Measured fundamental frequency and sound pressure level L_p of the sound emitted 20 cm away from the open end of the duct as functions of the applied fingering, for L_cav = 3 cm and for both cases of a feedback loop that uses either a microphone ROGA MI19 ($L_{p_{\max }}\approx$125 dB SPL) or a microphone GRAS 46BE ($L_{p_{\max }}\approx$160 dB SPL). For all fingerings the overall feedback gain is constant, with Γ₀ ≈ 1.5 when a ROGA MI19 is used as the feedback microphone, and Γ₀ ≈ 1.35 when a GRAS 46BE is used. The acoustic pressure p(t) in the cavity, the pressure p_rad(t) emitted 20 cm away from the open end of the duct, and the current i(t) supplied to the loudspeaker are also plotted for three different fingerings.