A jet mill in which crushed material introduced into a crushing chamber is comminuted by gas being sprayed from a plurality of gas-jet nozzles, wherein high-efficiency pulverization is performed by optimizing various pulverization conditions according to the type of crushed material or other such properties. Accordingly, a recordingreproducing apparatus that cannot recognize a predetermined function can properly use a medium having the predetermined function. The plate body includes a plurality of conduits configured as an array of acoustic horns, with a small throat opening at the first face and an intermediate horn section which diverges to a broad mouth opening at the second face. The framing comprises a fixed frame comprising displacement means and an intermediate element fixed to the panel, the intermediate element comprising at least two main longitudinal wings passing through the at least two through slots and cooperating with the displacement means so as to permit the displacement of the panel along the fixed frame.