Crosstalk Between Quartz Tuning Forks in Superfluid He II

We present experimental results on crosstalk of non-electrical origin between high frequency quartz tuning forks immersed in the same volume of helium gas, liquid or superfluid. We compare these results with various observations of other groups and propose an explanation of this puzzling phenomenon. To the best of our knowledge, notable crosstalk has only been observed in superfluid helium both in the two-fluid regime and at very low temperatures, but was rarely seen to behave in a systematic way. We demonstrate some of its most significant properties-amplitude dependence within a short time span, long-term temporal instability, effects of the geometry of the setup and of obstacles placed between the tuning forks. Although the results are not fully understood, as the most likely explanation, we ascribe the observations to the coupling of tuning forks to standing acoustic modes inside the experimental volume, emphasizing the importance of second sound for understanding the observations at temperatures within the two-fluid regime (1 K < T < 2.17 K). Finally, we suggest simple precautions leading to suppression of excessive acoustic crosstalk between oscillating objects in He II.