Most of the time, people don't digitize the acoustic signal unless wanting to measure a phase shift. Even at a 1MHz, a 500KHz signal is too fast to capture usable signal fidelity. I wouldn't want anything less than 3MHz sample rate.
I'm not sure this will help. However, if the acoustic transducer is pulsed, then you might use a detection circuit for timing the response. Perhaps simply a comparator circuit.
If it's a continuous wave, mix it with a known frequency and measure the difference frequency.