However, the acoustic transducer emission diagrams often show lobes of secondary emission that also send back a signal that is subject to the Doppler effect (see Figure 2.37). If the amplitude signal backscattered by the principal lobe is weak, i.e. if the medium has few particles in this direction, the secondary lobes can be detected and will induce errors in the measurement, particularly if the current meter is near to the surface, because signal amplitude reflected by secondary lobes is therefore considerable. The thickness of the water canal where measurements cannot