AN ADAPTIVE NOISE-REDUCTION STETHOSCOPE FOR AUSCULTATION IN HIGH NOISE ENVIRONMENTS

Auscultation of lung sounds in patient transport vehicles such as an a mbulance or aircraft is unachievable because of high ambient noise lev els. Aircraft noise levels of 90-100 dB SPL are common, while lung sou nds have been measured in the 22-30 dB SPL range in free space and 65- 70 dB SPL within a stethoscope coupler. Also, the bandwidth of lung so unds and vehicle noise typically has significant overlap, limiting the utility of traditional band-pass filtering. In this study, a passivel y shielded stethoscope coupler that contains one microphone to measure the (noise-corrupted) lung sounds and another to measure the ambient noise was constructed. Lung sound measurements were made on a healthy subject in a simulated USAF C-130 aircraft environment within an acous tic chamber at noise levels ranging from 80 to 100 dB SPL. Adaptive fi ltering schemes using a least-mean-squares (LMS) and a normalized leas t-mean-squares (NLMS) approach were employed to extract the lung sound s from the noise-corrupted signal. Approximately 15 dB of noise reduct ion over the 100-600 Hz frequency range was achieved with the LMS algo rithm, with the more complex NLMS algorithm providing faster convergen ce and up to 5 dB of additional noise reduction. These findings indica te that a combination of active and passive noise reduction can be use d to measure lung sounds in high noise environments. (C) 1998 Acoustic al Society of America.