Active noise reduction in an ear terminal

Abstract

An active ear terminal is being designed to obtain both hearing protection and radio communication in adverse environments. Passive damping is sufficient for high frequencies, but low frequencies must be damped by active methods. Even when low-frequency noise is not harmful to the hearing, it heavily increases the effort of communication. Active noise reduction may use the signal from an outer microphone, or from an inner microphone. Active noise reduction obtained by these two microphone positions are simulated and discussed. Analog and digital filters are considered. The time delay budget of the active system is drawn up. Time delays should be minimized to obtain the best performance of the active noise reduction system. The physical system of transducers with their acoustic surroundings contains time delays which cannot be compensated by filtering. These time delays can be determined from the transfer functions of the system. The physical design of the ear terminal is performed with a view to reduce these delays. The signal prosessing delay adds to the delay of the physical system. The total delay determines the upper frequency limit of the active noise reduction system.