Spectral analysis of lung sounds in normal adult by digital filtering technique : a preliminary study

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Spectral analysis of lung sounds in normal adult by digital filtering technique : a preliminary study


Author: Ng, Chi-keung
Title: Spectral analysis of lung sounds in normal adult by digital filtering technique : a preliminary study
Degree: M.Sc.
Year: 1997
Subject: Lungs -- Diseases
Sound -- Measurement
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Pages: v, 94 leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1403640
URI: http://theses.lib.polyu.edu.hk/handle/200/1868
Abstract: Chest auscultation and stethoscopy have been used in clinical practice for more than a century. However, the mechanism of lung sounds production is not fully understood. Moreover, the acoustic characteristic of stethoscope is not well investigated and documented. Thus, the assessment of lung sounds is still subjective. The aims of this study are to investigate the acoustic properties of the chestpiece of a conventional stethoscope and to develop a microphone-based lung sounds measurement system suitable for objective measurements by digital filtering technique. Chestpiece-microphone complex was modified from conventional chestpiece by embedding a condenser microphone at their outlet end. Their sensitivities were calibrated against a constant sound pressure level. The acoustic properties were studied by obtaining the transfer function and the coherence function. These functions are obtained by comparing the simultaneous measurements made by the chestpiece-microphone complex and a reference measurement microphone. An acoustic coupler fabricated by plastic was employed to provide a consistent environment for these measurements. The general acoustic characteristics of adult-size chestpiece with and without diaphragm was studied by repeated measurements. The acoustic characteristics of chestpiece at different sound pressure levels were further studied. In addition, the acoustics of chestpieces of different brands and different sizes were studied. Supplementary study on vibration characteristics of the diaphragms were carried out. Finally, in-vivo lung sound spectrum of an healthy male subject was evaluated by the proposed chestpiece-microphone complex. The calibration results showed that a larger size chestpiece has a greater sensitivity. Moreover, the diaphragm exhibited an attenuation effects. The adult-size chestpiece without diaphragm has a flat transfer function with high coherence function (帠2 >= 0.9) from 40 - 400 Hz. The presence of diaphragm reduced that flat transfer function range with greater variability. The frequency range of coherent measurements reduced as the sound pressure level of the input signal decreased. Moreover, the transfer functions of adult-size chestpiece without diaphragm exhibited a consistent flat transfer function while these became varied when diaphragm included. The smaller chestpiece having diaphragm exhibited a greater attenuation to low frequencies. The natural frequency of vibration of the diaphragms ranged from 133 - 208 Hz. Finally, frequency spectra were obtained at different measurement sites over the chest wall of a normal subject. Further development of this microphone-based stethoscope is feasible to provide an objective measurement for lung sounds.

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