Abstract:

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.