An apparatus for detecting inhomogeneities in panels (1), of the type suitable to be used for panels (1) in transit along a processing line and comprising a plurality of sound wave emitting devices (2), operating from one side of the panel (1) in transit, and a plurality of sound wave receiving devices (3), operating from the opposite side of the same panel (1) at a pre-established distance from the emitting device (2). The receiving devices (3) are situated at the ends of a plurality of channels (30), which are disposed in alignment with the respective emitting devices (2), are set side by side at a short distance from one another and have their other ends near to said panels (1); each of said channels (30) being formed in an external structure consisting of a set of partitions or layers (31) placed one on top of the other successively at pre-established distances.