On the linewidth of the ultrasonic Larsen effect in a reverberant body

A simple ultrasonic feedback circuit is applied to a high-Q elastic body. At sufficient gain, an ultrasonic howl - or Larsen effect, ensues. It is a pure tone with an extraordinarily narrow spectrum. Theoretical estimates are constructed that predict linewidths proportional to the ratio between the spectral power density of the background noise and the intensity of the howl. In these experiments, this is of the order of nano-Hz and is unmeasurable. By augmenting the absorption and adding noise, the width is brought into a measurable regime, and the theory's prediction of a wider line is confirmed. It is speculated and demonstrated that these narrow lines permit high precision measurements of tiny changes in structures.