Abstract: The present invention relates to a measurement of a flow by the use of Doppler ultrasound, and in particular, by analyzing the spectrum of the reflected ultrasonic signal. The reflected signal is mixed electronically with the transmitted signal in a product detector, demodulator or similar component (250) to produce a signal that is shifted in frequency. In another embodiment, the method includes analyzing the spectrum (280, 300) of the reflected ultrasonic beams by performing the step of: (a) determining a maximum frequency of the detector output signal which is representative of a maximum particle velocity; and (b) calculating a total flow of the particles based on the maximum particle velocity.