FIG. 4.

(a) Change in the magnitude of the acoustic velocity $\left|U_1\right|={(u_1+w_1)}^{1/2}$ at x = x_a for different modulation indices m over two different time cycles: t = 1/(4 f_m) representing the maximum amplitude (◇) and t = 3/(4 f_m) representing the minimum amplitude (×). Note that m = 0 represents the case where amplitude modulation is absent. The trendlines were added to aid visualization. (b) Average squared values of the acoustic velocity amplitude $|U_1{|}_{\mathit{a}\mathit{v}\mathit{g}}^2=(|U_1{|}_{t=1/(4f_m)}^2+|U_1{|}_{t=3/(4f_m)}^2)/2$ at x = x_a at the two different time cycles for different modulation indices. The dotted line represents a least square fit (R² = 0.991) and suggests that $U_{\text{dc}}\sim {10}^m$.