Worst frequency for EM interference?

[ChromeBit]

So I have some big speaker amplifiers at home, and they sometimes pick up radio interference from the CB enthusiast next door (he uses AM, so I'm guessing this is inducing a signal inside my amplifier which amplifies it into my speakers). It isn't really a big problem so I'm not looking for a solution, but I was curious:
-Is there a scientific reason why some frequencies are worse at causing interference than others?
-Does this relate to the capacitance and frequency formula (that was just a random guess so please forgive me if I sound like a loon)?

 

[nsaspook]

Some possible reasons.
1. Poorly designed equipment with little RF bypassing in the frequency range of the transmitter.
2. Field strength, nearby transmitter.
3. Lengths of common wiring (speaker interconnect or power wiring) being close to the wavelength of the transmission that can intercept signals strongly.
4. Signal input stages (mV audio signals) that operation at a low level that can be easily driven into a non-linear region causing RF demodulation by a high level signal.

 

[Drakkith]

-Is there a scientific reason why some frequencies are worse at causing interference than others?

Yes, but the reasons are actually quite complex. EMI (electromagnetic interference) comes from many sources and behaves very differently depending on both the source and receiver. Typically the very low and very high frequencies don't interfere with electronics since they are either too difficult for the circuit to absorb, or can't penetrate the shielding anyways. The mid-range frequencies, corresponding to the KHz through the lower GHz ranges, easily couple to the electronics and are emitted by a wide range of sources. You can get interference outside of this range, but that's usually only in specific circumstances.

Most electronics today incorporate some method of reducing EMI. This can be as simple as making the wires leading to/away from the device a twisted pair or as complicated as encasing the entire device in a faraday cage and having multiple filters and suppressors.

See the following links for more information:
http://en.wikipedia.org/wiki/Interference_(communication)
http://en.wikipedia.org/wiki/Electromagnetic_interference
http://en.wikipedia.org/wiki/Intermodulation

 

[sophiecentaur]

There is not a frequency that is generally 'worst' for causing interference. It depends upon the device being interfered with. Most Audio equipment will have some devices inside it that will amplify signals much higher than audio frequency but that signal needs to 'get into' the circuit.
CB uses around 27MHz and the equipment should be 'type approved', with a limited permitted output power and (believe) a limit to the gain of the antenna used, too. You are just unlucky to have your sensitive equipment where it is. Did you ever try moving its position within your home to improve things? You could even ask your neighbour nicely if he could try another position for his antenna. He might even find it an interesting exercise. I remember I had an ancient Hoover Vacuum cleaner which interfered `(the motor) with a young neighbour's FM recordings. I managed to make up a suppressor filter which just fitted inside the motor case and that sorted the problem.

The circuits in your speaker amplifiers will be making some attempt at limiting the levels of interference. This will be partly, as you suggest, by the capacitor decoupling of the power rails, which acts as a Low Pass Filter.
Lower RF frequencies are often the worst offenders. If you live near an MF broadcast transmitter, you can pick up AM signals easily because of the high field strengths around the transmitting site. Many pieces of equipment have 'RF chokes' in their power leads which can reduce incoming - and outgoing- interference. These chokes consist of ferrite cored inductors. You could always try the same sort of thing if you want to solve the problem - but it may entail a lot of suck it and see.