| Video Noise September 2005
Doug's Rules for Home
Theater: Part One -- The Room
When it comes to selecting or
building a room for your home theater, there are a few important rules to remember to get
the best sound. If you have to break a rule here or there, the results might not be
disastrous, but the fewer rules you break, the better your results will be. Bending a rule
is always better than breaking it, of course. Knowing your room and what it needs to
perform well will help you make the right choices.
Cubical rooms are the worst for sound because of
bass-response problems. The sound will be noticeably better in a square room with a
typical ceiling height; say, 13’ x 13’ x 8’. But a 14’ x 13’ x
8’ room will sound even better because all the dimensions are different and the room
therefore will not reinforce bass frequencies to the same degree. However, 14 and 8 are
both divisible by 2, so there will be some common bass reinforcement. Change these
dimensions to 15’ x 13’ x 8’ and there will be few, if any, overlaps in
bass reinforcement or cancellation.
Rooms with smooth, hard surfaces of bare wood, glass,
paneling, tile, and stone will sound more harsh and fatiguing than rooms with less
reflective materials. Conversely, rooms with lots of heavy draperies, overstuffed
furniture, thick carpets, and other sound-absorbent materials might sound dead and boring.
Vary the textures, surfaces, and materials within the room to spread out reflection and
absorption properties so that you don’t overly affect any one range of frequencies.
Rooms with ceilings or walls that reflect sound toward the
listeners will not sound as good as they might with different angles. Sloped ceilings that
direct sound toward the listening positions are notoriously bad for sound.
A room that has no parallel surfaces will sound the best,
but such rooms are almost impossible to have built. Most carpenters are averse to angles
of 75, 80, 100, and 105 degrees for wall construction; 90-90-90-90 is what they’re
usually willing to handle. If the room must be rectangular to control costs, it will sound
better if it is left/right symmetrical: If there is a window on the left wall, there
should be a window in the same position on the right wall; if the ceiling on the left side
is 8’ high, the ceiling on the right side should be 8’; if there is a bare wall
3’ behind the left speaker, there should be a bare wall 3’ behind the right
speaker. Everything you do to make the room left/right symmetrical will mean that each of
your ears will hear similar reflections with similar time delays. Similar frequencies will
be absorbed and reflected on both sides of the room. As a rule, the sound will be more
coherent and enveloping.
Rooms can be tuned to sound better with inexpensive DIY
materials, or more expensive -- and sometimes more effective -- commercial products. This
tuning can undo some, if not all, of the problems created by an asymmetrical room.
Anything a decorator or interior designer dictates will
probably compromise your room’s sound quality. If you’re building a home
theater, it should be set up like one. For instance, two large couches facing each other
about 8’ apart, with neither couch facing the large rear-projection video display
located against a side wall, is not a good setup. No seat in that home theater
would be in a proper position for viewing the screen or hearing the surround sound. While
dedicated home theaters can be breathtaking, they shouldn’t be furnished as if they
are just another room in the house. If you raise the floor for a second or third row of
seats, you should raise the ceiling (unless the ceiling is already 10’ or higher).
The stiffer the room, the better it will sound, up to a
point. Concrete bunkers don’t seem to sound as good as wood construction. Walls
framed with 2" x 6" lumber sound better than 2" x 4" walls, and
2" x 8" walls are even better. Floor and ceiling joists that are larger than
code will generally sound better. They could be thicker or they could be doubled at each
location, or laminated joists or engineered-wood I-beams could be used for joists. If
there is a basement or crawl space, use more rather than fewer columns to support the
floor. The more supporting beams or joists there are, the less flex the floor will have.
Two layers of subfloor, glued and screwed to the joists, sounds better than one layer.
Plywood under drywall makes the walls stiffer and the sound better.
Too much of anything can be bad for sound. Too much glass,
too much bare floor, too much soft and absorbent upholstery or curtains, too much tile,
too much space -- all can be bad for sound. The home theaters you see in
"lifestyle" magazines, in which the entire room is robed in heavy velvet
curtains, can sound dead and dull.
Do you have the space to do what you want to do?
In my experience, the smallest room that is practicable for
a minimalist home theater measures 13’ x 13’, with an 8’ ceiling. In that
room you could use five in-wall or on-wall speakers, and two people could share a single
sofa. A 14’ x 13’ by 8’ (182 square feet) room would have much better bass
quality. This is about the smallest room you’d want to use for a home theater if you
aspire to something with good sound. Still, a room this small won’t realistically
support back surround speakers, which would have to be too close to the listening
position.
In the smallest room feasible for a 7.1-channel surround
system you’d need, say, 6’ from the back surrounds to the listeners, 3’ for
the listeners to occupy, and 8’ from the front speakers, assuming they’re larger
than the rears and need more space for the sound to develop. You’ll still be using
on-wall or in-wall speakers. This gives us a room 17’ deep by 13’ wide, while
retaining a typical 8’ ceiling.
Moving up to the next level of sound quality: To sound
their best, most freestanding speakers need to be out into the room, away from the walls.
If you want a 7.1 system with freestanding front speakers but on-wall or in-wall surrounds
and back surrounds, you’ll need a room a minimum of 21’ deep. You’ll also
need some space to the sides of the front speakers, so figure the room will have to be at
least 17’ wide.
Freestanding speakers all around, with freestanding
bookshelf speakers for surrounds and back surrounds, will mean that the room will need to
be at least 23’ deep and 19’ wide.
If you want the sound to be great for five people,
you’ll need more space and larger speakers. The seats will have to be arranged in two
rows, the back row elevated 6" to 8" above the front. This room will probably
need to be at least 27’ deep by 23’ wide, with a 10’ ceiling.
Home theater is a special environment that doesn’t
follow the rules of typical living spaces. Know the rules and you’ll have a better
home theater.
...Doug Blackburn
db@hometheatersound.com |
