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Author Topic: Surround speaker cable run length - same? David E. Nedrow Master Film Handler Posts: 368 From: Columbus, OH, USA Registered: Oct 2008 posted 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.  |  IP: Logged Fred Schoenfeld Film Handler Posts: 49 From: PORTSMOUTH, VA Registered: Aug 2004 posted 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.  |  IP: Logged Demetris Thoupis Phenomenal Film Handler Posts: 1240 From: Aradippou, Larnaca, Cyprus Registered: Apr 2001 posted 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  |  IP: Logged Bill Enos Film God Posts: 2081 From: Richmond, Virginia, USA Registered: Apr 2000 posted 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.  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 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.  |  IP: Logged Rick Raskin Phenomenal Film Handler Posts: 1100 From: Manassas Virginia Registered: Jan 2003 posted 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.  |  IP: Logged Robert Minichino Master Film Handler Posts: 350 From: Haskell, NJ, USA Registered: Dec 2005 posted 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.  |  IP: Logged Rick Raskin Phenomenal Film Handler Posts: 1100 From: Manassas Virginia Registered: Jan 2003 posted 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.  |  IP: Logged David Stambaugh Film God Posts: 4021 From: Eugene, Oregon Registered: Jan 2002 posted 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. †TM Widescreen Review  |  IP: Logged Bill Enos Film God Posts: 2081 From: Richmond, Virginia, USA Registered: Apr 2000 posted 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.  |  IP: Logged Tim Reed Better Projection Pays Posts: 5246 From: Northampton, PA Registered: Sep 1999 posted 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).  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 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.  |  IP: Logged Louis Bornwasser Film God Posts: 4441 From: prospect ky usa Registered: Mar 2005 posted 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?? Louis  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 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  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 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  |  IP: Logged

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