Interference or noise
Unwanted radio noise from electrical equipment or natural sources rather than a transmission.
What does it sound like?
Buzzing, hissing, crackling, whining or ticking.
Who uses it?
Not a station: power lines, electronics, appliances, lightning and so on.
Where and when can I hear it?
Anywhere; man-made noise is usually worst at the lower frequencies and near homes.
The science
The ITU, which coordinates the world's radio spectrum, sorts background radio noise by cause: lightning; the combined unintended radiation of electrical machinery, electronics, power lines and engine ignition; emissions from the atmosphere and rain; the ground; and radio sources in space. In the ITU's model, man-made noise falls by about 28 decibels for every tenfold increase in frequency, so it is strongest on the AM broadcast band and lower shortwave. The same model puts median city noise more than 20 decibels above a quiet rural site at the same frequency, and notes that man-made noise often has an impulsive part (clicks and crackles) as well as a smooth hiss. Its figures come from measurements made in the 1970s, and the ITU cautions that they may change over time.
Why it matters
Noise, not signal strength alone, sets how weak a station you can hear, so a quieter location or a quieter home can help more than a more sensitive radio. Because man-made noise is often local, the usual first step for listeners is to find out whether it comes from inside the house by switching off breakers while listening on a battery-powered radio.
Lore
The ITU's radio noise recommendation has a revision history reaching back to 1951, and its definition of noise includes a note that noise may carry information about its source, such as its nature and location, which is what noise hunters rely on.
Easily confused with
- Steady carrier, from the same family