Electromagnetic interference, often referred to as EMI, is a degradation of the performance of a piece of equipment or system caused by electromagnetic radiation.

Electromagnetic radiation is the travel of alternating electric and magnetic fields through space at 186,273 miles (about a billion feet) per second, or the speed of light. A wavelength, as shown in Figure 1, is the physical distance traveled by a wave during one cycle. A list of frequencies and corresponding wavelengths follows.

When the equipment or system is closer than about a sixth of a wavelength from the radiation source, it’s said to be in the nearfleld of the source, Most audio systems are in the nearfield at all frequencies below about 1 MHz, including 60 Hz (hum) and its harmonics (buzz).

In the nearfield, either the magnetic or electric field dominates, and each requires a different kind of treatment to eliminate interference, For example, twisting of signal pairs reduces pickup of magnetic fields while shielding reduces pickup of electric fields.

However, when radiation sources have frequencies above about 1 MHz, the magnetic and electric fields travel together and are collectively called a wave. Most audio systems are in the farfield of the radiation source at these “radio” frequencies, hence the older term RFI for radio-frequency interference. Generally, the newer term EMI also refers exclusively to radio-frequency interference.

Figure 1 - High-frequency magnetic and electric fields travel together as a “wave.” (click to enlarge)

RF interference may originate in either intentional or unintentional radiators. Familiar intentional radiators include broadcast stations, amateur transmitters, and cell phones, Unintentional radiators include devices such as electric welders, brush-type motors, relays, and switches that produce arcs, which are potent sources of brief bursts of wide-band interference.

Less obvious sources include arcing or corona discharge on power line insulators (common in coastal or high-humidity areas) or malfunctioning fluorescent or neon lighting, Of course, lightning (the ultimate spark) is a well-known producer of momentary RF interference to virtually anything electronic.

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