Experimental and Theoretical Determination of the Speed of Sound in Lung Parenchyma of Rabbits

The aim of this study was to develop an experimental and theoretical method for determining the speed of sound in the lung parenchyma. The method is based on measuring the impedance characteristics of the Helmholtz resonator; the studied lungs were placed in its cavity. The input impedance of the Helmholtz resonator corresponds to a parallel connection of the impedance of the lungs and a separate resonator. A modified two-microphone method was used to measure the resonant frequency and the input impedance characteristic of the resonator with lungs. The installation consisted of a loudspeaker, a waveguide, two measuring microphones, and an impedance termination with a tube for connecting a Helmholtz resonator with specified geometric characteristics. When the lungs were inserted into the volume of the Helmholtz resonator, the frequency did not increase in proportion to the decrease in volume, which indicated an abnormally low speed of sound in the lung parenchyma. The difference between the calculated and measured resonant frequencies was used to determine the speed of sound in the lung parenchyma. Measurements were carried out on the inflated and collapsed lungs of eight conditionally healthy rabbits. Two variants of sound transmission in the lung parenchyma corresponding to isothermal and adiabatic processes were considered. The results of measurements and calculations indicated that the speed of sound in the lung parenchyma of a conditionally healthy rabbit is 21–22 m/s, which corresponds to modern theoretical concepts.