Research On The Acoustic Field Of Ultrasonic Radiation From The Plate In The Air

The ultrasonic radiation system composed of the longitudinal vibration transducer and the flexural vibration radiator has obtained a wide range of applications in the areas of the gas medium,For example in the ultrasonic levitation,the drying and defoaming in food processing,ultrasonic distance measurement and so on.This kind of composite system not only has the high power and high efficiency of the longitudinal vibration transducer but also has the low impedance,large radiant surface and other characteristics of the bending vibration.The sound field of the ultrasonic radiation system composed of longitudinal vibration transducer and flexural vibration radiator is divided into the near field and the far field.The distribution of the sound pressure in the near field and the far field are completely different,so that it needs to make a choice according to the use demand.Such as near field acoustic levitation,the near field can be used in the technology of near field acoustic levitation.Its working principle is to use the high intensity acoustic field between the transducer and the suspended solids to balance the gravity of the suspended solids.Meanwhile,the far field can be used in the ultrasonic level meter,ultrasonic distance measurement and other ultrasonic detection.The research conclusions indicate that the distribution of the sound field caused by the flexural vibration radiator has intense directivity,that is to say,in some direction the sound intensity radiating is weak,but in the some direction the sound intensity radiating is strong.The side-lobe can caused echo interference reflects by the non-measured target and interfere the measurement results.Therefore,the sharp directivity is the precondition of ultrasonic detection.There are rectangular and circular two kinds of geometrical shapes of bending vibration radiators in common,and the same geometrical shape radiator can be divided into flat type and stepped type.In this paper,the flat and the stepped circular radiator,the flat and the stepped rectangular radiator and the flat and the stepped square radiator,a total of six kinds of bending vibration radiator are studied,the specific work is as follows:(1)By using the Rayleigh integral and the superposition method,the formula of calculating directivity pattern of the flat circular plate radiator and the stepped ones are derived.Taking steel flat and stepped circular radiators for example,the vibration mode and the directivity pattern of the radiators are calculated and tested.For the circular plate radiators,the major lobe of the stepped circular plate radiator is sharper than the flat circular plate radiator,it exhibits good directionality.(2)The rectangular plate with free edges has no analytical solution and its flexural vibration mode is complex,but there is stripe vibration mode which the nodal line is parallel to the long edge or wide side.In this paper,the mode of stripe vibration is selected as working mode,by using the Rayleigh integral,the finite element method and numerical method,the formula of calculating directivity pattern and axial pressure of the flat rectangular radiator and the stepped ones are derived.Finally,taking steel flat and stepped rectangular radiators for example,the vibration mode,the directivity pattern and the axial pressure of the radiators are calculated and tested.It shows that the vibration mode of stepped rectangular radiator has not changed and the stepped radiator exhibits better directionality than that of the flat rectangular plate radiator.Both the flat and the stepped radiator show that in its near field area the axial pressure along with the increase of distance has larger fluctuation change and in its far field the sound pressurewith the increase of the distance have slow attenuation.(3)A square flat plate radiator driven by a longitudinal vibrator at the resonant frequency of its fourth-order intrinsic vibration mode at its center is able to create a bending vibration mode,whose four straight nodal lines are located at a 45° angle on each side.To get highly directional sound,the surrounding area by four right isosceles triangles is modified to be raised one by a half-wavelength of sound radiated in the propagation medium,so that the flat square radiator is changed into the stepped ones.By using the finite element method and numerical method,the mode of vibration,the directivity pattern,the axial pressure and the relationship of geometrical dimension with directivity pattern of the flat square plate radiator and the stepped square plate radiator are studied.It shows that the nodal lines will be expanded outside for the stepped square ones.The stepped square radiator exhibits better directionality,for its major lobe is sharper and its side lobes are small.Both the flat and the stepped radiator show that in its near field area the axial pressure along with the increase of distance has larger fluctuation change and in its far field the sound pressure with the increase of the distance have slow attenuation.Finally,the mode of vibration,the directivity pattern and the axial pressure of the fabricated radiators are measured.It shows that the calculated result is in good agreement with the tested one.