An acoustic particle counter—Preliminary results

The detection of dust particles in air by an acoustic phenomenon is described. The particles are passed through a sensor in which they are gradually accelerated to about 100 m./sec. and then the particles are suddenly projected into a wide exit cavity. At this point a pressure pulse is generated by a particle and gives an audible click. The sound pulse lasts 2 to 20 milliseconds, depending on the entrance design, and has an optimum signal-to-noise ratio of $\text{50}\text{1}$. This sensor in its present state detects particles down to 5 microns with no change in signal amplitude with size. It has been applied in the laboratory to count ice crystals in supercooled clouds.