Make A Room Sing - for $3,500
by Tom Hidley
We've all heard someone say (if only in jest), "I've got a barn. Let's put
on a play." For better or worse, it's an epidemic attitude in recording
these days, with a new generation of high-quality, low cost gear enabling
sound pros to build rooms in their own offices, homes, garages - and yes,
barns.
But no matter how good the equipment and gifted the musicians, it all comes
to naught if the acoustic environment isn't tailored to deliver true and
honest sounds, free of distortion and wild-flying sound reflections that
will fool the engineer into making bad musical decisions.
Acoustic design is an exacting - and to some, almost mystical - science,
and it is probably the most neglected aspect of sound control in today's
modern studio. Time and again, I've seen people build rooms with the best
of everything, only to be stumped in the end when it all sounds non-musical.
And that's sad, because the principles needed to make a decent basic and
clear acoustic (musical) sound space are fairly simple and inexpensive.
The Right Room
So how do you do it? Well, in the first place, we have to assume that you
have a room to begin with. It shouldn't be a warehouse or an airplane
hangar, or some other enormous structure; your sound will reflect
uncontrolled - everywhere. So assume you already have a room with a floor,
ceiling and four walls - ideally, it's symmetrical. For our example, I've
picked an 18x24 foot room (see the accompanying diagram).
The room itself should be reasonably isolated from the neighbors, for your
and their peace of mind. The best situation is to have a concrete floor on
ground level.
|
A. Exterior Wall B. Flank Blanket Cover 1" Fiberglass C. Open Frame Wall Cover Grille Cloth D. 2' Minimum Dense Rockwool
E. Rockwool Cover Grille Cloth |
Preferably the floor isn't a wood joist system, and preferably it has no
windows (if so, let's hope they're double glazed). Try your best to avoid
using a room with a tin or "live" roof, and in the best case scenario,
there's an attic overhead or some such isolation space. If you don't have
a ground floor site, then putting your room on the top floor is a better
choice than using a middle-floor room where people will be walking overhead.
The size of the room is an important consideration-and it isn't always a
factor over which you have absolute control. Look for a room that would
complement the lowest frequency of the monitoring system that you intend to
use.
Basically, you need to look for a room with a physical long dimension of
half the wavelength of the lowest frequency your monitor can produce - so
for a 50 Hz monitor, you need 14 feet of room front to back dimension. A
rule of thumb to remember is that every time the frequency reduces an
octave, the distance needed doubles. Thus, for 25Hz, you need a 28-foot
room. Here's a table to help you (in half wavelength - monitor low
frequency to room):
60 Hz - 12ft
40 Hz - 16ft
30Hz - 24ft
20Hz - 32ft
So let's assume that before you even start spending any money on acoustic
construction, you already have a space and set of monitors that match. Set
up the longest dimension of the room as the length dimension from the
monitors to the rear wall. The side-to-side room distance should ideally be
the shortest, and higher ceilings are better than lower ones.
Now, since we're working on a limited budget, we have to get our priorities
straight from the beginning. Believe me, the largest potential problem in
any listening room is uncontrolled and certain unwanted sound reflections.
Obviously, reflections are needed in any human listening indoor environment,
but if they fly loose, the result is a dishonest room - and you'll end up
making incorrect musical decisions.
Sound Floor
|
For essential "in room" gear, I suggest a wooden "low boy" portable rack construction whose front mounting panel geometry reflects arriving sound away from the engineer/producer. Dim. given in cm. Scale- 1:10 |
If your priority is to control the reflections (and it should be), start
with the floor. This will be the prime source of room reflection in our
room. In the final analysis, you want structural stability in your floor.
When completed you want to end up walking on a rigid concrete, hardwood
finished floor. Ideally, it's finish is butt-jointed, without ridges and no
beveled edges, and polished to a sparkle. I use very stable laminate woods
manufactured in Scandinavia by Kahrs and Tarkett. Both floorings are about
15mm thick. An inexpensive alternative is using a prefabricated, 1/8-inch
hardwood parquet available at most flooring stores.
Of course, what's underneath that wood surface is important as well.
First, you use mastic to fill in any pits and holes you may have in your
base cement floor, making sure it's perfectly flat. Cover it all with glue,
and lay down a 3/4-inch tongue-and-groove moisture-proof sub-flooring of
something like chipboard or particle board over the entire area. It's best
to shoot this into the concrete with Hilti - the trade name for a tool that
shoots special Hilti-hardened nails into concrete or stone. When the
sub-floor is secure, glue and blind tack your finish hardwood floor onto it.
Don't lose all hope if you don't have a concrete floor. In many cases,
it's possible to lay 3-inch dense Rockwool and 1-inch shutter plywood on top
of the existing wood floor and pour concrete (at least four inches) on top
of the ply to achieve a float slab condition. I would advise consultation
with a structural engineer first, or you may end up with a collapsed
building. If you can't pour a new floor, another alternative is to put the
monitors up on a coil-type spring whose resonant frequency is at least one
octave lower than the lowest frequency the monitor is capable of. But this
is not a cheap way out - purchasing 1 Hz to 10Hz industrial viscous damped
springs can be costly.
One U.S. company, Kinetics, makes neoprene isolators that can get down to
about 10 to 12 Hz. If you provide Kinetics with the needed data, they can
tell you what kind of base to build and how to place the pads (and which
pads to use) for best results. Air pressure from the monitor will still
create unwanted motion in a wooden frame floor, but the right monitor base
can go a long way in minimizing impact transmission to the internal wood
floor structure of the room.
Wall Hanging
The next step is to deal with the walls.
Basically, it is necessary to construct and hang four walls of 1/2-inch
flanking blankets and then visually hide them with acoustically transparent
grille cloth. These "flanks" help negate the horizontal standing wave
pressure buildup and keep the room sounding smooth. They are hung 6 to 12
inches into the room parallel to the building walls and hung from the
ceiling with eye hooks and wire, allowing them to swing free to the floor,
clearing it by a couple of inches. They should be free at the top a
little bit as well. Suspending the flanking blankets lets them dissipate
pressure build up.
Construct the flanking blankets of 1/2-inch particle board, which usually
comes in 4x8 foot sheets. Let the boards overlap at the edges, and bolt all
edges together for the full wall length (and height). Then staple 1-inch
fiberglass on the insides (room-facing surface) of the 1/2-inch board,
keeping the shiny aluminum foil or paper backing next to the board and
letting the fluffy fiberglass face the room. The fiberglass will keep mid-
and high-frequency reflections down to a reasonable room content. Finally,
build an open frame 2x4 wall in front of the flank blankets (which are now
covered with a 1-inch soft fiberglass) and cover the open frame 2x4 wall
with acoustically transparent, synthetic, 100 percent all-polyester open-weave grille cloth to hide the fiberglass from view.
The monitors need to look down the length of the room - leave about 2 feet
between the monitor and the wall behind it. Put the monitors on an
appropriate center commensurate with the width of your finished room (see
room diagram) and focus them horizontally at the engineer.
Meanwhile, pack the space between the monitors solid with at least 2 feet
minimum (and preferably 3 feet) of Rockwool or dense fiberglass.
The monitors should be sitting on solid concrete blocks, keeping a piece of
sound board underneath them to prevent scratches. In my listening room I
sit on the floor, so the bottom of the monitors are about 8 inches from the
floor. If you're in a chair for listening, keep the bottom of the monitors
about 3 to 4 feet off the floor. Never put grille cloth over the front of
the speakers. Now you're ready to bring in the gear and fit it into the
acoustic environment. Clearly, you want it to interfere as little as
possible with the sound field. It's always better if the console is
open-bottomed so that the low frequencies won't be obstructed as they pass to
the back of the room. Of course, not all consoles are built this way, so
you do what you can.
Cover the back of the console with a 3-inch soft fiberglass or 3-inch
Sonex. Sonex, a porous and spongy eggcrate-like material is nonreflective
to mid and high frequencies (as is 3-inch soft fiberglass). Cover the
fiberglass or Sonex with grille cloth. If you can't put the fiberglass
or Sonex against the back of the board because of heat dissipation, keep it
a few inches away on standoffs.
As much as possible, keep the outboard gear out of the room. For essential
"in-room" gear, I recommend a "lowboy" (2 feet high) wooden housing
with an equipment mounting front having a 45 to 55 degree angle that deflects
sound upward and away from the engineer (see diagrams above).
Now you're ready to go. The room should have a clean, honest sound, and
the bill should be between $3,500, and $5,000. That investment in acoustic
treatment can save you thousands in buying new processors to compensate for
a room that doesn't sing.