Once a colleague showed me a sound measurement setup in a reverberation room, meant to measure (unwanted) sound from magnetostriction in ferrite cores. There was a problem creating a true "zero" measurement:
- Of course in that case a coil without any ferrite was used as device under test (DUT).
- Also the Earth's magnetic field was neutralized by DC-fed Helmholtz coils for that purpose (see drawing).
- Harmonic frequencies were discarded (possible by self-interaction forces between coil windings, which must at least be quadratic in the current).
Mysteriously, there still was some remaining sound at the fundamental frequency of the AC current in the DUT! It did depend on the orientation of the DUT, but didn't have a very deep minimum. The level was low, and not a practical problem, but every good experimenter wants to have the setup as close to perfection as possible. And this was apparently still not a setup that was exclusively sensitive for magnetostriction. It was puzzling what actually created the sound...
Much later I met the experimenter again and by then I had thought of one explanation we might have overlooked (although it is in plain sight in the drawing!) Unfortunately by then the measurements had already been finished and we could not check it. So the question remains: what could have caused the remaining sound?
Possible explanation: (see further down...) $$ \begin{matrix} \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \end{matrix} $$
Analysis and possible explanation:
First note that the reverberation room cannot localize the sound, so we have to consider the whole setup as possible source. The DUT creates an AC field at the fundamental frequency but will not feel any torque or other force from an external $B$-field since there is none: the Helmholtz coils completely cancel the Earth's field (checked with a sensitive magnetometer at the DUT position). Consequently, the external world cannot feel any torque from the DUT.
But that doesn't mean that magnetic effects are ruled out! Because we can still say that if we look separately at the Helmholtz coil set, and the rest of the Earth, then each of those individually feels a nonzero, but opposite torque from the DUT. The Helmholtz coils could easily get some wobbling motion at the fundamental frequency, whereas the rest of the Earth is much too heavy to react to the torque.
So this leaves the possibility that we get no sound from the DUT, but we do get it from the Helmholtz coils. This could have been verified by replacing them by heavier ones, or bigger ones at further distance from the DUT, preferably outside the reverberation room, but this was not tested.
