AV:Measuring Magnetic Emissions

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Magnetic interference is a common issue that affects CRTs — particularly changing the image color or geometry. The most common source of magnetic interference is from unshielded speakers nearby. In general, modern speakers aren't shielded as a cost saving measure since CRTs are not prevalent any longer, thus you may encounter immediately apparent issues or ones that develop over time. This page aims to provide a way to gauge whether a speaker may cause issues over time.

Smartphone Apps

Any smartphone that has a compass or GPS built in has a magnetometer. This sensor can be used instead to loosely measure the strength of a magnetic field.

  1. Download a "Gauss meter" app from your phone's app store.
  2. Hold still and note the ambient magnetic field (e.g. 0.550 Gauss). Depending where you live, you may have a vastly different ambient value. If this value is greater than 2 Gauss, your environment may be highly magnetic.
  3. Move your phone to the side of the speaker, the same distance away as you would have your CRT. Note the new value shown.
  4. If the measured value is greater than 2 Gauss, it will likely cause issues.
  • For reference, a weak fridge magnet is approximately 20 Gauss.
  • Professional or broadcast monitors often have magnetic shielding built into their shells and will affect the magnetometer's reading if you are very close to it.

Practical Test

You can test any set of speakers manually to see what effect it has on your CRT. This should not cause any permanent issues with your CRT, as generally permanent issues develop over a long period of time of being exposed to unshielded speakers.

  1. Mount a video camera on a tripod or stable surface pointed at your CRT screen.
  2. Display a pure white screen on your CRT, such as with the 240p Test Suite.
  3. Start recording video. Move the speakers near the CRT at different many different angles, including setting it where you plan on putting them.
  4. Stop recording the video. Scrub through the video, especially comparing the beginning and end of the video, to verify that the white screen does not change. If it does change, they are unshielded speakers and should not be used.