Active Noise Reduction: Their Interaction with Very Low Frequency Acoustic Energy

摘要

The technique of Active Noise Reduction (ANR) uses interfering sound waves toreduce noise exposure. ANR systems have become commonplace in transportation equipment as a method for creating a favorable environment to perform auditory tasks. Recent field experience has shown that high-amplitude low-frequency sound encountered in helicopters and tracked vehicles causes some ANR systems to overload or saturate. This is perceived as the presence of extraneous noise at the ear. A technique is described wherein low frequency ANR performance may be assessed by measuring the threshold of overload. The results of this procedure indicated large differences in the saturation thresholds among systems tested. A strong dependence upon the integrity of the ear seal was also noted. Those systems offering active attenuation into the infrasound region tended to saturate most easily, but did create the best listening condition for the user when operated below the saturation threshold.