Non linear down-frequency conversion effects in high intensity Vibration of plate transducers and piezoelectric structures

Driving airborne ultrasound plate transducers at high intensity levels is an important point in many industrial applications, that meets with unavoidable limits due to nonlinear modal interactions. Subharmonic and fractional-harmonic frequency pairs generation, internal modulation and chaotic behaviour are all effects that greatly compromise the transducers efficiency and their use. In the present paper, similarities are put in evidence between a complex plate transducer used for ultrasound generation in air and a simple piezoelectric structure. The latter case, however, could be experimented through a very wide input power range, such that a drop over of the above mentioned effects is set in evidence above a given power level. This suggests that a similar behaviour could be met in the former case, that would greatly advantage the use of such kind of transducers. INTRODUCTION The excitation of high intensity ultrasound waves in air is a valuable means employed in many industrial applications, spanning from food drying procedures [1] to smoke and foam abatement [2], and is usually performed through large aperture plates set into vibration by the coupling with piezoelectric structures, properly designed. High vibration amplitudes is a must for good performance of the transducers, due to the low impedance of air where waves are generated, that meets with the requirement that vibration be confined to one and single mode and not be flowing to other modes, particularly to subharmonic ones. In that case, indeed, lost of coherence in the radiated field is produced and the efficiency of any industrial procedure is drastically reduced. In addition, since coupling of modes is caused by inhomogeneities in the transducer, local heating of the transducer may be easily achieved and damage to the transducer will follow. At high amplitudes of vibration, indeed, any region of the transducer where frictional movements of single parts may be present is a risky source of heat production and anharmonicity, that may easily drive the vibration out of its fundamental frequency. Subharmonic generation, as it is well known, could be the road through which chaotic behaviour may develop, that spoils the performance of the transducer. In the present paper, the experimental evidence is given of the production of harmonic and subharmonic vibrations in high power plate transducers, commonly used in industrial applications, as they are driven by piezoelectric elements properly fixed to the plates. Evolution in the spectral response of the transducers vs. input power is compared with the behaviour of cylindrical piezoelectric structures, driven separately, and analogies in the genesis of suharmonic vibrations is set in evidence as a possible common road to the chaotic behaviour of both structures. That may suggests the existence of linearity regions alternating with chaotic ones, as the input power to the elements is smoothly increased towards high levels.