Dept. of Elec. Eng. Syst., Faculty of Eng., Tel-Aviv Univ., Tel-Aviv 69978, Israel
Most hearing tests today are done in soundproof rooms. A method is proposed for conducting tests in a regular room while using a hearing test probe with active acoustic noise attenuation. The system consists of passive and active attenuation parts. The passive attenuation is achieved by a foam plug placed in the ear canal. The active attenuation is achieved by using two mini-microphones, a receiver and a DSP board. The first microphone is placed at the ear canal entrance and receives the acoustic noise. It then transfers the noise to the DSP board (with the TMS320, placed in a PC), which by using an adaptive algorithm transmits the antinoise signal to the receiver placed in the ear canal. The second microphone is placed near the receiver and produces the error signal to the adaptive process. The system was analyzed on a larger scale acoustic model and a 20- to 40-dB noise attenuation was achieved for tones up to 4 kHz (not including the plug passive attenuation). Theoretical analysis of the acoustic filter transfer function (from the outer microphone to the inner microphone) was done using computer simulation, and showed good agreement with measurement. Implementation of the system for broadband noise attenuation will be discussed. Preliminary results have shown that the characteristics and configuration of the acoustic probe and the DSP controller specifications have to be considered.