Receivers are arranged to detect a corresponding observed phase shift, observed attenuation or other observed signal parameters for its respective microphone. An electronic data processor is adapted to estimate a distribution or quantity of material on the sieve based on the observed phase shift, the observed attenuation or the other observed signal parameters relative to a reference phase shift, a reference attenuation or other reference signal parameter. The operator can be alerted via a user interface if the material on the sieve is unevenly distributed or matches a preestablished distribution profile, or the sieve can be adjusted by an actuator to promote a generally uniform distribution.