| Video Noise January 2006
Room Construction for
Improved Acoustics and Soundproofing
If you have the luxury of being able to build a new home
theater from scratch or remodel an existing room, there are techniques and practices you
can use to make the final result something that people will know is special the minute
they enter it, even before the wallboard goes up. I’ve had contractors, homeowners,
and real-estate agents all comment that a home-theater room designed according to these
guidelines makes them feel different from every other room in the house, even as
it’s being built. It’s all in the acoustics, the dimensions, and the
construction.
Symmetry is your friend
The first thing to keep in mind when designing a room for
good sound is symmetry. The left and right sides of the room should be identical. If there
is a window on the right wall, there should be a window of the same size in the same place
on the left wall. If there is an object on the right side of the rear wall, there should
be an identical object on the left side of the rear wall in the same relative location. If
there is a bookshelf behind the right front speaker, there should be an identical
bookshelf behind the left front speaker.
The front and rear halves of the room don’t have to be
identical, but too radical a difference is not a good idea. For example, if a third of the
front wall is a ceiling-to-floor fireplace of stone or brick, the rear wall won’t
need a similar structure. But if the front wall is relatively flat and three-fourths of
the rear wall is open to the rest of the house, that’s not a good situation for good
sound.
It would be a bad idea to have the right side of the
ceiling 8’ high while the left side slopes up to 14’. This would make the
reflections reaching a listener on the left side of the room very different from those
reaching a listener on the right. A sloped ceiling 8’ high in the front of the room
and 14’ high in the rear would be fine. But the reverse -- a low rear half and a high
front half -- is not so good, because too many reflected sounds will be directed toward
the listeners.
Controlling sound reflections
Sound is reflected by walls the same way light is reflected
by a mirror. As far as sound is concerned, every surface is a mirror. But just as there
are normal mirrors and funhouse mirrors, not all surfaces reflect sound the same way.
Sound reflected by glass or plastic usually sounds more harsh, aggressive, or brittle,
while reflecting surfaces of wood or painted wallboard are more neutral in their sonic
effects. Some softer woods can actually sound softer than harder woods because they absorb
more high-frequency energy.
A large framed print with glass is not a good acoustic
choice for a home theater. A bare or painted wooden object, or the same print with no
glass, would be much better choices. Glass doors on cabinets are less desirable than
wooden doors. Windows should be covered while watching movies or listening to music, to
damp as much as possible any sounds reflected by the glass.
However, covering all wall surfaces with padding and
fabric or heavy drapes will deaden the sound by damping too much midrange and
high-frequency energy. The result would sound plodding and boring. A home theater with
many yards of heavy velvet curtains may look like a real theater, but it won’t sound
very good. The best-sounding rooms have a mix of reflecting and absorbing surfaces and
objects. Upholstered sofas or cinema loungers are appropriate for a home-theater room when
there aren’t a lot of other absorbent materials. If the room is covered with heavy
drapes because there are many windows that need to be covered to block daylight and sound
reflections, using leather or vinyl instead of cloth for some or all of the furniture can
help balance the sound.
Stoutness
If you’re building a home-theater room, stiffer is
better -- up to a point. A stiff wooden frame is preferable to concrete, for example.
Concrete is very stiff and makes for quiet rooms that can be surprisingly soundproof, but
the sound can be troublesome to get right. Sufficient bass impact can be difficult to
achieve without some flex in at least some of the six room surfaces, which can recompress
the room following a bass transient.
When using conventional wood framing, consider using studs
and floor and ceiling joists that are wider than code, as well as thicker drywall.
Doubling or tripling the subfloor material adds worthwhile stiffness to the floor without
being as unyielding as a concrete slab. Adding a layer or two of subfloor material to the
ceiling and walls will substantially stiffen those surfaces as well, especially if
they’re finished with wallboard that’s thicker than normal. Layers should be
glued together to eliminate possible resonances in the tiny gaps between layers. An
adhesive that never becomes brittle but stays pliable long after application will work
best. The glue should be runny enough to be squeezed smooth by the pressure of the
drywall, rather than create unsightly bulges in the wallboard.
Soundproofing
Soundproofing a home theater’s ceiling, floor, or one
or more of its walls is best done by building a second ceiling, floor, or wall with a gap
of a few inches between the two separate sets of studs or joists. The gap can then be
filled with conventional fiberglass insulation to effectively decouple the sound.
Additional sound deadening can be accomplished by using
layers of different building materials in the walls, ceiling, or floor. In a wall, you
might use a layer of subfloor against the studs, then a layer of Homasote 440 SoundBarrier
on top of that, and extra-thick wallboard atop the Homasote. The sound will then have to
travel through wallboard, SoundBarrier, subfloor, studs, and fiberglass insulation. This
method is relatively effective even without two completely separate walls.
Using these ideas can help you make your custom
home-theater room really special.
...Doug Blackburn
db@hometheatersound.com |
