Do spaeker wire lengths need to be the same in 7.1 setup?

Discussion in 'Archived Threads 2001-2004' started by Marty D., Mar 17, 2002.

  1. Marty D.

    Marty D. Agent

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    I am wiring in my HT for 7.1. My left rear is 50 feet from my receiver. That is the farthest with the right rear being closest at 20 feet. Should all my runs be similiar in length. If I wire my right rear fifty feet I would have 30 feet excess. Would I just put it in the wall?
     
  2. RAF

    RAF Lead Actor

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    While theory would seem to suggest that wires should be approximately the same length, in practice it doesn't really make a difference, even with a 30' disparity.

    I can't find it right now, but some time back someone did all of the calculations involved and concluded that any effects introduced by different lengths of wire would not be audible nor would there be any timing differences that mattered.

    Perhaps someone else here will be able to reference the thread(s) or information I'm talking about.
     
  3. Jim Mc

    Jim Mc Agent

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    Robert,

    I did those calculations again last night and an extra 50' of decent cable causes a .0000008sec delay in the signal.(Less than 1 microsecond) Actually moving a speaker 1" had a 100x more impact than 50' of decent wire.

    I say decent wire like a #14 cable not Cat 5 where you can tell how long the cable run is by measuring the looped resistance.
     
  4. Deane Johnson

    Deane Johnson Supporting Actor

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    I suspect the myth that speaker wires need to be the same length might have arisen from the fact that wires in the RF spectrum frequently are affected dramatically by length.

    As a for instance, when AM radio stations are built with directional antennas, the stations with more than one tower, all of the transmission lines are the same length. This means that the nearer towers have hugh coils of the extra cable rolled up inside the little building at the base.

    As pointed out above, the effect on audio frequencies is so immaterial it isn't considered.

    Deane
     
  5. RAF

    RAF Lead Actor

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    Thanks for the calculations, Jim. I knew that lengths like we are talking about here wouldn't make a difference.
     
  6. tony randall

    tony randall Stunt Coordinator

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    if ya can live without "Monster" cable, check out the Liberty cable at onecall.com. It too is oxygen free, and you can get it in 12-16 gauge as well as in-wall options. It is no more than half the cost of the monsters
     
  7. Cees Alons

    Cees Alons Moderator
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    Cees Alons
    It may even prove to be disadvantageous to use same sizes when the physical distances are different: you have to leave the extra length somewhere, and "coiling" it up (if you get the point), may in itself lead to unwanted audible effects!

    Cees
     
  8. ace peterson

    ace peterson Second Unit

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  9. Deane Johnson

    Deane Johnson Supporting Actor

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    There is a magnetic field around a wire with AC current in it. When wire is coiled up, the magnetic fields from various wraps of the wire interact on each other.

    That's how transformers, generators, etc. work. A "matching transformer" is nothing more than two coils of wire placed together so that the current flow and magnetic field of one causes current flow in the other coil.

    A "choke" that is used in certain electronic circuits is nothing but wire coiled up in a way that gives a predictable effect on the circuit it is in.

    Probably someone else can give you a more simplified answer, but that's the explanation from a guy who learned electronics before "transistors", back when transformers and coils were everywhere (and CinemaScope was introduced). Luckily, solid state has eliminated many of these things.

    Deane
     
  10. Westly T

    Westly T Second Unit

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    I saved this from this forum some time back...
    The following was written by Harv Jeane from www.DigitalTheater.com His bios is as follows...
    Harvey was formally trained as an Electrical Engineer with an emphasis on digital systems and computer science. He has always been interested in technology and reads extensively in a variety of technical disciplines.
    Harvey is currently retired from the computer software industry. Most recently, he was president and CEO of Optika, a Colorado based software firm. Prior to that, he was vice president of R&D at Ashton-Tate, the developers of dBASE. Harvey has been around computers and electronic equipment his entire life. Before getting into computer software, he designed computers, communication equipment and other electronic systems. For 13 years he worked at the Jet Propulsion Laboratory (Caltech/NASA) designing flight and ground computers for spacecraft. All the pictures you see from Jupiter, Mars, Saturn, Venus, etc. were taken from spacecraft designed and launched by JPL. Harvey worked on the design teams for about 10 of these planetary missions. This was borrowed from http://www.digitaltheater.com/pictures/harv/
    Equal Length Speaker Cables - Are They Necessary?
    For several years, it was almost impossible to talk with any salesperson in the audio/video industry without a conversation that went something like this…
    "You do know that you must match the length of speaker cables in you system to insure "an equal delay" in all channels?"
    Of course, you didn’t know this until just then when the salesperson posed this "rhetorical question". On the surface, this sounds perfectly reasonable. Your power amplifier generates a set of signals that are sent to your speakers (to the right main, to the left main, to the center, to the right surround, to the left surround, to the subwoofers, etc.). This set of signals have been created in such a way to be synchronized with each other – in engineering terms, they are "in-phase" with each other. Therefore, it seems perfectly reasonable to maintain this "in-phase condition" by keeping all cables the same length. Unfortunately, this means that you will have to use the same length cable to the speaker setting right next to your equipment rack (less than 10 ft. away) as you use to the speaker setting in the back of the room (greater than 60 ft. away). As this realization takes hold, you look up at the salesperson and see this smile come cross his face – he’s "got you". You are about to spend 3 to 5 times more that you had anticipated (or budgeted) for speaker cables to compensate for the "DREADED PHASE DELAY". Even if you are an electrical engineer, you probably won’t take the time to think through the "sales pitch" you’ve just been given – it sounds so logical on the surface.
    When Einstein first conceived the theory of relativity and before he attempted to codify everything with mathematics, he performed a set of "thought experiments" that helped him visualize the concept that would totally revolutionize our view of physics in the 20th century. I’m no "Einstein" and what I’m attempting to explain is not the "general theory or relativity", but I do find that using "thought experiments" are extremely useful in making complex subjects a bit more understandable. I’d like to use this approach to dispel the concept that speaker cables need to be cut to the same length to prevent audible phase delays in a HT system.
    Let’s assume that you have a stereo audio system with a speaker attached to the left channel and a speaker attached to the right channel. Let’s assume that the cable between the amplifier and the left speaker is 10 ft. long; let’s assume that the cable between the amplifier and the right speaker is 60 ft. long. You are seated at a position that is equal distance from each speaker. One speaker (the left speaker) receives its signal approximately 50 ns (nanoseconds) before the right speaker. This is because the signal to the right speaker must travel through an additional 50 feet of cable (i.e., 60 ft. – 10 ft.). A good "rule of thumb" is that an electrical signal takes approximately 1ns to travel 1 ft. through a cable. According to the salesperson we met back in paragraph 2, your system has a severe phase delay problem because of this 50 ns difference between the left and right channels. With this as our premise, let’s examine our ability as a listener to detect this unbalanced condition. I will attempt to do this through three thought experiments.
    Thought Experiment #1:
    Let us assume that the sound reaching your ears from the two sources (the left channel and the right channel) is indeed out of phase by 50 ns; can the audio sensory system of a human detect this difference? Fifty nanoseconds is a very short period of time - 0.000000050 seconds. Your audio sensory system is a chemical based relay that converts the movement of your eardrum into a chemical change that eventually triggers electrical impulses to the brain. Although the brain in an extremely powerful relational computer, it is very slow compared to even the simplest desktop personal computer. Even the best human hearing is limited to an upper frequency response of 20 to 22 kHz. This corresponds to a signal with a duration (i.e., period) of .00045 seconds or 1000 times longer than the 50ns phase difference generated by the cable delay in our hypothetical system. If you were able to detect this 50 ns phase difference, your upper hearing limit would be somewhere between 20 and 22 MHz. If this were the case, you could receive most of the world’s short wave radio broadcasts directly without having to resort to a radio receiver. You will not find any physiologists that will testify that human hearing is capable of anything approaching this level of differentiation. In other words, the human ear cannot come even close to detecting a 50 ns phase difference.
    Thought Experiment #2:
    For the purpose of a second thought experiment, let’s assume the same setup and also assume that the human audio sensory system is capable of differentiating a 50ns phase difference between two sound sources. In our hypothetical example, energy from our amplifier reaches the left speaker 50 ns before energy from the amplifier reaches the right speaker. Therefore, sound from the left speaker should reach our ears 50ns before sound from the right speaker reaches our ears – Right? The answer to this question is almost always, "No". Speakers are electromechanical devices; an electrical current flowing through a coil surrounding a magnet moves a speaker cone to produce a sonic wave. As such these devices exhibit all of the inherent inertia effects of any mechanical system. From the time an electrical signal is applied to the speaker and the time the cone actually begins to move will vary considerable from speaker to speaker. I don’t care how much money you pay for your speakers (i.e., $10,000 each) or how well they are matched from the factory, no two speakers will "come alive" with exactly the same delay. Because of this, you have no guarantee that the right speaker which gets the electrical signal 50 ns late will not actually be the first speaker to start moving a column of air. You go to all of the trouble to cut wires to the same length, and the mechanical inertia associated with each speaker totally negates your careful preparations. In fact, there is a 50/50 chance you will increase the phase difference between speakers by timing the cables (i.e., cutting them to the same length) and make matters even worse.
    Thought Experiment #3:
    For the purpose of a third thought experiment, let’s assume the same setup, assume that the human audio sensory system is capable of differentiating a 50ns phase difference between two sound sources and that the speakers are perfectly matched and start moving with identical delays. In our hypothetical example, energy from our amplifier reaches the left speaker 50 ns before energy from the amplifier reaches the right speaker. The two speakers "mimic" their respective electrical signals and the left speaker starts moving a column of air 50 ns before the right speaker does the same thing. Therefore, sound from the left speaker should reach our ears 50ns before sound from the right speaker reaches our ears – Right? Again, the answer to this question is almost always, "No". To understand why this is the case, we need to understand something about how sound travels through the air. Sound travels approximately 1100 ft. in one second through the air. This means that a sound wave will travel approximately 0.00066 inches in 50 ns. This distance is equivalent to approximately 1/10th the thickness of a human hair. If you, as the user of our hypothetical sound system, aren’t capable of placing your ears to within 1/10th of the thickness of a human hair ever time you set down to listen, you are introducing far more phase difference that you could ever introduce through unequal lengths of cable. To make this point, let’s assume your ears are always within ½ inch of some nominal location everything you listen to your audio system. This would represent a phase shift that would be equivalent to having one speaker cable 3,156 ft. longer than the other speaker cable.
    I don’t know how many of you have reached this point without going to sleep. But, for those of you that have, I think you’ll see the absurdity of the "equal speaker cable length" argument as it pertains to phase delay. Anyone of the above thought experiments, by itself, is sufficient to negate the "equal speaker cable length" argument. A better argument for maintaining cable lengths that are similar might be to achieve consistent damping factors between speakers. This is not to say that there aren’t places where attempting to match cable lengths is not important. If you were building a s-video cable or a set of component video cables from RG-6 cable, matching cable length becomes more critical because of the high frequency (i.e., short period) nature of these signals. Therefore, the next time a salesperson attempts to sell you more speaker cable with the "equal speaker cable length" argument, you can simply smile and say, "Thanks for this excellent piece of advice, but I think I can spend my HT budget elsewhere and get infinitely more bang for the buck."
     
  11. RAF

    RAF Lead Actor

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    Wes,
    Thanks for sharing Harv's commentary. It is excellent and I've saved a copy of it on my computer for future use. I might even put it up on my website with proper credit, of course.
    This is exactly the article I was referring to. All this talk about wire lengths and time delay are fine on paper. But the words that Harv has put down on paper make a most eloquent case for trusting your ears instead of trusting statistics. What I like to term "Spec Speak" is just that - a pleasant area of discussion that should be used as a guide, but never used as a replacement for the real thing.
    Besides, since you have MA700's and Harv has M&K S150's you are both obviously very, very astute individuals.
    [​IMG]
    Take care.
     

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