  In a duct-flute such as the recorder, steady-state oscillations are
controlled by two parameters, the blowing pressure and the frequency of the
acoustic resonator. As in most feedback oscillators, the oscillation amplitude
is determined by gain-saturation of the amplifier, and thus it cannot be
controlled independently of blowing pressure and frequency unless the feedback
loop is opened. In this work, the loop is opened by replacing the recorder body
with a waveguide reflectometer: a section of transmission line with
microphones, a signal source, and an absorbing termination. When the mean flow
from the air-jet into the transmission line is not blocked, the air-jet
amplifier is unstable to edge-tone oscillations through a feedback path that
does not involve the acoustic resonator. When it is blocked, the air-jet is
deflected somewhat outward and the system becomes stable. It is then possible
to measure the reflection coefficient of the air-jet amplifier versus blowing
pressure and acoustic frequency under linear response conditions, avoiding the
complication of gain-saturation. The results provide a revealing test of flute
drive models under the simplest conditions and with few unknown parameters. The
strengths and weaknesses of flute drive models are discussed.
