| Video Noise October 2005
Doug's Rules for Home
Theater: Part Two -- Speakers
Last month’s column covered my rules for your home
theater’s room. This month’s list of rules will help you pick the right type and
size of speakers and subwoofer(s) for your home theater.
Speakers standing free in the room sound better than
speakers mounted on the wall. Speakers mounted on the wall sound better than
speakers mounted in the wall. The first two rules don’t mean that in-wall
speakers sound bad; they just won’t reach the same levels of performance as similarly
sized freestanding speakers or even on-wall speakers. In-wall and on-wall speakers may
also have limitations concerning where they can be placed and aimed. Room considerations
may limit where on-wall or in-wall speakers can be located. If a speaker can’t be
mounted in exactly the right spot, some in/on-wall models can be partially or entirely
aimed to compensate for this limitation.
The surround speakers should be slightly above the
listeners’ ears for the best "disappearing" act. If no rear surround
speakers are used, side surrounds should be slightly behind the listeners. If rear
surrounds are used, the side surrounds should be more directly to the sides of the
listeners.
Modern Dolby Digital and DTS 5.1-channel surround sound, as
well as Dolby Digital Surround EX and DTS-ES 6.1-channel surround (which adds a rear
channel), work best with direct-radiating surround speakers. Years ago, when the
surround channels were simple matrix-derived surround and not directional, dipole surround
speakers were the best setup. These were designed to fire forward and backward, though not
directly at the listener, to create a diffuse surround soundfield. Today’s DVD
soundtracks and high-definition all-digital sound allow engineers to put any sound in any
channel and pan it to any point in the room using the imaging power of the five or seven
speakers. Dipole speakers have thus become a handicap because they can’t precisely
position these discrete sounds. But because some manufacturers still make dipole
surrounds, they fluff them up with all sorts of hot air about why you might still want
dipoles. Forget it. Stick with direct-radiating speakers for your home theater. If you
need a more diffuse image, add more direct-radiating surround speakers, such as rear
surrounds. With direct-radiating speakers in every location, you’ll get precise
imaging of every sound in the rear half of your room.
For those who want an authentic experience listening to
laserdiscs or VHS tapes with matrix soundtracks, switchable surround speakers are
available that let you select different modes at the speaker; for instance, direct or
dipole response. Most people don’t need this feature, but it can solve an otherwise
unsolvable problem for those who do.
Tiny 5.1- and 7.1-channel surround speaker systems often
come with a "subwoofer" that is a sub in name only, and probably won’t be
able to produce bass frequencies below 40Hz. You’ll miss the room-shaking bass that
can make a home theater so exciting. A true subwoofer should be able to reproduce the
lowest bass frequencies, down to around 20Hz, that are found in movie soundtracks and
music. Of course, subwoofers that can produce such bass are large and more expensive.
However, five to seven little speakers and a single big
subwoofer that goes down to 20Hz will not be a good sonic match. The output of the
speakers should overlap the output of the subwoofer by about an octave (a doubling of the
frequency, as expressed in hertz, or Hz) for a good blend of sound. If the small speakers
were down by 3 decibels (expressed as -3dB) at 100Hz, the subwoofer would need to be rated
at -3dB at 200Hz to work well with the main and surround speakers. That would make for a
bad subwoofer, because a sub needs to produce low frequencies with power. 200Hz is in the
lower-midrange octave, above the low-bass region. You’d be better off with speakers
that are -3dB at 60Hz and a subwoofer that is -3dB at 120Hz. Even better would be speakers
that are -3dB at 40Hz with a subwoofer that is -3dB at 80Hz. The rule of thumb here is
that the higher the top-end -3dB point of the subwoofer, the worse its
low-frequency extension will be. Of course, speakers that produce bass down to 60Hz or
40Hz will be larger and more expensive than speakers that begin to roll off at 100Hz.
Whenever possible, stick to the same speaker manufacturer
for all of your full-range channels. This would almost guarantee that all the
speakers’ drivers and crossovers are of equal performance and quality. Using a
different brand of subwoofer, however, can often produce better results than a sub made by
the manufacturer of your other speakers -- many makers of great speakers don’t
necessarily make great subwoofers. Hsu Research and SVS are good examples of companies
that specialize in subwoofers; their models usually excel. For example, Hsu’s VTF-2,
which lists for $500, is so good that it performs better than other manufacturers’
subs costing $1500 to $1800. Scrimping on a subwoofer can really sink the sound, so to
speak -- but spending more on a subwoofer doesn’t necessarily guarantee better
bass.
Having identical speakers in each location would make the
ideal home-theater sound system. This is rarely practical, however, which is why
center-channel, surround, and main speakers are often of very different sizes and shapes.
Sometimes the rules aren’t easy to follow, and why truly great home-theater sound is
hard to come by.
...Doug Blackburn
db@hometheatersound.com |
