Rob wrote in :

Deviation is something different than bandwidth. To receive an FM signal,
as a rule of thumb you require a receiver with a bandwith of about
twice the deviation plus twice the maximal audio frequency. So to receive
a signal with 15kHz audio and 75kHz deviation, you require a bandwitdh
of about 180kHz.

This is not "to make things less critical". It is a requirement because
of the characteristics of FM modulation, where sideband frequencies are
determined by a Bessel function.

The ratio between deviation and maximal audio frequency, the modulation
index, determines the signal/noise ratio after demodulation for a strong
signal.

That's useful. It ties in with things I have read about phase modulation
synthesis. At risk of opening a wild (but fun) bit of off-topic, I wonder if
given a reference carrier at precise fixed frequency, a phase modulation
technique might allow advantages to signal transmission that FM lacks.