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By coincidence, I posted something about this on another site yesterday:
The 10 kHz channel spacing in the US dictates that an RF bandwidth of 10 kHz (~5 kHz of audio bandwidth) is the maximum that can be transmitted using conventional AM without creating interference to the sidebands of adjacent channels. Here is the reason why.
Receivers can limit their RF bandwidth to 10 kHz, yielding an audio response to 5 kHz. But that will not remove modulation from adjacent channels that exceeded 10 kHz of RF bandwidth on the adjacent channel. Those modulation components will lie in the RF passband of the receiver, and cause interference to the desired signal. The higher the audio frequency response on the adjacent channel, the more it will interfere with a receiver tuned one channel away.
For most locations, the only time that 10 or 15 kHz audio response would be usable for AM broadcast would be in the daytime (not that many of today’s receivers could deal with it). At night, all stations with that audio response would have to cut it back to ~5 kHz if they wanted to avoid creating skywave interference to stations on adjacent channels. This is just a reality of physics.
Probably most people would choose listening to clear, intelligible program audio with less audio bandwidth than listening to “wideband” audio with a lot of interference in it.