  Controlling acoustic fields is crucial in diverse applications such as
loudspeaker design, ultrasound imaging and therapy, or acoustic particle
manipulation. The current approaches use fixed lenses or expensive phased
arrays. Here, using a process of analogue-to-digital conversion and wavelet
decomposition, we develop the notion of quantal meta-surfaces. The quanta here
are small, pre-manufactured three-dimensional units - which we call
metamaterial bricks - each encoding a specific phase delay. These bricks can be
assembled into meta-surfaces to generate any diffraction-limited acoustic
field. We apply this methodology to show experimental examples of acoustic
focusing, steering and, after stacking single meta-surfaces into layers, the
more complex field of an acoustic tractor beam. We demonstrate experimentally
single-sided air-borne acoustic levitation using meta-layers at various
bit-rates: from a 4-bit uniform to 3-bit non-uniform quantization in phase.
This powerful methodology dramatically simplifies the design of acoustic
devices and provides a key-step towards realising spatial sound modulators.
