The capability for differentiating particles according to size
is provided in an improved acoustical detection apparatus and
method for detecting particulate matter in air or other gases. This
is accomplished by controlling the flow of the gas through the
capillary section of an acoustic sensor such that the flow conforms
to a predetermined Reynolds number selected according to the
particle size of interest. It has been found that acoustical
sensors exhibit a threshold for particle size detection which
varies with the Reynolds number of the flow of a gas through it.
Thus, the detection of particles above a given size is achieved by
controlling the flow parameters to yield a Reynolds number
appropriate for detecting particles of such size.