Apparatus is provided for assessing a characteristic of a first acoustic field (22) at a first frequency in a region (24) of a medium (26), the first acoustic field generating oscillatory motion of scatterers (28) disposed within the medium, at the first frequency. An acoustic transducer (30) (a) generates a second acoustic field (32) at a second frequency in the region, the second frequency being higher than the first frequency, and (b) receives echo data of the second acoustic field scattering off the oscillating scatterers in the medium, the echo data containing Doppler-shifted frequencies related to the oscillations of the scatterers, resulting in a time-dependent Doppler shift that oscillates at a frequency that is related to the first frequency. Control circuitry (36) (a) extracts the oscillating time-dependent Doppler shift from the received echo data, and (b) converts the extracted Doppler shift into particle-velocity of the first acoustic field.