Frequencies are often times found in circuits where most would not expect it.
I deal with RF Amplifiers, and we have one circuit that uses a simple ferrite that is rectangular with a 75-ohm strip of wire through the center of it. As we increase the power out of the amplifier, the ferrites literally sing. Albiet this is slightly a different cause than the article, it is an interesting phenomenon that can be found in many places.
By the way, what was ever done to completely remove the ringing, or was the epoxy solution the final one?
The resonance of ferrite winds up being very useful in ultrasonic cleaners in that it allows a higher level of energy transfer than would be available at a nonresonant frequency. Some disk capacitors exhibit a similar ringing due to a piezoelectric effect.
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