4aEA8. Acoustical simulation of active noise control telephone earpiece system.
Session: Thursday Morning, June 19
Author: John C. Baumhauer, Jr. Location: Lucent Technologies, Bell Labs, 6602 E. 75th St., Indianapolis, IN 46250
A lumped-parameter equivalent circuit representation of an active noise
control (ANC) feedback system has been developed for telephone handset earpiece
application. Since the feedback controller must be stable to avoid ``squealing''
in the user's ear, its filter must work with known and robust transducer
transfer functions. The earpiece loudspeaker (receiver) is the weak link. As
opposed to the feedback electret microphone located between the earpiece and
pinna, the moving-coil receiver's response undergoes a large, negative phase
change with increasing frequency owing to multiple, highly coupled
degrees-of-freedom. Moreover, the magnitude and phase of the receiver's transfer
function are shown to depend on its electroacoustic parameter tolerances, such
as cone stiffness, coil B1, acoustic damping, and handset porting. Another
variation is due to the acoustic leak that occurs naturally between the earpiece
and the ear. This is not just a function of the earpiece design, but also of the
varying handset-to-ear force applied. Not only is low-frequency response in the
ear lost with an increased leak, but the high gain feedback magnitude is reduced
and the overall phase lag in the receiver becomes greater. Results with
simulated ITU-T coupler equivalent circuits demonstrate these effects. A
prototype ANC system is discussed.