Acoustic devices that break reciprocity, for instance acoustic isolators or circulators, may find exciting applications in a variety of fields, including imaging, acoustic communication systems, and noise control. Non-reciprocal acoustic propagation has typically been achieved using non-linear phenomena, which require high input power levels and introduce distorsions. In contrast, we have recently demonstrated compact linear isolation for audible airborne sound by means of angular momentum bias [Fleury et al., Science 343, 516 (2014)], exploiting modal splitting in a ring cavity polarized by an internal, constantly circulating fluid, whose motion is imparted using low- noise CPU fans. We present here an improved design with no moving parts, which is directly scalable to ultrasonic frequencies and fully integrable. Instead of imparting angular momentum in the form of a moving medium as in our previous approach, we make use of spatio-temporal acoustic modulation of three coupled acoustic cavities, a strategy that can be readily implemented in integrated ultrasonic devices, for instance, using piezoelectric effects. In this new paradigm, the required modulation frequency is orders of magnitude lower than the signal frequency, and the modulation efficiency is maximized. This constitutes a pivotal step towards practically realizing compact, linear, noise-free, tunable non-reciprocal acoustic components for full-duplex acoustic communications and isolation.