Audio sound system wireing...

[1793]

My son is a sound designer at our local theatre and they are about to re-wire the space for their sound system. He asked me what type of wire to use, MN, MC etc.

My first thought is that MC should be used. No NM, Article 334.12, 2005. They are thinking of 14 gauge throughout the system. These are for the speakers, I have very limited information at this time.

I have looked at 640 Part II and I'm not sure if Article 725 comes into play.

I would appreciate any information that could help them in the planning of this change-over.

Thanks as always.

 

[mhulbert]

Well, you need to be mindful of limitations imposed by art 518 and 520, if they apply to your building.

If the runs are long, 10 or 12 gauge (or higher) would be prudent, especially for high wattage loads (Subs and LF drivers). Most theaters are run in EMT, at least the original installations. Typically, stranded wire is used because it can be terminated on amps, speakers and typical connectors (Neutrik "speakon" NL4's and NL8's). If you can use MC, I'd look into a custom stranded reel of it. You may also be able to use listed speaker cable, with an outer jacket (see Belden cables). What are the wattages of your amps? They go from class 2 to class 1 for some of the large ones (which can have 250+ volts at the output terminals!). A chapter 3 wiring method may be required, and may be easiest anyways!

IMO, a Cadillac install would have spare lines from your amp room/area to your speaker areas, with neutrik connectors available to patch in temp speakers. You can never have to many extra lines, and this keeps the "creative" solutions from getting out of hand down the road.

What size theater is this?

-Mike

 

[hbiss]

NM in a theater? I don't think so.

All I have ever seen is #10 THHN stranded twisted pairs run in pipe from the projection room sound rack to behind the screen and also to the surrounds. One pair per speaker.

Clifford can twist em' up for you.

-Hal

 

[1793]

NM in a theater? I don't think so.

All I have ever seen is #10 THHN stranded twisted pairs run in pipe from the projection room sound rack to behind the screen and also to the surrounds. One pair per speaker.

Clifford can twist em' up for you.

-Hal

This is a Theatrical theatre, not a motion picture theater. However I do agree NO NM.

 

[hbiss]

You should have been more specific. Regardless, its a place of assembly.

See Article 640.9 and the referenced sections of 725. It will be necessary for you to know what the wiring requirements for your amplifier outputs are in terms of Class 1, Class 3 or Class 2.


-Hal

 

[1793]

I'm planning on getting into the theatre this afternoon. I will try to get some photos of the existing system.

They are talking about having conduit of various sizes run. If they find that a 14/4 cable(s) are used, I assume conduit fill requirements come into play with the Audio System? What about derating?

 

[hbiss]

This is sound system work, not electrical work. It's a different animal. There is no derating because you don't get involved with current carrying and these are not continuous loads anyway. There is no "14/4", wiring should be paired. Read the articles I mentioned above and look at the back of the amps to see what class of wiring is permitted. Conduit is good and depending on what class wiring you use and what code cycle you are using there may or may not be NEC fill requirements. If not you would then look to the manufacturer of the cable for recommended conduit size and fill. If you do use twisted THHN then you would use the NEC tables.

-Hal

 

[Smart $]

If you do use twisted THHN then you would use the NEC tables.

Personally, I would never use THHN for any professional sound reinforcement speaker wiring for two reasons: 1) because skin effect comes into play (20 ~ 20,000Hz frequency spectrum) and 2) the final connection to speaker cabinets is likely to be exposed. I won't make any recommendations because my experience is dated some 20 years now. Nonetheless, because of skin effect and its associated voltage drop, when setting up a sound reinforcement systems for club-size, live performances (rock bands), I have always placed the amplifiers back or side stage—sometimes even on stage—to minimize the the effect on the amplified-level audio transmission conductors.

Consider this: Professional LF amps easily approach 1000 wrms output rating or more, with a typical load of 4 ohms nominal or less. If the 1000 wrms rating is at 8 ohms nominal, it is likely that its output at 4 ohms nominal is 2000 wrms. Since the transducers are reactive, their Re can go down to say 75% of the nominal rating at low frequencies (e.g. 3 ohms for a 4 ohm nominal driver).

P = I? ? R
I? = P ? R
I = √(P ? R) = √(2000 ? 3) = 25.8A(rms)​

 

[mhulbert]

Smart $,
Twisted pair (or not) THHN is used all the time. The research I have seen says that skin effect is a non-effect at 20KHz. As you mention, distance vs wire size is the key factor here. If I recall, the Crystal Cathedral in Garden Grove, CA, has twisted pair #6 going froma ground floor amp room up to their clusters. In practice, this is probably overkill, the amin reason being that even with a 1000W amp, it is almost never putting out that power for more than a brief moment on peaks (bass drum, etc). In practice, long runs of #12 produce good results. The touring world typically uses #12 SO cable and these runs may be over 100' with 200' not being unheard of. You lose some power in the cable, but it's negligible in practice. We tried some A/B tests quite a while ago between THHN (#12) and the super fine stranded "oxygen free copper" wire, we could not tell a difference.

While hardly a definitive article (from http://www.audioholics.com/educatio...n-effect-relevance-in-speaker-cables-page-3):
"The bottom line is Skin Effect is not a relevant factor of concern when choosing / designing high performance loudspeaker cables for hifi audio systems. The DC resistance and inductance of the cable are far more important factors"

Like all things related to audio, there is a lot of subjective factors, and two people may never agree on a particular aspect of an installation. The true crafts people can look past this and maximize the performance of the particular system.

Mike

 

[1793]

Wire............

Wire............

Ok so today I have received an e-mail with the data sheet, http://www.westpenn-wpw.com/pdfs/cm_PDF/C207.pdf

After review I would like to hear how I should calculate the conduit fill for this arrangement. The "Overall Cable Diameter 0.260 Nom."

We are looking at 30 of these from the source out to the "house" and then branch out to the different speaker locations.

.260/2=0.13
Pi R*2
0.13*0.13=0.0169*3.147=0.05318
30*0.05318=1.595529
Chapter 9 Table 4 (EMT)
30 conductors 40% fill 2-1/2"

Thanks for all of the input so far and I do appreciate the insights.

 

[hbiss]

I would never use THHN for any professional sound reinforcement speaker wiring for two reasons: 1) because skin effect comes into play (20 ~ 20,000Hz frequency spectrum)

There is no skin effect at audio frequencies even beyond 20khz. Even if there were, what difference would THHN make as opposed to anything else? Copper wire is copper wire, end of story.

You have to remember one thing. There is professional audio and installed sound and there are the lunatic audiophiles. The people who do professional audio and installed sound have an education and experience in what they do. About all audiophiles have are big bank accounts, little knowledge and an active imagination.

-Hal

 

[RayS]

Smart $,
In practice, long runs of #12 produce good results. The touring world typically uses #12 SO cable and these runs may be over 100' with 200' not being unheard of. You lose some power in the cable, but it's negligible in practice.
Mike

one often overlooked issue with speaker wire sizes is damping factor- those big cones pushing back from the air compressed in the box will drive current back into the amp, and since the impedances on ea end are low, wire resistance (as in DC, not reactance) becomes significant in keeping it "tight". I'd really try to oversize the wire, especially for the lo's/subs. You don't want to lose the benefit of an amps damping factor (say DF 50 to 100, impedance of .16 to .08 ohm) by going cheap on the wire.

 

[mhulbert]

Looks like you got it under control. I would up size the conduit, at least to 3", you will find that once you are installing this stuff, there may be something you forgot, or you will want spares for traveling shows, etc (Maybe you already accounted for this?). A lot of times we would put a few "tie-lines" from a patch panel backstage to a patch panel up in the rafters, allowing for additional speakers to be temporarily put in for certain shows (some shows want centers or surrounds or sidefill or balcony fill, etc). The idea was that you could roll up a rental rack of amps, patch them in, and then temporarily rig your speakers and run cables (SO cable) to the speakers.

Mike

 

[dbuckley]

one often overlooked issue with speaker wire sizes is damping factor

On a related note, run a separate cable for each enclosure, and if you need to parallel them (common with subs) parallel them at the amp end.

 

[Smart $]

Smart $,
Twisted pair (or not) THHN is used all the time.

I did not say it wasn't. I said I personally would (will) not use THHN.

The research I have seen says that skin effect is a non-effect at 20KHz.

While hardly a definitive article (from http://www.audioholics.com/educatio...n-effect-relevance-in-speaker-cables-page-3):
"The bottom line is Skin Effect is not a relevant factor of concern when choosing / designing high performance loudspeaker cables for hifi audio systems. The DC resistance and inductance of the cable are far more important factors"

Please note the wire length used for that article was a whopping 10 feet long and reports a less than 3% loss—I'm going to assume that also means greater than 2.5% loss :grin: —from skin-effect resistance at 20kHz. So what would be the loss at 100 feet? (rhetorical)...

Then if you follow the link (for a working link to the page you referenced, click here) to the calculations page at the bottom, one paragraph says, "As you recall, we calculated that Skin Effect does attribute about a 34% increase in cable resistance at 20kHz. This again assumes solid core wire. In actuality, multistranded wire has less of an issue with skin effect because each individual strand has a smaller cross sectional area than the skin depth at the particular frequency. Although un-insulated multi stranded wire is not considered true Litz wire, it still behaves somewhat as such and thus reduces the overall skin effect problem. However, for argument sake we will ignore this factor and thus we will ultimately yield a more conservate estimate of the skin effect problem. Recall that actual measured increase in AC resistance at 20kHz due to skin effect was only about 3% from our Cable Face Off article." Just the difference between their calculated and measured results makes one wonder about the report's credibility! I know, I know... you said it was hardly a definitive article. I concur :grin:

Anyway, even though I brought up skin effect, it was with respect to signal loss and not intended to be confused with signal degradation.

As you mention, distance vs wire size is the key factor here.

Bottom line no matter what factors contribute to signal loss.

We tried some A/B tests quite a while ago between THHN (#12) and the super fine stranded "oxygen free copper" wire, we could not tell a difference.

As I mentioned before, signal loss and signal degradation are not to be confused. Conductors are twisted to minimize both signal loss and degradation (non-linearity) resulting from conductor capacitance [and magnetic flux]. Some would say the difference between parallel and twisted is negligible... but that goes to your conclusion...

Like all things related to audio, there is a lot of subjective factors, and two people may never agree on a particular aspect of an installation. The true crafts people can look past this and maximize the performance of the particular system.

I could go on for quite some time on the matter, but I agree that maximized system performance to one person may only pass as tolerable performance to another. I usually find myself on the latter end of that stick when discussing the matter


EDIT in brackets

 

[mxslick]

THHN is not evil....

THHN is not evil....

In fact it is used extensively in cinema installs. It has a lot of advantages including easy identification (by use of all the available colors) for the multi-channels of speakers behind the screen. For runs up to 100 feet or so, #12 is the norm. Over 100 feet I spec #10awg as a minimum. A single 1.5" to 2" conduit is run from the booth to behind the screen.

Never had any complaints and my THX rooms passed their certifications.

But, having said that, the best practice is to install the amplifiers as close as possible to the speakers, and run high quality shielded wire from the source to the amplifiers. (Belden 8451/9451 are the most common.) Use #12 THHN or jacketed speaker wire from the amps to speakers.

Unfortunately, in cinemas putting the amps behind the screen is a very bad idea. The area gets used for junk storage all too often, and the janitorial staff loves to use blowers to get all the trash to the front by the screen (which also does wonders for the life of the screen.)

So the only place I can usually install amps behind the screen is in some high-end home screening rooms.

For legit theaters and live performance venues, it is sometimes possible to get the amps close to the speaker clusters, which makes for a "cleaner" install.