Mazda 2015 5 Owner's Manual page 209

Signals from an FM transmitter are similar
to beams of light because they do not
bend around corners, but they do reflect.
Unlike AM signals, FM signals cannot
travel beyond the horizon. Therefore, FM
stations cannot be received at the great
distances possible with AM reception.
Ionosphere
FM wave
FM wave
100—200 km (60—120 miles)
Atmospheric conditions can also affect
FM reception. High humidity will cause
poor reception. However, cloudy days
may provide better reception than clear
days.
Multipath noise
Since FM signals can be reflected by
obstructions, it is possible to receive both
the direct signal and the reflected signal at
the same time. This causes a slight delay
in reception and may be heard as a broken
sound or a distortion. This problem may
also be encountered when in close
proximity to the transmitter.
Direct
AM wave
Reflected wave
Flutter/Skip noise
Signals from an FM transmitter move in
straight lines and become weak in valleys
between tall buildings, mountains, and
other obstacles. When a vehicle passes
through such an area, the reception
conditions may change suddenly, resulting
in annoying noise.
Weak signal noise
In suburban areas, broadcast signals
become weak because of distance from
the transmitter. Reception in such fringe
areas is characterized by sound breakup.
Interior Comfort
Audio System
6-13