Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 392 08 9(47)
Noise
Noise is an undesirable interference in the audio system.
Noise can enter the audio system one of two ways,
conducted or radiated.
•
Conducted noises will have a physical connection to
the audio system through one of the audio system
circuits.
•
Radiated noises have no physical connection to
the audio system, but the electrical or magnetic
fields reaching the audio system or wiring induce an
electrical effect causing noise.
In diagnosing noise complaints, technicians may need
to identify noise sources that are physically connected
(back-way noise) or sources affected only by radiation.
The vehicle contains many “noisy” components which
radiate an electromagnetic signal. As the distance
from the radio transmitter station increases, on-board
components may begin to compete with desired radio
signals. The following pages will help isolate the cause of
radio noise and suggest a fix.
EMI / RFI
EMI (Electromagnetic Interference) is the undesirable
interference of an electrical system via radiated emissions
from another electrical system.
The term “EMI” includes all disturbances in the
electromagnetic spectrum. Within the EMI category
is a smaller group known as RFI (Radio Frequency
Interference). This is the frequency group of EMI that is
the radio frequency band.
RFI characteristics:
•
RF (Radio Frequency) waves are radiated when a
high-frequency current and proper geometry exist in
a circuit.
•
Waves travel in all directions and pass easily through
non-conductive materials.
•
Waves are reflected by conductive materials unless the
geometry is correct for absorption. If so, then the wave
is converted into electrical potential.
The energy radiated relates to the amount of energy in
the circuit and the geometry.
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