Abstract: Comb filter having a comb like band pass characteristic curve by using an ultrasonic delay line (9) is improved to reduce its size. An input signal is given through an impedance element 13 to a single terminal 10a of an improved delay line 9, wherein the single terminal serves as the input terminal and at the same time as the output terminal. Signals at both ends of the impedance element 13 are given to a mixing circuit 15 for summing or subtracting thereby. In the improved delay line 9, the ultrasonic wave radiated from the input terminal travels twice the conventional delay line and comes back to the original terminal, and thereby a sufficient propagation distance is obtainable in a small delay line.

Abstract: An ultrasonic ceramic microphone having a wide operating frequency range, including piezoelectric ceramic plates disposed on upper and lower surfaces of a metal plate, wherein the ultrasonic ceramic microphone is operated near two or three adjacent resonance frequencies and in a frequency band covering those resonance frequencies, of a fundamental wave and respective higher order harmonics of the deflection vibration of a transducer comprising the metal plate and the pair of piezoelectric ceramic plates.