In an air‐filled porous medium, the slow acoustic mode may potentially be a useful probe of the geometrical properties of the rigid frame. Use of the acoustic slow wave for probing a two‐component granular system is studied. In a mixture of two granule types of disparate size, experimental results show that the slow wave velocity depends sensitively upon the relative concentrations of the two types of granules. This may be reasonably interpreted as resulting from the varying tortuosity associated with the pores of the granular mixture, with change in relative concentration. From the experiment described, it is concluded that the acoustic slow wave measurement may be used as a sensitive, noninvasive probe of relative concentration in such a multicomponent system.