Helmholtz resonators (HR) to control tonal noise propagation inside a duct can be made adaptive with an adjustable volume to allow their resonance frequency to track changes in the frequency of the tone to be controlled. As the resonator is intended to minimise the downstream transmitted sound power, a measure of this quantity would be an ideal cost- function to tune a resonator. As it is desirable to have a completely self-contained resonator with no external transducers, it is necessary to be able to estimate the sound power transmission from measurements inside the resonator...