Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why does CB radio sometimes interfere with speakers?

  1. Jun 3, 2014 #1
    I have experienced first hand, and have heard many cases of CB radio causing interference in speakers. I had a couple of questions related to it:

    1. What type of modulation (AM or FM or both) causes this interference?
    2. How big does the antennae need to be for this to happen?
    3. My neighbour doesn't get it for some reason but I do - any reason why his speakers don't get the interference but I do?
    4. Why do we actually hear the voice of the radio user and not some noise - surely if we were hearing the modulated signal it would just sound like a bunch of noise?
  2. jcsd
  3. Jun 3, 2014 #2
    You didn't describe your setup in detail. Is the CB plugged into an outlet in the same house where you get the interference?

    A radio signal shouldn't normally be able create a voice on a set of speakers which is not connected to a tuner. The radio signal has to be demodulated from a high frequency to a low frequency before a voice can be heard.

    Your CB radio will have an amplifier that magnifies the audio from the microphone before it sends the audio to a modulator. Your speakers are going to be powered by some kind of amplifier too. I suspect that the CB amplifier is leaking some signal into your house wiring which makes its way to the amplifier for your speakers.
  4. Jun 3, 2014 #3


    User Avatar
    Science Advisor
    Gold Member

    I'd bet it's an RF problem. A high power RF signal can cross modulate onto an audio signal when there is a nonlinearity. Although the audio circuitry may be very linear at AF frequencies, the linearisation may not work at RF. Perhaps the transmit antenna is not far enough away from your speaker amp or, probably even more likely, the feeder from your transmitter to the antenna has a poor continuity from amplifier ground to the cable outer. Try tidying up all your earths and re-routing cables.
    You can buy or make notch filters for power leads, to deal with RF interference if you know the frequency.
  5. Jun 3, 2014 #4


    User Avatar
    Science Advisor
    Homework Helper

    I agree it is most likely the amplifier for your speakers that is "receiving" and demodulating the CB radio signal, not anything coming through the mains wiring.

    This used to be quite a common problem with electronic musical instruments back in the 1970s and 80s, when the instruments used analog electronics not digital. There were stories in the newspapers about electronic organs installed in churches that picked up radio signals from local taxi companies, etc.

    I had a more modern version of it recently, after rearranging my home computing setup. The neatest place to put the WI-FI hub was on top of the subwoofer for the sound system. That wasn't a good plan, because the audio amps in he subwoofer cabinet were picking up faint "clicks and pops" from the WI-FI. The noises were not affected by the amplifier volume controls, so presumably they were being picked up directly by the amplifier circuit, not getting into the audio input cables. It was easy to prove where the interference was coming from, by moving the hub around, and switching it on and off.
  6. Jun 3, 2014 #5


    User Avatar
    Science Advisor

    If the transceiver is close enough, any speaker leads will pick up sufficient RF signal to be detected.

    The non-linearity of the amplifier output transistors is differentially rectifying the RF signal picked up in the speaker leads. The resultant DC component is flowing through the speaker coils. Put RF chokes in the speaker leads, or thread the leads through an RF ferrite toroid, close to the amplifier.

    UHF on about 480MHz with FM is not usually a problem, it tends to cause key clicks. The FM is narrow band with constant amplitude so it does not demodulate the voice FM component to an audio signal.

    HF on 27MHz with AM modulation can be a real problem. You can often listen to one side of the conversation.
  7. Jun 4, 2014 #6


    User Avatar
    Science Advisor
    Gold Member

    It's strange that we all accept the chirping on audio equipment due to mobile phones. I find it annoying - from the standpoint of the Engineering and the Politics involved with the standards.
    You are referring to 'active' speakers (with an amp) here, I think. The power level needed to get a passive drive unit to respond to RF would need to be a hazard to human life. (Although, the signal could get down the speaker lead into the amp circuitry. It is more likely that RF amplification would take place at a stage earlier than the output, I think but Power devices, these days, have high gain bandwidth products so you never know.

    On the whole, I feel it would be better to tackle the transceiver setup, rather than the Audio equipment first. You could be laying down 'illegal' levels of interference with your rig, without knowing it. The neighbours could be getting some of your shash too. :wink:
  8. Jun 4, 2014 #7


    User Avatar
    Science Advisor
    Gold Member

    No definitely not. I have seen it often in the past with just normal amplifier systems picking up RF from 26/27MHz CB and other mobile transmitters up to ~ 80MHz
    It was mainly all AM TX gear ( Land Mobile gear) transmitter powers from 10 - 25W range

    All "in another life" ( aka years ago) as the sound tech for a couple organisations

    Long speaker lines and amplifier gear with inadequate RF bypassing were the 2 main causes

    As Baluncore said .... for the long speaker lines, the RF being picked up and fed back into the audio amp output transistors, amplified and sent out to the speakers

    Else the RF could enter almost anywhere in the microphone or other input lines, preamp-amp, or main am stages, get rectified, amplified and sent to the speakers <---- all that indicative of either crappy/non-existent RF suppression or just plain brute force RF overload.
    ie. .... the transmitter in a vehicle sitting out on the street 10 - 20 odd metres from the audio gear

  9. Jun 4, 2014 #8


    User Avatar
    Science Advisor
    Gold Member

    I know you know about speakers. I was making the point, for the benefit of others, that it isn't the passive speaker box that let's the interference through. The effect will go away with the amp switched off.
    You don't often come across RF filters in speaker leads. Surprising when you think that they can constitute a dipole, resonant in the VHF region.
  10. Jun 4, 2014 #9


    User Avatar

    Staff: Mentor


    I know it is completely off topic, but I had similar experience in the past. One day my Mom called and we talked for a while using and old fashioned analog cordless phone like the one attached. Few minutes later my neighbor (and a good friend) came laughing and he reported whole conversation, he even took notes. He was listening to the radio and he have heard whole our talk loud and clear.

    Attached Files:

  11. Jun 6, 2014 #10


    User Avatar
    Science Advisor
    Gold Member

    Iirc, the base unit used an MF Tx frequency and the hand unit used UHF. A gift for the CIA. Lol.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Why does CB radio sometimes interfere with speakers?
  1. Why do speakers click? (Replies: 2)