The measured intensities of separate ultrasonic frequency components furnished by respective bandpass filters of the spectrum analyzer portion of an audio frequency Doppler-effect flow meter are converted into digital values and the related values of noise intensity are calculated by applying weighting factor to these digital intensity values. The noise intensity values are subtracted from the observed intensity values to provide corrected signal intensity values these corrected values are then compared by a digital processor to determine, from the frequency of maximum corrective intensity value, the mean rate of flow which the ultrasonic transducer and receiver has observed. The output of the bandpass filters of lowest and highest frequency range are respectively multiplied by a first weighting factor of between 0.65 and 0.85 and by a second weighting factor of between 0.35 and 0.45 and weighting factors for other frequency components are then calculated by a function generator by reference to a monotonic curve having extreme values equal to the first and second weighting factors.