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My car's entertainment system is only able to play audio CDs and receive AM/FM radio. I am thinking of getting a Bluetooth FM transmitter so that I could use my car's speakers to play my smartphone's music and make hands-free calls.

It is my understanding that the Bluetooth connection between my phone and Bluetooth FM transmitter is encrypted. However, the FM radio transmission between the Bluetooth FM transmitter and my car's FM radio receiver seems to be unencrypted. Does this mean that anyone within range of the FM transmission will be able to listen to my hands-free calls? Does this mean that I must not talk about sensitive topics (e.g. banking, medical, etc.) when making hands-free calls through the Bluetooth FM transmitter?

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Yes and Yes.

Try it out! Grab a handheld FM radio (these still exists I'm sure) and walk around the car seeing just how far the transmission can be heard from.

Now, FM is a licensed and presumably still profitable band, so the FCC heavily scrutinizes these products (if it's been scrutinized as it should). As such it may have a pretty short transmission range. Maybe grab the device and see just how far you can walk from the vehicle and still be heard.

Finally, FM is a very noisy band filled with multi-kilowatt profit motive machines. The device may be like whispering your SSN in a crowded and noisy room. Probably fine unless your arch nemesis is standing right next to you.

But in general it's best not to broadcast one's personal details.

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Yes. FM isn't encrypted and everything transmitted over FM can be listened by anyone close enough.

If you are concerned, you could use a Bluetooth receiver with line-out output. There are several options on your favorite online shop.

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  • "Bluetooth receiver with line-out output" — How would that be useful in my case? My car's entertainment system has no input ports.
    – Flux
    Commented Apr 12, 2023 at 16:12
  • If you don't have audio input, sadly no.
    – ThoriumBR
    Commented Apr 12, 2023 at 17:07

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