Ultrasonic-wave irradiation unit

An ultrasonic-wave irradiation unit comprising:

• a diaphragm having an irradiation surface for irradiating ultrasonic waves and a non-irradiation surface located on an opposite side of the irradiation surface, the diaphragm being rectangular in plan view and having bolts projecting from the non-irradiation surface;
• a plurality of bolt-clamped Langevin-type ultrasonic transducers, each including a front plate joined to the non-irradiation surface of the diaphragm;
• and
• a plurality of resonant rods that resonate at the same frequency and in the same longitudinal oscillation mode as the ultrasonic transducers,
• wherein the plurality of ultrasonic transducers and the plurality of resonant rods are fastened and fixed to the diaphragm via the plurality of bolts,
• wherein the plurality of ultrasonic transducers each include four or more piezoelectric elements made of lead-free piezoelectric ceramics and are arranged at respective vertices of at least one regular hexagon assumed at a central portion of the diaphragm,
• wherein the plurality of resonant rods include first resonant rods having a relatively large cross-sectional area and second resonant rods having a relatively small cross-sectional area,
• wherein the first resonant rods are disposed at a center of the regular hexagon and at positions located on the short-side portion of the diaphragm and externally adjacent to the regular hexagon, and
• wherein the second resonant rods are disposed on the long-side portion of the diaphragm and externally adjacent to the regular hexagon.