I had my check up today and me and doctor were talking..i asked his a advice about using machines and what not..he told me this:
It is not 100 percent safe to use a doppler...because what it does..it transmits the waves into your belly and then recieves an echo that is coming back from the waves..in order to hear you babys heartbeat..it's called my baybs beat..Brilliant!!
he explained that using the app for hearing your babys heartbeat he so much safer because it's based only on recieving and listening and not transmiting!!! (by putting it on airplain mode) so it does not harm your baby..
I really thought i knew things...but this is news to me..