|
Early acoustical theaters were just that –
acoustic. The good news and the bad news are
usually the same news when a venue sounds
incredibly good at the start. An auditorium that
projects natural sound well is most often
over-powered and overloaded by modern musical
performances and the line array sound systems
that reinforce them. That’s the bad news in the
good news. This type of good room will need to
be modified to handle high-powered sound from
modern music performances while keeping its
sound-enhancing properties. All efforts can be
directed into the “how” of doing the job when
everyone has heard the “why” it needs to be
done.
Re-engineering
reinforcement – Modifications for “loud”
The hard back wall of the stage is a significant
source for monitor splash-back into the
performers’ microphones, reducing
gain-before-feedback and enhancing opportunities
for system squeals. This is in addition to
promoting timing confusion due to slap-back that
is usually out of sync with the music. This
disturbance and annoyance can be overcome by
using materials from the province of industrial
noise control. A factory finish, that is a
finish for the factory, is also both
“roadie-proof” and “on-the-road” compatible.
Yes, you can take it with you. This allows one
set of materials to follow the performances from
one venue to the next.
Curtain call – Reflecting on the stage
The
industrial curtains called
QFM for
Quilted Fiberglass Materials accomplish
multiple functions:
Bass control from an internal limp mass,
Absorption from
quilted fiberglass, and Resistance to abuse
from a tough vinyl cover.
The covering is thin enough to avoid reduced
effectiveness at all but the highest frequencies
and strong enough to withstand stage and road
wear. Hanging mass (at one time plywood) with an
absorptive cover is a long-standing studio
technique to control low frequencies. The newer,
non-rigid barriers allow a curtain configuration
that is invisible to the audience, while
providing a clean sound source for the both the
performers and the listeners.
Overhead, not overheard - Many are baffled
Above the stage, there is almost always a large
cavity designed for lights and to accommodate
rigging. This space can act as an unintended
echo chamber. Being out of harms way, the area
allows for a lighter and less costly sound
treatment with
acoustical baffles. Besides the obvious
requirement that they work acoustically, they
need only to be invisible (usually black) and
pass the proper fire code. Acoustically, they
have about twice the exposed sound absorbing
surface as a wall-mounted panel, by hanging in
free space. It’s more surface, less
reverberation, out-of-sight and within budget.
They are light enough and small enough to travel
well if strung in a way that allows easy removal
for relocation, such as threaded onto aircraft
cable and hung in a line from side-to-side.
Adding a fabric finish to the baffles, produces
a more decorative product, suitable to the
audience side of the auditorium when a more
permanent
ceiling solution is required.
Stage One – Separating Sound
Stage one of acoustical control often is the
stage. Both on-stage and in-studio
sound
isolation usually begin with structure borne
sound traveling through the floor. It is always
wise to implement isolation between instruments
from the beginning, where it is a “cheap” fix
rather a costly solution. This can be
accomplished by floating the stage surface, and
doing it in several separate sections. As noted
previously with the hanging back of stage
curtains, mass matters. Mass can come from many
different materials whose properties are heavy
and dense. They can be common materials such as
gypsum or sand as well as more acoustically
specific items like sheet lead or
mass
loaded vinyl barrier. (BlockAid®
is a readily available example.) Added mass
damps the damage of vibration and reduces
ringing resonance.
Once the stage
goes “thud” when hit due to its added mass
rather than a cartoonish “boing”, it is time to
handle the hollow space beneath the stage and
fill it with fluffy stuff. This can be whatever
attic insulation that is on sale at the local
home improvement store. It need only trap the
air to prevent its becoming a big bass drum when
stomped upon.
Way back in the
days of Disco (or Disco daze), complications
arose in the studio from the required
“lead-foot” kick drum getting into the
acoustical piano by traveling through the studio
floor as vibration and transmitted up the piano
legs. Although studio floors are usually
isolated from other rooms, they can still
connect within a room. This problem was solved
by floating the drum booth independent of the
common recording studio floor. At that time this
author’s studio went so far as to construct a
sand-filled floor set on nine truck tires. The
sand provided mass and inertia while the tires
created de-coupling from the common structure.
Today it is accomplished with high mass
materials and off-the-shelf
vibration pads,
at about the same cost. Independent and
transportable compact structures can be created
for the individual instruments and be moved with
very little heavy lifting.
After stage
resonance is reduced by adding a layer of
mass
loaded vinyl to its surface and the cavity
below is stuffed with fiberglass to prevent its
ringing or singing along with the music, a
second stage may be layered on top of the
original and floated on
ribbed
neoprene pads every 12 inches along
standard, 16” on-center bracing. This keeps the
guitar amp’s sound out of the vocal microphone
stand, bass drum out of the piano legs, and so
on, to create increased clarity and improved
separation in the live performance.
Islands in the
stage will stop transmission transit and are
relatively cheap to build into the plan.
Separate sections for drums, piano, singer,
bassist and guitar amplifier can be buffered
with half-inch strips of
flexible
resilient neoprene without being seen. Much
like vocals can be modulated when source through
the same speaker as the bass, surfing the bass
wave in the stage floor can also add an
undesirable tremolo (or vibrato) effect to voice
or other wind instrument. (This effect can be
demonstrated by auditioning a vocal through the
bass player’s amplifier while playing.)
Dome details – Round and around
One technique used in early acoustical
performance theaters was the overhead dome. This
feature captured wasted sound energy and focused
it back to the audience to reinforce sonic
energy in areas where it had diminished with
distance from the source. With new systems the
level is electronically reinforced, not needing
further enhancement, which confuses rather than
clarifies. In addition, the dome creates a sonic
racetrack where the sound moves around the edge
in a swirling motion. Anyone who has been in a
domed facility during a thunderstorm has heard
how sound travels around the perimeter. The RCA
dome in Indianapolis provided a good example to
CEDIA attendees a few years ago . This
phenomenon of raceway runaway can be abated with
acoustical “speed bumps” of
Melamine
foam which easily bends to conform to
curves*, keeping the look while truncating the
travel of the fast moving sound waves. In this
case being unfocused is a desirable trait.
To reduce sound
getting into the dome from the line arrays and
the like,
hanging baffles can be placed around the
front half of the perimeter of the ellipse.
These may be fabric covered to blend with the
décor of the audience area and made from
two-inch, seven pound per cubic foot density
acoustical fiberglass to extend its absorption
range. Being hanging baffles they do not
permanently change the original architecture,
where that is a concern.
(Don’t Look) Behind the curtain - Unseen,
Unheard
When acoustical treatments must be essentially
permanent, high efficiency at low cost can be
achieved with utility finishes that can be
field-cut to fit spaces in cavities behind
auditorium side curtains. Factory fit panels
require precise measurements to install within
curves. Field cutting skips this step as it is,
by definition being, done in real time to
as-built measurements rather than made to
out-of-date plans. Savings derived from the
unseen, utilitarian treatments can be applied to
upscale finishes for panels in plain sight.
Balcony bounce-back
Another common problem for an older theater in
the modern world is sound returning from the
balcony face. These are usually concave surfaces
that not only send sound back but focus it for
feedback as well. Convex curves such as
polycylindrical “barrel
shapes or semi-reflective half-round,
hollow traps can control concave
characteristics when interspersed with thicker,
flat acoustical
wall panels to achieve a combined “Flat”
curve.
Definition by Diffusion
Sound intensity can be reduced by the decision
to destroy or
diffuse. Absorption is the destructive
choice, eliminating the problem by eliminating
the sound. Care needs to be taken to use only
what is necessary and no more.
The other
alternative is to spread the sound over a larger
area to reduce intensity. This can be likened to
spreading peanut butter on bread – it becomes
easier to swallow although it is the same
quantity as the original lump from the jar. With
diffusion,
a little goes long way. A single
barrel
shaped diffuser can clear up the cacophony
of a board room without the deadness of
absorption required for the same amount sound
clarification.
Check back After Launch
With venue retrofits, some tweaks can be made
after opening. Covering all walls before there
is an evaluation with performers and audience,
is not always a good idea. While it may be
theoretically possible to model and predict
acoustical performance, it can be more
economical and efficient to get the room in a
reasonable range and polish to the real world
result. An informed conclusion, upon hearing the
room in use, can produce an optimum result.
*Contrary
to popular belief,
acoustical foam can be painted to match
décor without affecting its performance. (The
author has a copy of the independent lab report
comparing painted to unpainted natural. Painted
measured better, but not significantly.)
The Author
Nick Colleran is past-president of SPARS
(Society of Professional Audio Recording
Services), past president of the VPSA (Virginia
Productions Services Association), a former
recording artist and audio engineer.
Starting in 1978,
his company began supplying unique acoustical
materials. Nick now leads a “quiet life” as a
principal of Acoustics First Corporation.
The company holds patents for several innovative
acoustical products.
Acoustics First
designs, manufactures and distributes products
to control sound and eliminate noise for
commercial, residential and industrial uses.
Materials
Mentioned:
Vib-X™
vibration pads |
BlockAid® mass loaded vinyl noise barrier |
Stratiquilt® quilted industrial blankets |
Cloudscape® Baffles hanging acoustical baffles
| Sonora®
acoustical wall and ceiling panels | Select
Sound™ black fiberglass board |
Geometrix™ half-round broadband absorbers |
