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I was shopping for a bluetooth meat thermometer. Since this device would also be used in my combo (conventional and microwave) oven, which is shielded for microwaves, I expected the device to not work.

So, I decided to make the following test before ordering:

  • play a song over bluetooth on my headset
  • put my cell phone in the combo (conventional and microwave) oven
  • close the door

I expected the bluetooth connectivity to drop, since the oven is shielded to the microwave spectrum. Yet, I could still hear the song just fine on my headset.

So, what physics principle explain the bluetooth signal being able to exit by microwave oven ?

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    $\begingroup$ We have a microwave that killed my PC's internet access any time it was running. The PC was running a 2.4 GHz WiFi card which was right in the range of the microwaves coming out of the oven. Switching to a 5 GHz card solved the problems. The point is that even microwaves created by the oven aren't all blocked by the shielding. $\endgroup$
    – MichaelS
    Commented Feb 1, 2022 at 9:49

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Just some rough numbers: say the oven produces ~1kW=60dBm RF power of which only 1mW=0dBm is allowed to leak out then the window's leakage is about -60dB. If your Bluetooth is radiating about 1mW =0dBm and your receiver has about -90dBm operating threshold meaning it will receive 0-60=-60dBm still having some 30dB (1,000X) margin above that value. A modern RF receiver is an amazingly sensitive device.

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    $\begingroup$ So, the ability to receive is more explained by the high sensitivity and acceptable leakage than it is by the two spectrums being different in some ways? $\endgroup$
    – Jeffrey
    Commented Jan 31, 2022 at 20:10
  • $\begingroup$ "acceptable" leakage is determined by the FCC and OSHA, and it has nothing to do with waveforms and some such. The spectrum of the magnetron is a very noisy tone with its harmonics; BT being a spread spectrum signal has no dominant "tone" or fundamental frequency and it is roughly uniformly distributed in its occupied frequency . The leakage is primarily determined by the sizes of the roughly circular holes on the oven's window through which you can see your food while being cooked. The receiver sensitivity is determined primarily by its RF amplifier's own noise and antenna connection losses. $\endgroup$
    – hyportnex
    Commented Jan 31, 2022 at 20:51

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