Subclass 1/38: Details of transducers (diaphragms H04R 7/00; characterised by the nature of the transducer, see the relevant group of main groups H04R 9/00-H04R 23/00; mounting radio sets or communication systems in helmets A42B 3/3; mountings specially adapted for telephone equipment H04M 1/02) Arrangements for obtaining desired frequency or directional characteristics (for stereophonic purposes H04R 5/00; speech enhancement by processing of the speech signal G10L 21/02) for obtaining desired directional characteristic only by using a single transducer with sound reflecting, diffracting, directing or guiding means in which sound waves act upon both sides of a diaphragm and incorporating acoustic phase-shifting means, e.g. pressure-gradient microphone