We demonstrate the existence of Bloch oscillations of acoustic fields in sound propagation through a superlattice of water cavities and layers of methyl methacrylate. To obtain the acoustic equivalent of a Wannier-Stark ladder, we have employed a set of cavities with different thicknesses. Bloch oscillations are observed as time-resolved oscillations of transmission in a direct analogy to electronic Bloch oscillations in biased semiconductor superlattices. Moreover, for a particular gradient of cavity thicknesses, an overlap of two acoustic minibands occurs, which results in resonant Zener-like transmission enhancement.PACS numbers: 43.20.+g, 43.40.+s, 46.40.Cd Bloch oscillations (BO) and Zener tunneling are fundamental transport effects appearing when electrons in a periodic potential are accelerated by an external DC electric field [1, 2]. Both phenomena have been impressively demonstrated in a number of experiments after the advent of semiconductor superlattices (SSL) [3]. The frequency domain counterpart of BO is the equidistant Wannier-Stark ladders (WSL) of the electronic states in a biased superlattice, leading to resonances of the density of states which were observed in optical spectra [4]. In time-resolved optical experiments, BO were first observed as oscillations of electron wave packets in biased SSL [5,6,7,8,9], and later as a periodic motion of ensembles of ultracold atoms [10,11] and Bose-Einstein condensates [12,13]. The related high-field phenomena, the nonresonant Zener tunneling between neighboring minibands and resonant Zener-like tunneling between the anticrossing Wannier-Stark states of neighboring minibands, were also observed in SSL [14,15].In general, the Bragg reflection can cause BO of a wave of any nature (electronic, optical, acoustical or matter wave) in a lattice with a weak linear gradient of the lattice potential, which can be caused in turn by an external field or perturbation of any nature (electric, magnetic, acceleration or gravitation field), see, e.g., [16]. Optical BO and Zener tunneling of light waves have been recently observed in time-resolved experiments in 2D [17] and 1D[18] optical superlattices with refractive index gradient along the growth direction. Phonon BO and Raman spectra of phononic WSL states were described in semiconductor multilayer solid structure based on acousticphonon cavities with different thicknesses [19]. More recently, WSL have been also observed in one-dimensional elastic systems [20].In this Letter we predict analytically and confirm experimentally the existence of acoustic Bloch oscillations in two-component ultrasonic superlattice made of layers of Methyl Methacrylate (Plexiglas) and water cavities. The system of Plexiglas' layers, all having the same chosen thickness, separated by water cavities with a given gradient of thicknesses, is equivalent to a set of acousticwave cavities with a gradient of local resonant frequencies. In this system the water cavities act as resonant cavities, the resonant acoustic modes being well confined, whi...
