2
$\begingroup$

After reading about cable screens to prevent electric field interference such as:

“Most of our audio cables are screened, and this does a decent job in keeping the Radio Frequencies at bay, but doesn’t do anything much to keep out the signals that we can hear, because these attack our audio largely by long wave length electromagnetic fields which go straight through the expensive foil wrap and braided screen protection around our cables.”

How can low frequency interference such as 50/60Hz long wave due to mains electricity pass through the a coaxial cable shield but not high freq. EM waves like radio waves?

$\endgroup$
4
$\begingroup$

After having used high-impedance shielded cable runs for audio and live on-stage performance for 48 years, I can say from direct experience that the assertion you referenced is wrong. in fact the primary purpose of shielded cables in audio apps is to block

  1. radiated interference from things like fluorescent light fixtures and some neon bulbs (60Hz plus higher harmonics),
  2. radiated interference from silicon-controlled rectifiers ("light dimmers", also 60Hz plus higher harmonics) and
  3. crosstalk from adjacent audio cables carrying signals to and from speakers, amplifiers and preamplifiers (in the full audio range).

in fact, the way you detect a shielding fault when it occurs is a sudden and very audible increase in the amount of audible hum and buzz getting into the signal path.

The RF shielding effect is important too, but only if you are in the vicinity of a large radio transmitting antenna or if your upstairs neighbor is a poorly-educated CB or ham radio enthusiast.

$\endgroup$
  • $\begingroup$ At 60Hz there is no wave there is electric field or magnetic field. At that freq the interference is not EM wva but the field. You need a huge antenna to receive 60Hz as a wave. $\endgroup$ – user1999 Oct 6 '18 at 22:05
  • 1
    $\begingroup$ no, you don't. this is easily demonstrated with a guitar amp and an electric guitar with unshielded pickups, feeding an oscilloscope. $\endgroup$ – niels nielsen Oct 6 '18 at 22:08
  • 1
    $\begingroup$ I am a ham radio operator. we call any signal carried by electromagnetic waves (which are time-varying electromagnetic fields) "radiation". $\endgroup$ – niels nielsen Oct 6 '18 at 22:12
  • 2
    $\begingroup$ to get a good impedance match between the antenna and free space, that is true. you do not need a good impedance match to radiate. a poor match will work. $\endgroup$ – niels nielsen Oct 6 '18 at 22:35
  • 1
    $\begingroup$ @newage2000 That's one helpful way to think in some cases for sure, but even for evanescent waves of any wavelength, it's also okay to call it "radiation". It may not really radiate in a vacuum at infinity, but if stuff picks it up, receives energy from it, it's okay to call it radiation. 1, 2, 3 $\endgroup$ – uhoh Oct 7 '18 at 2:31
2
$\begingroup$

The skin effect means that fields of high frequency penetrate less deep into conductors than fields of low frequency.

$\endgroup$
1
$\begingroup$

You can radiate 60Hz EM quite easily at 120v and 10amps for example, but as Nielsen shows in many instances it is from ac switching devices that actually produce a white noise (composed of many high frequencies) similar to a spark noise (but not a spark). Most home speakers do not use shielded cable, it's only on the inputs from dvd players etc where coax is present, and this prevents rf as well as 60Hz. If these input cables get too long you can pick up the 60Hz and the white noise which when amplified is bad. I have no experience in stage wiring but these cables can be long which would help pickup even on speaker cables, also there may me a mix of speaker cables and microphone cables that could cross talk. Also many times the white noise gets carried on the powervlines directly into amps.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.