| Video Noise December 2002
Setting Speaker Distance
The menu system for most home-theater receivers and
surround processors can be so intimidating for average, non-technical users that many of
the important adjustments go unused just because they are not understood. I’ll tackle
one of those adjustments in this column: speaker distance -- also known as delay settings
in some menu systems.
Why does speaker distance or delay matter?
Movie sound engineers place sound in each of the six or
seven audio channels at specific times so that the soundtrack seems realistic to you. They
try to convince your brain that what you are hearing is correct for what is happening on
the screen, even if the sound was recorded far from the original event. If the soundtrack
is played back on a system that does not have all the speakers placed at the same distance
from the people who are watching the movie, the soundtrack can sound off synch to varying
degrees. You may hear too little or too much echo in a large space. You may hear sounds
moving from front to back imprecisely, or the effect may be reduced. Ambient sounds like
crickets may seem too focused or too distracting.
The reality of home theater is that rooms need to
have the various speakers placed in different locations to fit the room. When all the
speakers, or many of them, are at different distances from the listening position, you
really do have to compensate to keep the soundtrack sounding the way the engineers
intended.
Distance matters because time and distance are inescapable
elements of sound. Sound moves through the air at about 750 miles per hour -- sometimes
faster or slower, depending on atmospheric pressure, humidity, and temperature. Even in
your own home theater, sound will travel at different speeds at different times as the air
itself changes. But because the speed of sound changes equally for all the speakers in
your room, we can forget about the changes in speed.
Speed implies distance. And 750 miles per hour may be a
hard quantity to identify with. Let’s see how far sound travels in 1 second: Doing
all the calculations, you get 1100 feet per second -- a bit less than one quarter of a
mile.
Breaking 1100 feet per second down further, in 1/1000th of
a second, sound will travel approximately 13". That means if your surround speakers
are 3’ farther from your "sweet spot" than your front speakers, the sound
from the surround speakers will be delayed in reaching your ear by about 3/1000ths of a
second. This is enough delay for your ears to be fooled into thinking that the echo in a
medium-sized space like a foyer or other largish room in a house is much larger, like the
echo you might hear in a public library instead. The surround-sound listening experience
becomes less convincing when things like this happen.
If the surround speakers were closer than the front
speakers to the viewer, you would hear less echo or reverberation than you should. That
will tend to make larger spaces seem smaller. Through the miracle of modern digital
processing, you can dial-in correction factors for each speaker so that sounds from all
six or seven speakers reach your ears at the same time. These corrections will delay the
sound to various speakers so that sounds from every speaker reach the listener at
the right instant in time.
Getting the distances
Newer receivers and surround processors want you to enter
the distance of each speaker from the viewer. How do you know the distances? Measure them!
Get a tape measure that’s at least 15' or 20' long and measure the distance. If you
don’t have a tape measure, use a yardstick. You want to measure from the ear height
of a listener to the midrange driver in a three-way speaker system, or halfway between the
tweeter and woofer with two-way speakers. Be sure you measure to the same point for each
speaker. Enter these distances in the receiver/processor menu system in the speaker-setup
section and you’re all set to enjoy more realistic surround effects. If you
don’t have a second person to help, you may want to set up some sort of reference
point in the "sweet spot" so you have something realistic to measure to instead
of empty air. Stack books or boxes up to ear height, and then use something to mark the
reference point you will use for all measurements. Guessing where the ear positions are
may introduce measurement errors.
What about the subwoofer?
If the subwoofer distance/delay is wrong, you won’t
have good blending among the bass of the subwoofer and the upper-bass and midrange
frequencies of the other speakers in the system. This will usually cause the bass to sound
slow or sluggish, and maybe even bloated and soft. You may also find that the midrange
appears thick and opaque when it should be clean and transparent -- all because the
subwoofer delay is not correct. So don’t forget the subwoofer when setting up speaker
distances.
What if my speaker setup menu doesn’t ask for
speaker distances?
If you have an older receiver or processor, the menu system
may ask you for the delay time for each speaker in milliseconds. A millisecond is 1/1000th
of one second and we already know this represents approximately 1’ of distance. You
will still need to start by measuring the distances to each speaker. Let’s say you
measure your main speakers and they are 9’ from your listening position in the center
of the viewing/listening area. Your center-channel speaker is 8’ away and your
surround speakers are 12’ away. Read the instructions in your owner’s manual to
see exactly how to deal with these different dimensions. Most of the time the main left
and right speakers are your "0" point and you have to manually calculate the
delays for each speaker based on the differences in dimensions between the various
speakers. Your center-channel speaker in this example has a difference of -1ms since it is
closer to you than the main speakers. The surround speakers have a difference of +3ms
since they are farther away than the main speakers. In this case, your receiver/processor
should delay the center-channel 1ms and make the surrounds start 3ms earlier than the
front speakers.
Some older receivers or processors may be limited in what
they will allow you to do. For example, you may not be able to set delays correctly if the
surround speakers are farther from the listener than the main front speakers. This means
that, in your room with your current layout you’ll never get the surround sound quite
right. Your choices would be to lay out the room again so the rear surround speakers and
the main front speakers are equidistant from the listeners, or purchase a newer receiver
or surround processor with more flexible setup facilities to accommodate your room.
What if all my speakers are at different distances?
For some people, the reality in their home-theater room may
be that every speaker is a different distance from the primary viewer. This may be due to
decorating decisions, or the size and shape of the room, or the amount of use the room
gets when the TV is off. With an informed purchase, you can definitely improve your movie
sound. You simply need to be certain that the home-theater receiver or surround processor
you purchase allows different distance settings for every speaker in the system. In
general, the more you spend on the receiver/processor, the more likely it is to have this
feature. The more expensive receivers and processors allow you to set different distances
for up to seven speakers and two subwoofers. Some receivers may allow you to set distances
in increments of 6" versus the more standard 12" distance settings in most
products.
As you come down in price, receivers and processors will
probably give you just one distance for the two back speakers (in a 7.1 system) assuming
they will be side-by-side behind the listeners (a fair bet, but perhaps incorrect for some
people). The next rung down the price ladder will probably force you to set the left and
right surround speakers to the same distance: If one is 12’ and the other is
14’, use 13’ as your distance setting. The least-expensive receivers may not
have a distance/delay setting for the surround speakers. These systems may require you to
place the main speakers, center-channel, and subwoofer all at the same distance from the
primary viewer to avoid time errors that can degrade sonic effects. Home-theater-in-a-box
systems tend to have very simplistic distance/delay capabilities and may not meet your
needs for achieving the best surround sound in your room.
Because receivers and surround processors have added
features at just about every price point, it is easier than ever to find receivers under
$1000 that allow individual adjustments for each speaker. When you hit the $600 mark, or
so, you may see some adjustment capabilities disappear on some models. But simply paying
over $1000 for a receiver or processor does not guarantee that you will have all the
adjustments you need, either. Always be sure to check the speaker-setup feature by
attempting to set every speaker in the system to a different distance. You may find, as I
have, that some of these products have distance-setup limitations that are not documented
in the owner’s manual!
What to listen for when everything is "right"
Once your speaker-distance settings are right, you should
be able to hear subtle improvements throughout your favorite movie soundtracks. You
won’t hear new things, but the things you do hear will sound more like what the sound
engineers had in mind when they recorded the soundtrack. The most obvious differences will
be in where things are placed in space, whether the amount of echo sounds right for the
space the characters occupy, and tighter, speedier-sounding bass with more clarity of
other sounds when there is bass in the soundtrack. There won’t be a huge difference,
but you should hear subtle changes that will make movies more convincing and enjoyable.
...Doug Blackburn
db@hometheatersound.com |
