There are often questions regarding the length of your speaker
wires. Related to this are issues of time delay, supposedly caused
by wires of differing lengths as well as other factors in your
home theater such as speaker placement, etc. Much has been made
of adjusting the time delay via your pre/pro. The following article,
written by Harv Jeane addresses these issues and explains
the difference between the theory involved and the actual effects
in your Home Theater.
Some background:
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.
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."