Audio loudness is correlated to signal strength, so it is clear a very strong signal is from a source that is close, important to know in aircraft. Two transmi

Audio loudness is correlated to signal strength, so it is clear a very strong signal is from a source that is close, important to know in aircraft. Two transmissions will produce a heterodyne (whine) due to the offset of carrier frequencies, so you hear both and the tone to know there are two transmitters. On FM, you’d hear only one, the strongest who captured your receiver.