(a) The current i(t) and the acoustic pressure u(t)/S_μ sensed by the microphone as functions of time, for various amplitudes of the assigned current (the frequency is 124 Hz). (b) Fundamental and higher harmonics components of the apparent pressure, u(t)/S_μ, as functions of the peak amplitude P_pk of the actual pressure in the cavity p(t) (known from i(t)). Experimental data are shown with filled markers, and theoretical data calculated from equation (6) are plotted with dashed lines.