like combinations for dual-receivers or other multi-receiver combinations.

Following Mr. Llamas-Young�s talk, visitors got to see highlights of several development and test labs featuring such items as production testing of balanced armature receivers, prototype assembly of Knowles� SiSonicTM surface mount microphones, and other activities. An enjoyable time was had by all attendees, and the Chicago Section thanks Knowles for opening it�s doors to us for this special event.

Dan Mapes-Riordan (center) and Jeff Segota (right) look on as John Beard demonstrates the KEMAR head and torso

simulator in the Knowles anechoic chamber.