An ultrasonic probe serving as a sensor array used in a transmitting/receiving device includes a substrate made of a packing material. A plurality of piezoelectric vibrators shaped like a rectangular parallelepiped are fixed in a matrix on one principal surface of the substrate. Each of the piezoelectric vibrators includes a plurality of piezoelectric layers stacked in a direction that crosses two adjoining side faces of the piezoelectric vibrator at an angle of approximately 45.degree.. Inner electrodes are formed between the piezoelectric layers, and outer electrodes are formed on both end faces of the piezoelectric layers.

A method and apparatus are described for harmonic imaging using a transducer operated in the k.sub.31 mode. In one embodiment, the invention includes a transducer operative in a k.sub.31 mode, a transmitter for transmitting first signals to the transducer assembly, and a receiver for processing second signals received by the transducer assembly at a harmonic of the frequency of the first signals. In another embodiment, a method of operating an ultrasound system includes emitting first signals in a first frequency range from a transducer operating in the k.sub.31 mode, projecting the signals into a body, and detecting second signals confined to a second frequency range.

The invention provides a multi-frequency transducer capable of obtaining desired directivity regardless of frequencies. Parallelepiped-shaped transducer elements (1) are used as transducer elements and a plurality of such transducer elements (1) are arranged in an array. The transducer elements (1) are caused to resonate using a resonant frequency in fundamental vibration mode and a resonant frequency in harmonic vibration mode determined by the dimensions (A, B) of a short edge (1a) and a long edge (1b) perpendicular to a longitudinal edge (1c) of each transducer element (1) to transmit and receive at multiple frequencies. The angle of directivity (.theta.) of the transducer elements is controlled by the dimension (C) of the longitudinal edge which is set to a value which does not affect resonance in either the fundamental vibration mode or the harmonic vibration mode.

In k.sub.31 mode, a vibration is along an axis or orthogonal to the poling or electric field orientation. The direction of vibration is toward a face of an ultrasound transducer array. For each element of the array, electrodes are formed perpendicular to the face of the array, such as along the sides of the elements. Piezoelectric material is poled along a dimension parallel with the face of the transducer and perpendicular to the direction of acoustic energy propagation. Using elements designed for k.sub.31 resonant mode operation may provide for a better electrical impedance match, such as where small elements sizes are provided for a multi-dimensional transducer arrays. For additional impedance matching, the elements may be made from multiple layers of piezoelectric ceramic. Since the elements operate from a k.sub.31 mode, the layers are stacked along the poling direction or perpendicular to a face of the transducer array for transmitting or receiving acoustical energy.