Running long PA speaker cables parallel to AC 120v lines -

Question

I'm setting up a PA system (LabGruppen 2k watt power amp to two Bose 802 Panaray 8ohm speakers) over 120' 12GA speaker cables in a temporary outdoor tent for graduation ceremonies over three days. Fire code indicates that the tent must be equipped with emergency exit signs, (even though there are no side panels,) so 120v AC power lines are distributed around the inside perimeter of the tent to power the signs, with a few upward floods to provide ambient illumination.

In years past, I've laid the speaker cables (50') on the ground for each event, and removed them in between to stow securely and prevent damage, etc.

This year our school has doubled the footprint of the tent, in addition to being square vs. rectangular, so I need a far longer cable run. Installing the speaker cables on the inside perimeter of the tent alongside the power lines would save me a lot of time in setup and breakdown, and the electricians can to the heavy lifting for me!

I've done a fair amount of research on the net about running lengthy speaker cables parallel to AC cable and the possibility of interference with the sound, (buzz, etc,) and most of my findings indicate that there won't be a problem with this. In essence, the shielding in the cable and the signal level is high enough to prevent interference, and the 8ohm characteristic of the speakers help as well.

Barring human error (electricians by accident cutting the cable casing and exposing the shielding) and acts of nature (unexpected violent thunderstorms) should I, or should I not, have the speaker cables installed alongside the power cables on inside perimeter of the tent?

Answer 1

You are talking about cables powering (passive) speakers directly. Those are not shielded, nor do they need to be. The amount of electrical energy transfering between adjacent return cable pairs (namely with a net current of zero per pair) is small. In addition, you are talking about the power for exit signs which don't take a lot of current (and it is current that creates the magnetic fields primarily responsible for energy transfer).

Now this would be a different situation if you had powered speakers rather than passive ones because then small energies injected into the speaker cables can already cause significant troubles. However, powered speakers for venue use take balanced input cables, and if you have a proper balanced sound source at the other side, the cabling it quite impervious to noise injection (basically, any non-trivial noise gets injected in both strands of the balanced cable causing it to get cancelled by the differential input circuitry of the speaker).

So short of improper cabling and/or home stereo devices (or instrument amps) with unbalanced inputs driven with long cables or mobile phones placed right next to a cable (those send bursts of several Watts of power in regular intervals even in "noiseless" modes), most stuff intended for venue use is pretty impervious to interference.

Answer 2

Out of personal experience (I work 9 years as a system technician) I would say "you won't have a problem".

If you look at it from a "more scientific" point of view the way buzzes (and in general interference) bleeds into cables is, most of the time, through (electro)magnetic induction (for more information if you are unfamiliar with the phenomenon look for "Faraday's Law" in a textbook or at Wikipedia). The phenomenon exhibits an intensity decay proportional to the increasing distance from the source (magnetic field property, see Wikipedia for quick reference).

This means that on top of the already decreased transferred energy due to the induction properties, the magnetic field is already a bit attenuated. Now, taking into account that you are talking about high voltage signals, the crosstalk from the power line will most probably be negligible, compared to the original signal, and buried into the signal to indistinguishable levels (due to masking effect mostly).

What you could possibly do (depending on the available space) is separate the power line and audio cables by some distance (some centimeters can make a difference) and avoid parallel runs.

In general you could find good information in Handbook for Sound Engineers by Glen Ballou. I have the fourth edition and in this edition you can look at section "14.27.2 Crosstalk".