Invariance of Doppler bandwidth with flow axis displacement

It is known that if single-frequency, continuous-wave ultrasound or electromagnetic energy is reflected from straight line flow, defined here as one or more scatterers moving with constant velocity along an infinite straight line, the Doppler effect will shift the echo spectrum center frequency from the transmitted value, and broaden its bandwidth. It is shown here that if such straight line flow is shifted anywhere in the field without change of orientation, the absolute echo bandwidth remains unchanged. This result is unexpected, since the bandwidth arises from amplitude modulation of the echo from the scatterers as they pass through the incident field, and the shape of this field varies with position. The Doppler bandwidth may therefore be used to estimate the transverse component of the flow velocity anywhere in the sound beam. The effect of the system relative aperture on the minimum spectral broadening and maximum frequency indication may also be assessed at any range