You need a hardware LPF, though. The trouble is that the sampler in the ADC down-converts harmonics of the passband into the passband. To make this concrete:

You need a hardware LPF, though. The trouble is that the sampler in the ADC down-converts harmonics of the passband into the passband. To make this concrete: you might sample at f0 to capture all frequencies from 0 to f0/2, but you also get all the energy in frequency bands f0 * 3/2 -- f0 * 5/2, f0 * 7/2 -- f0 * 9/2, etc. Unless you pre-filter that before it gets to the sampler, that is. By the way, a variation of this trick was used by HP and Tektronix in 1960's oscilloscopes to display multi-GHz signals. Depending on the power of the ultrasonic source, the hardware LPF might need to have a quite fast roll-off, which might imply a large number of stages, which is hard to design and expensive to manufacture.