ABSTRACT This paper experimentally deals with the vibration characteristics of flat ring transducers used for ultrasonic sensors and actuators. Radial vibration mode, which is the fundamental mode of a thin piezoelectric transducer, was measured by a laser in-plane vibrometer. An impedance analyzer was used to measure natural frequencies. The results measured by experiments verified theoretical predictions. The vibration characteristics of ring transducers were identified according to the outer di-ameter size. The shape of the fundamental mode is almost uniform but slightly decreases from the inner to the outer circumferential surfaces. The natural frequency of the fundamental mode decreases as the outer diameter increases. It appears that the ring type transducer is suitable to excite uni-formly distributed vibration on a flat surface. * 1. 서 론 초음파 센서 또는 액추에이터에서 역학적 진동과 전기적 신호 간의 상호 변환에 압전 변환기 (transdu-cer)가 많이 사용된다 (1) . 변환기의 압전소자는 대개 원판형이다. 압전소자를 적층할 필요가 있을 때, 중심을 관통하는 볼트로 체결하므로 중앙이 비어있는 링형 압전 원판을 사용한다