The concept of a transducer diffraction constant as the ratio of the blocked diaphragm pressure to the incident free‐field pressure is examined for meaning, definition, and application in the general case. The concept is extended to a transmitting transducer, and a transmitting diffraction constant that is equal in value but reciprocal in concept to the conventional or receiving constant is developed from the acoustical reciprocity theorem. The relationship among the diffraction constantD, the radiation resistanceR, and the directivity factor δ is given asD2=Rδ/R0, whereR0is the radiation resistance of a simple source. The electrical analog of the diffraction constantDis found to be an ideal transformer with the turns ratioD:1. The electrical analog of a complete and reciprocal projector medium hydrophone system is presented.