FOCUSING PROPERTIES OF ACOUSTIC FRESNEL LENSES

The focusing properties of some types of acoustic Fresnel lenses,such as bevel-shaped,arc-shaped and phase compensation plates,,are studied theoretically and experimetally.A formula,which is convenient for numerical calculation of pressure and intensity distributions of focused fields formed by acoustic lenses with large aperture,is derived from the Huygens-Fresnel Principle by using a higher order of approximatio than Fresnel's.Both theoretical values and experimental results show that these types of lenses are of good focusing properties and their field distributions near the focus are approximately the same,but their gains vary a lot.For a case in which the F-number is 1.1 and the wavelength is 1.4 mm,the lateral resolution is 2 mm,the focal depth (3 dB) is 17 mm,and the first side lobe is 17 dB lower than the main lobe.