Expression [2.141] in this case enables an assessment of the value of the signal spectral variance σ2 , which contains data on the dispersion of measured speeds. This dispersion is due to the random positioning of particles in the medium, introducing fluctuations in the amplitude and phase of the echoes detected, and also producing Doppler noise. For “pulse-pair” signals, the limits ±∞ of the integral must be replaced by ±1/2τ0 that correspond to the Nyquist limits. Beyond these limits, the signal is subject to aliasing.