Measurement and estimation of human forced expiratory noise parameters using a microphone with a stethoscope head and a lapel microphone

Abstract

Objective: The time signature of tracheal noise in the 200–2000 Hz frequency band recorded during a forced expiratory manoeuvre (FETa) is a promising tool for diagnosis of bronchial obstructions. FETa is typically registered using a microphone with a stethoscope head placed on the neck over the trachea (stethoscope sensor). We studied basic time and spectral parameters of forced expiratory tracheal noises with a microphone placed near the mouth (lapel microphone, outside the flow of exhaled air) and compared these with measurements via stethoscope sensor, with the aim of providing patient home monitoring via standard personal computer facilities. Approach: FETa, 200 Hz band pass times, and frequency responses of signals recorded simultaneously with both sensors were analysed in a sample of 24 healthy volunteers. Main results: Averaged real transfer function of signals from a stethoscope sensor and a lapel microphone in the frequency range 200–1800 Hz was characterized by a slope of −7.2 dB/octave. This is near the slope of −6 dB/octave predicted via theoretical models of both sensors. The lapel microphone and stethoscope sensor were not interchangeable regarding spectral characteristics of forced expiratory tracheal noises. However, FETa measurements in healthy volunteers via stethoscope sensor and lapel microphone showed no significant differences according to U-Mann–Whitney test for independent samples. Significance: The ability to measure FETa successfully with a lapel microphone placed near the mouth was experimentally demonstrated in healthy volunteers. Additional studies are needed to verify whether FETa measured near the mouth is acceptable for monitoring pulmonary status in patients with asthma or chronic obstructive pulmonary disease.