The basic formula for measuring sound absorption is the Sabine formula, a simple algebraic equation named for the man who developed it a hundred years ago. The Sabine equation is used with the desired reverberation time to yield the necessary absorption for that room. The difference between the necessary absorption and the existing absorption indicates the degree of requirement for acoustical panels. So, with a loud impact noise and a good stopwatch, absorption requirements can be estimated with reasonable accuracy.
There are, of course, several factors that keep this from being too simple. First, if there is already absorption on the wall that will be covered or removed, the panels will not change the acoustical characteristics of the room by their full rated value as they would over hard plaster. Second, if the reverberation is not particularly high and clarity is a problem, the answer may be diffusion of sound, rather than absorption. The difference is that while absorption reduces reverberation by controlling reflected sound, diffusion lowers the perceived sound level by scattering (diffusing) sound over a wide area. Diffusion can help achieve a space that is not too dry for the choir, nor too reverberant for the minister's message. (A full discussion of diffusion is reserved for another article.) A third consideration is that absorption characteristics depend on the sound frequency. Some acoustical problems must be resolved by addressing absorption or diffusion of selected frequencies.
Built-In Acoustics
Before building a new worship space, it is vital
to involve an acoustician along with the
architect. In the acoustics world, there is a
saying: "Architects have vision; but you don't
hear with your eyes." Thinking about acoustics
first rather than last will allow the installed
sound system to be effective with less signal
processing (and the attendant distortion) and at
less cost. It will certainly be more economical
than replacing or upgrading the sound system in
attempts to fix acoustical problems.
For
acoustical panels, the
package often weighs more than the product,
especially with
cellular Melamine (foam) panels.
This can bring about the specter of the dreaded
"dimensional weight," where freight is charged
by volume rather than by weight. By personal
experience, I've found it painful to learn that
someone has shipped a box of lightweight foam by
air at a shipping cost greater than the value of
the product.
Here is a sampling of the
panel choices available:
POP.
The "Plain Old Panel" made of a fabric covering
over a sound-absorbing, high-density fiberglass
board. The
fabric covering is acoustically
transparent. There are other variations with at
least one manufacturer using mineral fiber
(mineral wool) in place of fiberglass. The
fiberglass board is often mistakenly called
"703," the model number for one supplier's
original acoustical fiberglass. The original
"703" fiberglass boards were of lower density
than commonly used today. Most fiberglass board
used in acoustical panels is six to seven pounds
per cubic foot.
Flat diffusion panel.
POP with a membrane below the fabric surface.
The membrane has a pattern of holes that varies
reflection and absorption over the panel
surface. The result is a scattering effect of
the sound. This is an interesting item for a
confined space but less necessary for a
sanctuary where a traditional diffuser can be
used to greater advantage for sound and, with
custom wood finishes, for appearance as well.
Barrier sandwich. Two
POPs with a layer of
heavy vinyl in between to
block sound passing through the acoustical
panels. (Absorbing panels prevent sound
reflection but do little to block penetration
through walls from room to room or inside to
outside.) This item has been used to replace
composite foams in machinery enclosures. The
barrier sandwich can be made into a removable
insert for framed glass partitions to provide
additional isolation when conflicting events are
on opposite sides of the window. For permanent
installations, standard panels with separately
supplied
barrier may be more effective for sound
and budget.
High impact surface. POP
with a 1/8-inch layer of high-density fiberglass
below the fabric. These are particularly useful
in fellowship halls or gymnasium applications to
avoid the craters created by wayward basketballs
in softer fiberglass surfaces. A side effect of
the high impact surface is its improved
absorption in the lower frequencies, more than
twice the absorption at 125 Hz for a one-inch
panel.
Tackable. Handy for the church classroom, tackable panels allow pushpins without destroying the underlying acoustical material. Of course, too many hard hanging items will block the acoustical effect or, at least erode high frequency performance.
Cellular Melamine. The "foam" alternative: a white foam-like product that has become popular as a substrate for acoustical panels. Cellular Melamine should not be confused with polyurethane foam, which is restricted in use by fire codes. When selecting fabric cover colors, keep in mind that cellular Melamine is very white and will affect the perceived fabric color. It also conforms easily to curves, with the ability to bend around curved surfaces without the need to "score" the back surface.Panels with images. Custom or stock images printed on acoustically transparent fabric. Recent improvements in computer printing and acoustically transparent fabrics allow hi-resolution photographs to be used in addition to artwork and graphics printed by conventional means.
Stretch wall. Raw acoustical panels are mounted to the wall, then fabric is stretched over the panels. These custom on-site installations are useful for covering contoured or irregular surfaces.Full-, half- and quarter-round. A flat panel made to conform to a curve. By creating a trapped air cavity of varied depth behind a one-inch panel, absorption is extended well into the bass frequencies. One manufacturer patented a device that is tunable by the addition of a membrane over half of the surface. These devices are useful where low frequency energy is a problem and panels four-inches thick or greater are impractical.
Substrate and Fabric ChoicesThe color of the substrate is important because it can "tint" the color of the fabric covering. For example, a yellow substrate can produce a desirable vintage off-white or be totally wrong for a room that needs a white as bright as that of a toothpaste ad. It is always best to get a sample of the recommended acoustical panel in advance. To avoid having the substrate color enhance or discolor the finish, a panel may be under-wrapped with a scrim material to block the color below the fabric. Scrims are usually white but are available in black when that produces a better effect as with an image panel.
Acoustical wall panels can usually be made to blend with the sanctuary interior using the 48 standard colors available to all panel manufacturers. Custom fabrics can be used but usually at additional cost. When choosing custom fabrics, make sure they are acoustically transparent or have known sound absorbing properties of their own. Also make sure the fabric is made of synthetic fibers so it will not be affected by moisture and temperature changes. Fabrics should also be visually non-directional: color and texture should appear the same from all angles.Some manufacturers make panels to custom shapes and sizes. Again, this will usually involve extra costs. Panels have been made in the shapes of wild animals, spaceship interiors, corporate logos, art deco theater shapes, and much more. Conforming an acoustical panel to the curves of a church window is possible. However, as with all custom work, accurate drawings and descriptions are essential.
Advanced planning for acoustics (before building) can save much time, effort, and money later. Acoustical panels can be part of a planned sound-control strategy for a new building, or as solutions to sound problems in existing rooms. With the principles of acoustics well developed over the past 100 years, it is possible to predict acoustic results and costs in advance. There is every reason to expect to get it right the first time.Nick Colleran is a member of the Acoustical Society of America, past president of the Society of Professional Audio Recording Studios (SPARS), and former president of the Virginia Production Services Association (VPSA). Nick is currently active in acoustical design for houses of worship, new acoustical products and performance venues.
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