EMC Question of the Week: April 11, 2022

Fitbit lying on aluminum foil

A smart watch exhibits an audible hum when it's near AC power lines. Completely wrapping the watch in aluminum foil does not reduce the interference but placing the watch in a steel box does. The interference is almost certainly due to  

  1. low-frequency magnetic fields
  2. low-frequency electric fields
  3. high-frequency EM fields
  4. a software problem


The best answer is “a.” An aluminum enclosure would significantly reduce any electric-field coupling or coupling from a high-frequency magnetic field. On the other hand, the same enclosure would be practically invisible to a 50-Hz or 60-Hz magnetic field.  Aluminum is a non-magnetic material. At foil thicknesses, it has virtually no impact on low-frequency (e.g., <100 Hz) magnetic fields.

Most, but not all, steel enclosures would provide some degree of low-frequency magnetic field shielding. Generally, if a magnet will stick to the side of the box, it is made of a magnetic material that will reroute incident magnetic fields around the contents.

If the hum is at the power frequency, it may be due to vibration caused by magnetostriction in a component or circuit. It's also possible that the interfering field simply causes a malfunction that triggers an audible hum. In this case, there may be a software fix; but either way, the interference is due to a low-frequency magnetic field.

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