title: An adaptive noise reduction stethoscope for auscultation in high noise environments.
abstract: Auscultation of lung sounds in patient transport vehicles such as an ambulance or aircraft is unachievable because of high ambient noise levels. Aircraft noise levels of 90-100 dB SPL are common, while lung sounds 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 sounds and vehicle noise typically has significant overlap, limiting the utility of traditional band-pass filtering. In this study, a passively shielded stethoscope coupler that contains one microphone to measure the (noise-corrupted) lung sound 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 acoustic chamber at noise levels ranging from 80 to 100 dB SPL. Adaptive filtering schemes using a least-mean-squares (LMS) and a normalized least-mean-squares (NLMS) approach were employed to extract the lung sounds from the noise-corrupted signal. Approximately 15 dB of noise reduction over the 100-600 Hz frequency range was achieved with the LMS algorithm, with the more complex NLMS algorithm providing faster convergence and up to 5 dB of additional noise reduction. These findings indicate that a combination of active and passive noise reduction can be used to measure lung sounds in high noise environments.
authors.affiliation: School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana 47907-1285, USA.
authors.forename: S B
authors.initials: SB
authors.lastname: Patel
authors.forename: T F
authors.initials: TF
authors.lastname: Callahan
authors.forename: M G
authors.initials: MG
authors.lastname: Callahan
authors.forename: J T
authors.initials: JT
authors.lastname: Jones
authors.forename: G P
authors.initials: GP
authors.lastname: Graber
authors.forename: K S
authors.initials: KS
authors.lastname: Foster
authors.forename: K
authors.initials: K
authors.lastname: Glifort
authors.forename: G R
authors.initials: GR
authors.lastname: Wodicka
mesh_terms: D001314:Auscultation; D004777:Environment; D006801:Humans; D008168:Lung / Q000502:physiology; D008954:Models, Biological; D009622:Noise; D019023:Stethoscopes; D013997:Time Factors