On the Larsen effect to monitor small fast changes in materials

At sufficient gain an ultrasonic feedback circuit rings with a "Larsen" tone that depends on the acoustic properties of the solid body to which it is attached. Because the frequency of this tone may be measured virtually continuously and with high precision, it is potentially capable of responding to fast small changes in materials. Here a tentative theory for Larsen dynamics is introduced and compared with laboratory measurements. Larsen monitoring is then applied to observation of the curing process of a cement paste sample and to studies of "slow dynamics" in which mesoscale nonlinear materials subjected to modest loads experience a drop in modulus but then recover in a characteristic manner like log(t). The present technique, using as it does higher frequencies and the Larsen effect and brief transient loads, extends investigations of slow dynamics to earlier times. For the materials and loads investigated, it is found that log(t) behavior fails at the shortest times, recovery being faster over the first several milliseconds.