GMC 2007 Acadia Owner's Manual page 327

AM
The range for most AM stations is greater than for
FM, especially at night. The longer range can
cause station frequencies to interfere with each
other. For better radio reception, most AM
radio stations boosts the power levels during the
day, and then reduce these levels during the night.
Static can also occur when things like storms
and power lines interfere with radio reception.
When this happens, try reducing the treble on
your radio.
FM Stereo
FM stereo gives the best sound, but FM signals
only reach about 10 to 40 miles (16 to 65 km).
Tall buildings or hills can interfere with FM signals,
causing the sound to fade in and out.
XM™ Satellite Radio Service
XM™ Satellite Radio Service gives digital radio
reception from coast-to-coast in the 48 contiguous
United States, and in Canada. Just as with FM,
tall buildings or hills can interfere with satellite
radio signals, causing the sound to fade in and out.
In addition, traveling or standing under heavy
foliage, bridges, garages, or tunnels could cause
loss of the XM™ signal for a period of time.
The radio might display NO XM SIGNAL to
indicate interference.
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