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."
