Abstract
The aim of the paper is to delineate the performance of the lung sound measurement using a air-coupled microphone on the chest wall, more specifically, how the frequency characteristics of the measurement is determined, and how the spatial resolution of the measurement is determined. The system is modelled as the microphone on the free surface of the semi-infinite medium in which the point source is embedded. The theoretical analysis yielded a relatively simple expression for the sensitivity distribution, which implies 1) the sensitivity at the observation point right above the source decreases inversely proportional to the square of the source depth, 2) lateral distribution of the sensitivity on the surface is such that it diminishes to 1/2.83 at the point laterally displaced by the distance equal to the source depth. This means that we can expect fairly good spatial resolution when the source is close to the body surface. The sound pressure transfer function from the embedded spherical source to the air-coupled microphone on the surface is shown to be of the low-pass 2nd order characteristics, of which cut-off frequency is determined by the coupling-chamber dimension. If we want to have the cut-off frequency above 1kHz using the microphone with the aperture of 3.5mm radius, then the depth of the coupling air-chamber must be below 2mm. All those theoretical results were verified with reasonable accuracy on the experimental model using water as the medium. The study seems to give us the answer or clue to most of the fundamental problems in the lung sound measurement on the chest wall, although the model was quite simple.
