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In this paper we show how appropriate superpositions of Bessel beams can besuccessfully used to obtain arbitrary longitudinal intensity patterns ofnondiffracting ultrasonic wavefields with very high transverse localization.  More precisely, the method here described allows generating longitudinalacoustic pressure fields, whose longitudinal intensity patterns can assume, inprinciple, any desired shape within a freely chosen interval 0 < z < L of thepropagation axis, and that can be endowed in particular with a s t a t i cenvelope (within which only the carrier wave propagates). Indeed, it is heredemonstrated by computer evaluations that these very special beams ofnon-attenuated ultrasonic field can be generated in water-like media by meansof annular transducers. Such fields "at rest" have been called by us <<Acoustic Frozen Waves >> (FW).  The paper presents various cases of FWs in water, and investigates theiraperture characteristics, such as minimum required size and ring dimensioning,as well as the influence they have on the proper generation of the desired FWpatterns.  The FWs are particular Localized Solutions to the wave equation that can beused in many applications, like new kinds of devices, such as, e.g., acoustictweezers or scalpels, and especially various ultrasound medical apparatus; e.g.for attempting the destruction of tumor cells without affecting the precedingand subsequent (and surrounding) tissues.  Keywords: Ultrasound; Frozen Waves; Bessel beam superpositions;Non-diffractive waves; Localized Waves; Annular transducers.

Producing Acoustic 'Frozen Waves': Simulated experiments