| Video Noise April 2004
Time and Distance In Your Home Theater
In a home-theater speaker array, time
and distance must be dealt with accurately or the sound will suffer. Although sound moves
through air at a relatively fixed speed, that speed is affected by atmospheric pressure
and temperature. For example, at average temperatures, the speed of sound is 760mph at an
altitude of 500 feet, but 745mph at 6000 feet.
That speed of 760mph can also be thought of as one mile in
just less than five seconds (useful for estimating the distance of a lightning strike,
counting from flash to thunder), 1115 feet per second, or one foot in 1/1115 of a second
(this is easier to remember if you round off to one foot per 1/1000 second -- 0.001
second, or 1 millisecond).
In your home theater, your preamplifier-processor or
receiver probably has settings for compensating for the speakers’ distances from the
listening-viewing position. If you didn’t measure this distance for each speaker when
you programmed your system, you may not be getting the best possible sound from your home
theater.
| Time- and phase-correct speakers Any conventional loudspeaker that has more than a single driver
contains a crossover. A crossover is needed to divide the sound into the different
frequency ranges that are then sent to the tweeter, midrange, and woofer. However, most
crossovers introduce time-delay errors of 1 to 3ms, which is equivalent to having the
drivers spaced 1’ to 3’ apart instead of all close together. One type of
crossover minimizes such time delays -- the "first-order," or "time- and
phase-correct," crossover. Only a few companies make speakers that include
first-order crossovers; the rest consider the inevitable time delays insignificant,
preferring to concentrate on other aspects of speaker design.
If you adjust the delays in your system in 1' increments
and you can hear subtle changes in the sound, perhaps you should also consider changing to
time- and phase-correct speakers to eliminate another time-delay issue. If you hear
nothing at all when changing the delays in 1' increments, you may not be sensitive to such
small timing errors, and thus shouldn’t worry about changing speakers -- or the
timing errors in the speakers are large enough to mask improvements in speaker setup
dimensions so you can't really tell if time- and phase-correct speakers would be a further
improvement
...Doug Blackburn
db@hometheatersound.com |
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Let’s say you’ve estimated that your left rear
speaker is 8’ away when it’s actually 9’ away. That error of 1’
translates to an arrival-time error of about 1ms at your ear. If two or three people watch
movies together all the time, this won’t be very significant -- the people will be
several feet apart, each receiving sounds from each speaker at slightly different times.
But although it has "surround" sound, your home theater actually has only one
seat at which the sound is best. If you sit in that sweet spot by yourself to enjoy the
ultimate movie and music experience, you’ll want each speaker’s sound to reach
your ears at the right instant. That means you’ll need to pay close attention to your
speaker-distance setup.
When movie sound is mixed by the studio engineers, the
mixer presumes that all speakers are the same distance from the listener. If your speakers
are not all equidistant from you, there will be errors that will affect the quality of all
multichannel sounds.
The only way to optimize the sound of your system is to
physically measure the distance of the speakers to the centrally located listener-viewer:
You need someone seated in the sweet spot, centered in front of the screen at the normal
viewing distance. Measure from the left main and surround speakers to the person’s
left ear, and from the right speakers to his right ear. If, say, the left front speaker is
8’ away and the right front speaker is 10’ away, be sure you enter those numbers
in the setup menu for your receiver or processor. The receiver or processor will then
compensate for the speakers’ different distances from the listener by applying the
correct amount of time delay so that sounds from all the speakers will arrive at the
listener simultaneously. When measuring the front three channels, don’t assume that
the center speaker is the same distance from the sweet spot as the left and right -- in
many home-theater setups, the center-channel is half a foot to a whole foot closer to the
sweet spot. Similarly, the back surrounds are often closer to the listener than the left
and right surrounds.
Some processors and receivers can be set in increments of
half a foot. This is to be preferred to the coarser 1’ settings found in many setup
menus. If one of your speakers is 8’ 3" from the sweet spot but your
pre-pro is set to increments of 6", you can select 8’ and have a time-delay
error in one direction, or select 8’ 6" and have a small error in the other
direction. Or you can reposition the speaker so that it’s exactly 8’ or
8’ 6" from the sweet spot. Whenever you can, make the speaker distance
match the figure entered in the setup menu.
Dig out those tape measures, get those speaker distances
set up right, and give another listen to your favorite movie or surround music. You might
be pleasantly surprised.
...Doug Blackburn
db@hometheatersound.com |
