Experimental and theoretical determination of the speed of sound in lung parenchyma of rabbits

Abstract

The purpose of the study was to develop an experimental-theoretical method for determining the speed of sound in the lung parenchyma. The method is based on the measurement of the characteristics of the impedance of the Helmholtz resonator, that contains lungs under study. The input impedance of the Helmholtz resonator corresponds to the parallel connection of the lung impedance and a resonator. The modified two-microphone method is used to measure the resonant frequency and input impedance response of the product of the resonator chamber volume (lungs). The experimental system consisted of a loud speaker, a wave guide, two microphones for the measurement, and the end empedance node in the form of a connecting tube extended to the inside of the Helmholtz resonator with specified geometric characteristics. When the lungs are added to the Helmholtz resonator, the frequency increases, but not linearly with the decrease in their volume, indicating that the speed of sound is abnormal low in the lung parenchyma. The difference between the calculated and measured resonant frequencies is used to determine the speed of sound in the lung parenchyma. The measurements were carried out on air and collapsed lungs of 8 apparently healthy rabbits. Two variants of sound transmission in the lung parenchyma, corresponding to isothermal and adiabatic processes, are considered. The results of measurement and calculation demonstrate that the speed of sound in the lung parenchyma of an apparently healthy rabbit is 21-22 m/s, what is in accord with modern theoretical concepts.