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What radio interference sounds like

This page has audio and video demonstrations of what different types of radio interference sound like.

Weak signals on AM and FM radio

Radio reception becomes noisy and distorted.

Multi-path

FM

With FM, a common symptom of this problem is a harsh edge or loud scratching noise distorting the sound, often accompanied by the stereo light flashing on the radio receiver.

It may also cause a flutter effect on car radio FM reception when the vehicle is moving.

Audio example of multi-path interference [MP3, 361 KB]

AM

With AM there are 2 types of effect noticed. In both cases the sound continuously varies between clear and distorted:

The sound fades away into a hiss.
The sound becomes very distorted and unpleasant to listen to.

Radio transmitter interference

On AM and FM, a common symptom of this problem is a buzzing, rasping-type noise — or you may hear distorted voices or other audio sounds. This is often accompanied by the stereo or signal light flashing on the radio receiver.

Co-channel interference

FM

For FM, vertical layering of moisture content and temperature in the atmosphere (inversion layers) can occasionally cause signals to travel hundreds or thousands of kilometres further than usual.

With FM radios — and in particular radios fitted in moving vehicles — the primary signal suddenly disappears and is replaced by the unwanted signal. This is often noticed as both signals alternatively taking over from each other on a random basis as the vehicle is moving. There is often scratchy distortion preceding the changeover.

AM

With AM radios, the interference appears as whistle (a beat note) or loud tone mixed in with the required audio.

Electrical interference

This is usually caused by power lines, electric motors/thermostats, microprocessors, switch mode power supplies, etc. Anything that uses electric power may cause interference.

On AM and FM radios, the interference is characteristically heard as a buzzing noise, whine or hiss. It affects both mains- and battery-operated radios.

AM reception is more prone to interference than FM reception. For AM the source of the interference may be many hundreds of metres away.

Example of power line interference [MP3, 421 KB]

LED radio interference

Importers, distributors and users should be aware that the increased use of LED lights in both fixed and mobile installations are producing greater risk of noise across the radio spectrum.

Consumers should ensure that their products are compliant with standards AS/NZS CISPR15 or EN55015, and check with their supplier if they have any concerns before buying.

Below are two videos showing the interference effects of poorly performing LED lighting.

Paul Salter demonstrates the practical effect of interference generated by LED lights.

Right. Hi my name is Paul Salter I work for Weezer radio in Whangarei and I’m just going to take a minute or two to demonstrate the practical effect of interference generated by LED lights we have a radio here that has just been installed in this vehicle we are on the local Whangarei channel the local Whangarei channel is generated from a hilltop called Maunu mountain and it’s by the way the crow files only about 10 k's away. This vehicle is in our workshop at Herekino St so you can look up both Maunu mountain and Herekino St and you'll see that they are actually relatively close. We're going to demonstrate what happens when we turn the LED lights on, the LED lights are that switch there, we are going to establish a test call and you'll be able to hear what happens to the incoming signal. Wes radio wes radio are you there for a radio copy please asset. Roger. Yeah lovely can I get you to do another test count 1 to 10 and back again please. 1 2 3 4 5. I'm going to turn the lights on [presses button to turn lights on] that’s what happens [the counting in the background has been replaced by static noise - then he pushed the button again to turn the lights off] 4 3 2 1 turn them off again and get our signal back again. Thank you asset that’s lovely, I night actually pay you a visit this afternoon just to give you an idea of what we are up to. Thank you bye for now. The LED lights are just here and these LED lights generate interference. Right next to the LED light I have a spectrum analyser which is capable of looking at the radio spectrum. The centre frequency is 150 megs which is right in the middle of the E band or locally known as the forestry band or the bush band. What we're going to do now is turn the lights on, we can turn the lights on, and you will see on the screen up here that interference from these lights is now generated right across the entire band centred at 150 megs and its a 50 meg span so 25 megs high and low of 150 megs is now effectively wiped out by radio interference. Now we'll just get the lights turned off and you will see that the interference signal disappears. These are little things that are popping up here, they are actually viable radio signals from other radios in the vicinity but just to prove it one more time we'll get the lights turned on and you can see the interference occurring right across the entire band. Lights off again, thank you.

Radio detective Mathias Coinchon from EBU Tech investigates how LED lights block (digital) radio reception.