Film-Tech Forum ARCHIVE: Surround speaker cable run length

This is topic Surround speaker cable run length - same? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by David E. Nedrow (Member # 4981) on 11-02-2009, 11:44 PM:

I'm geting ready to finish the walls in the auditorium -- duct liner, SoundFold, etc. -- and will be running new speaker wiring for the surrounds.

Should I make the cable length for each speaker equal -- ie. all equal to the length of the longest run? My thought is that this would create an "out of the box" environment in which the transmission delay is the same for all of the surrounds.

I've got plenty of cable, so conservation isn't a priority. Just wondered if it's worth having all the surrounds at the same distance, cable-wise, to start.

Posted by Fred Schoenfeld (Member # 2632) on 11-03-2009, 04:52 AM:

David,
For what you are doing the difference in cable runs will not be noticeable. The important point is speaker placement; i.e., height above the audience, number of speakers and speaker angle. The sound delay due to speaker distance from the listener is much more critical than the length of the cable runs.

Posted by Demetris Thoupis (Member # 833) on 11-03-2009, 05:03 AM:

And also the amount of surround you have, how will they be wired, series/parallel each speaker it's own amplifier e.t.c?
Demetris

Posted by Bill Enos (Member # 440) on 11-03-2009, 10:45 AM:

You couldn't get enough wire in the building to make a noticeable time delay due to travel time of the electrical signal, effects of the impedance of the wire length might be a different thing.

Posted by Randy Stankey (Member # 64) on 11-03-2009, 11:29 AM:

If you had enough wire to go all the way around the world at the equator you might calculate a delay of 0.13 seconds.

(24,900 mi. ÷ 186,282 mi./sec. = 0.133668309337 sec.)

However, there is something else. Even if you have a wire that goes 10 times around the world, there will be almost no delay.

As you push electrons into the wire, electrons will come out of the wire at the other end almost instantly. Imagine a wire as if it was a very long tube. Imagine that the electrons are billiard balls.
There are ALREADY electrons inside the wire. They are just standing still. When you push a new electron into the pipe, one of the electrons already in the pipe gets pushed out the other end. It takes almost no time for this to happen. There might be a delay but it would be so small that it would be virtually unmeasurable unless the wire was very, very long.

It would have to be a wire long enough to reach the moon for there to be a noticeable delay.

The only thing you would have to worry about is impedance in a long run of wire but, with a good amplifier which is hooked up correctly, this will not be a problem.

Posted by Rick Raskin (Member # 1561) on 11-03-2009, 03:19 PM:

Randy, I don't think that is correct.

What is actually occurring is the transfer of valence electrons between the outer orbits of successive atoms. There is a heck of a lot of space between these orbits and there is a finite transit time.

I'm sorry, but your illustration of a tube full of billiard balls, really doesn't compare.

Posted by Robert Minichino (Member # 3495) on 11-03-2009, 04:14 PM:

The propagation velocity in a wire is somewhat less than the speed of light. For some cables that have significant distributed capacitance and inductance, it can be considerably less; a common type of coaxial cable has a propagation velocity of 66% of c. Electrons travel even slower than the signal, and it's there that the tube of balls analogy is useful to explain the difference between the speed the electrons are traveling and the speed the signal is traveling.

Posted by Rick Raskin (Member # 1561) on 11-03-2009, 05:41 PM:

I could meet you half way on that one because we are not talking about the movement of a single electron over the length of the cable run. What we are talking about is the effect of electron transfer between atoms. And that is where I can agree with the billiard ball analogy. What I can't buy are a near instantaneous transfer rate regardless of cable length and electrons standing still; unless of course we are speaking of a temperature of absolute zero.

But back on topic, the dissimilar cable lengths you are likely to encounter in a theater will have no noticeable effect to the customer.

Posted by David Stambaugh (Member # 1102) on 11-03-2009, 05:57 PM:

Are you guys crazy? Any golden-eared audiophile worth his salt will tell you that if those cable runs aren't EXACTLY equal in length, you'll introduce all kinds of phase anomalies and spatial temporal holosonic† soundfield irregularities and god knows what else that will RUIN the sound!!!!

Just kidding. [Razz]

†TM Widescreen Review

Posted by Bill Enos (Member # 440) on 11-03-2009, 06:49 PM:

Our shortest surround cable is about 125 feet, the longest is just under 400 feet, anybody want to stop by and measure the delay? All 12g. stranded.

Posted by Tim Reed (Member # 137) on 11-03-2009, 07:34 PM:

The only instances I've ever seen where the conductor lengths needed to be equal (because of capacitive effects) was PEC cable on dual projector installations -- but we don't have to worry with those anymore (phototubes, that is). [Razz]

Posted by Randy Stankey (Member # 64) on 11-03-2009, 09:40 PM:

I'm looking it up in the book I read which is the source of my statement.

I believe the title is, "Secrets of the Night Sky" by Bob Berman.

LINK HERE

However, I can't find my copy of the book. As soon as I do, I'll look it up. If my information is not correct, either I read it wrong, or the author is wrong. In any case, as soon as I find it I'll let you know.

Posted by Louis Bornwasser (Member # 3063) on 11-04-2009, 08:55 AM:

Tim: Of course we still use them. It is necessary to use two for stereo. How else would we derive signal for Steve Levy's all tube copy of a CP-50?? [beer]

Louis

Posted by Mark Gulbrandsen (Member # 72) on 11-04-2009, 09:41 AM:

quote: Rick Raskin
But back on topic, the dissimilar cable lengths you are likely to encounter in a theater will have no noticeable effect to the customer.

Getting back on topic...

You'd actually have a very difficult time measuring the delay differences between cables in a given theater with out some very spohisticated equipment!! What you may actually need to take into consideration is the sound delay of the speakers themselves if this is a really long room of if you also have a balcony!!! Sound travels much slower then electrons do and so on and so forth...

I still personally reccomend a home run from each speaker position back to the booth. There are so many reasons for doing this that I might fill a book... Also wire groups of speakers in parallel. Running them series-parallel will cause you more phase shift problems than not. Nothing worse than sending the signal through one speaker to get to the next and so on. You can safely drive 2 ohm groups as there are lots of commercial amplifiers today that will drive 2 ohms all day and night with no problem. I would personally reccomend QSC over any other amplifier out there. Stay away from Crown, they have been Harmonized and are no longer the company they once were. Don't over do it with the duct liner either. Its very easy to make a room way too dead and this is done all to often! The services of a good acoustician doesn't cost that much on a big project like yours!! BTW: There are no acousticians here on this site... we can reccomend several that are experts at just analyzing cinemas if you do decide to go that way.

quote: Louis Bornwasser
Tim: Of course we still use them. It is necessary to use two for stereo. How else would we derive signal for Steve Levy's all tube copy of a CP-50?? Louis

Its much easier to get stereo from a Push-Pull Photo toobe than from two seperate ones. They did make them... I don't remember the RCA number though...Anyone else here remember the number of that toobe? Should make doing Steve's Toobe based CP-50 easy to accomplish!

Mark

Posted by Steve Guttag (Member # 268) on 11-04-2009, 08:55 PM:

The propagation delay in copper is quite measurable and when dealing with video, one can easily get "skew" if R, G and B color cables are not of the same length...but even at those higher frequencies, it actually has to be off by feet to see the skew (looks like things are not converged...because they aren't!).

For audio frequencies/times there isn't enough wire in the theatre to cause a audible cue to the problem. However, the impedance (which is the cumulative effects of resistive and reactive components) is something that can indeed add up. The intensity of the speakers could be different, due to cable length, if they are drastically different, even if all paralleled since the furthest speaker will have a higher impedance cable than the shortest distance speaker.

I have never personally balanced the length of speaker cables in a theatre though I have certainly balanced video cables off of a DA and ensured that RGB cables are equal length (or use skew-free UTP cable) for video.

To add to Mark's post about the QSC amps...many don't realize it and QSC doesn't advertise it very well but the DCA1222 will run into 1.6-Ohms just fine...in fact, it was designed to do that...so go ahead and parallel 5 8-ohm speakers on a DCA1222. None of the other DCA amps can do this however, just the DCA1222.

When doing a series parallel...have more of the speakers in parallel too. So if you have say, 6 surrounds in a group...parallel three, then parallel the other three and then series the two groups. In this manner, if one speaker fails, it has a much smaller effect on the rest of the group. Also you are only presenting a 2.67-Ohm series impedance to each sub-group. If you do it the other ways, you will increase the series impedance to any sub-group and also raise the severity of any single failure.

Steve

Posted by Mark Gulbrandsen (Member # 72) on 11-04-2009, 09:03 PM:

The higher the frequency the more likely you are to see propegation delay. I still disagree with series parallel and except for cheap skate installations there is no real excuse for doing it. You will still see lots of phase related errors happenning by sending sound through one set of speakers. Usually the phase errors are caused more by the x-over then any other part of the speaker.

Mark

Posted by Scott Norwood (Member # 30) on 11-04-2009, 09:17 PM:

So, when did 70v lines become unpopular for surround speakers? Other than the cost of the transformers, is there any disadvantage to this arrangement?

Posted by Jack Ondracek (Member # 1466) on 11-04-2009, 11:21 PM:

quote: Scott Norwood
So, when did 70v lines become unpopular for surround speakers? Other than the cost of the transformers, is there any disadvantage to this arrangement?

Offhand, I'd guess a couple of reasons. First, you can use the same amplifiers you're already using for your stage speakers. Of course, you could put a 70v transformer in the booth, but why bother? Installing an amp with a 70v output wouldn't make sense to today's highly efficient chains, because now you have 2 models of amp in your rack... 3, if you're running a different subwoofer amp.

Easier to tie them all together, or series/parallel them and hope Mark doesn't show up to service the booth. [Razz]

Posted by Steve Guttag (Member # 268) on 11-05-2009, 06:40 AM:

ANY amp can be made to run a 70V system so some of Jack's theory is gone. QSC even makes the OT300 and OT600 for their amps to run 70V.

So Scott hit upon it, mostly. It is a money thing. You are going to need an output transformer large enough that will handle the output wattage of the amp (for surrounds, nowadays, it is going to be at least 400-watts for even small theatres). The typical surround speaker in a digital performance is going to need to be able to handle 100-watts or more...so the speaker transformer is going to need to be big enough to handle that. Note these transformers are going to need to be of high quality or frequency response will suffer. You add up the two transformers in the rack and the 8 to say 24 transformers in the theatre and you could have bought a lot more amplifier(s) and larger speaker wire.

70V systems work well when each speaker doesn't have to play REAL loud. Like in distributed music systems. The typical ceiling speaker is going to be tapped at 15W or less (often just 1-3 watts). Thus, you can cut up your amplifier's power to power a great many speakers. Furthermore, with the higher voltage, you have lower current so your speaker wire can be relatively small with no ill effects.

Steve

Posted by Jack Ondracek (Member # 1466) on 11-05-2009, 10:30 AM:

quote: Steve Guttag
ANY amp can be made to run a 70V system so some of Jack's theory is gone. QSC even makes the OT300 and OT600 for their amps to run 70V.

I thought I said that, Steve. You can easily put a transformer in the rack and convert your amp for 70v operation... unless you're talking about something else. A few drive-ins have matched indoor theatre amplifiers to their field systems this way, even though there are plenty of amps on the market with 70v outputs.

Still, I doubt that many indoor operators will see enough advantage in this technique to use it.