An ultrasound imaging system with a slewing bandpass filter is disclosed for selective passage of time varying acoustic signals. The bandpass filter includes an input node to receive unprocessed signals at a certain frequency, with the unprocessed signals constituting an input signal. A signal source is configured to provide a reference signal, and a mixer is configured to receive the input signal and the reference signal, and to mix the input signal and the reference signal to a desired frequency range. A fixed bandpass filter is configured to have a passband frequency above or below the frequency range and to provide a filtered output signal with a scannable bandwidth passband characteristic useful for examining the time varying characteristics of the input signal. A controller is provided to control the signal source in accordance with time-varying characteristics of interest.