Digital filters used in the feedback loop of an ANR earplug

Abstract

Commercially available active hearing protectors are usually implemented as ear muffs with an analog driven feedback loop. The active attenuation bandwidth of these devices is limited to about 800 Hz. Moreover, it is not possible to adapt the analog system for specific noise events or to the user’s morphology. Active earplugs allow one to extend the bandwidth of the active protection to frequencies higher than 2 kHz. A digital feedback system has been implemented for active earplugs, in order to allow an adaptation to different noises and users. As the acoustic delays are too short to obtain a causal system when using FIR filters, the filters have been implemented as IIR type filters. The paper presents numerical simulations of different possible algorithms, used for the cascaded IIR filter of the ANR system. It shows how numerical errors are propagated in cascaded Bi-Quad implementations, and how an optimum signal-to-noise ratio may be obtained. The numerical predictions and experimental results will be presented and compared. As these filters will be used in a closed-loop feedback system, a numerical simulation of the behavior of the closed-loop system, including the electroacoustic transfer function of the ear plug, will be presented.