Department of Aeronautics and Astronautics, National Cheng Kung University, Tainan 701, Taiwan ROC
It is well known that behind the orifice in a pipe, flow velocity increases and pressure decreases simultaneously. The generated sound appears that is caused by the pressure fluctuations that occur as the flow passes through the orifice. Then the flow velocity is averaged over a pipe cross-section and is considered as a constant, it can be seen that the amplitude of sound increases with increases in the expiratory velocity. An experimental study of the quantitative analysis of sound pressure level correlated with expiratory velocity in a pipe was conducted using an apparatus that includes an air pump in conjunction with a pipe, a microphone, and an orifice plate, among other instruments. The regression and analysis of the results shows that the pressure fluctuation of sound spectra can be correlated to the expiratory velocity of a pipe. The experiment is conducted under conditions where the air passing through the orifice has an averaged expiratory velocity ranging from 0.88 m/sec to 1.35 m/sec, an inlet temperature of 298.15 K, and where the outlet pressure is that of the atmosphere. In this experiment, the Mach number is very low, and the compressibility effects can be ignored. The obstacle orifice plate was placed in the center of the pipe, and a microphone was mounted flush downstream to acquire the sound pressure data on the pipe wall. The measured results show that the approach for measuring the expiratory velocity using a microphone can be justified, and there exists a good correlation between the Power Spectral Density (PSD) of sound pressure fluctuation and the peak expiratory velocity.