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When treating a performance
space, auditorium, or church sanctuary, there
are several considerations. Some are often
obvious, but nevertheless often ignored.
What’s going on?
First among these is the type
of performance. This can range from natural
acoustic to high powered reinforced electric. Is
it music that is traditional or a cappella? Is
it to be speech only, music only, or a
compromise of acoustical ambiance to allow both?
One size doesn’t fit all. A well-designed
theater from the vaudeville era will project an
un-amplified performance but is easily
overwhelmed and acoustically overloaded in the
age of today’s high-powered line arrays. Sound
that did not reach to back wall, back in the
day, now hits it with storm force producing
annoying slap-back to interfere with the
musician’s timing and the speaker’s
concentration. If an existing space is to be a
natural acoustic venue, little treatment and
less sound reinforcement will be necessary.
What’s the
problem?
Next is there a sound problem
or a noise problem, with noise being the
sub-category of sound that is unwanted?
If there is a noise issue, is it noise
coming into the auditorium or leaving it to
disturb neighbors or others in the building?
Higher-pitched noise, will generally be less
intense, with lower energy content, and more
easily controlled with dense, heavy barrier
materials such as gypsum board and
mass loaded vinyl layers, Blockaid® being
one commercial example. Low frequencies have
longer wavelengths
and generally have more power. The longer
wavelengths take a greater distance to fully
develop and may explain why the neighbors
perceive a sound as louder than the folks close
to the source. Low frequencies tend to be
structure borne and usually require decoupling
from the source with resilient isolation. This
treatment is lower cost if implemented at the
start of construction where it is easier to
split a concrete slab or
float a floor on pads.
Is compromise
required?
The discussion of noise
pollution, especially the long waves is also a
long discussion, best reserved for a more
in-depth article. The primary consideration for
an auditorium is almost always its sound
quality. Again sound quality is not the same for
every source or every listener. A typical space
will sound best for spoken word if the
reverberation time, RT60, is between 0.90 second
and 1.00 second. (Reverberation time (RT60) is
the interval between when the sound is made and
its decay becomes inaudible, 60 dB down.)
Traditional music may want to hear the
reverberation in the same room at between 1.5
seconds and 1.6 seconds, while electronic music
may want to be “dry” with no reverberation or
echo other than what is added artificially by
the sound engineer. These situations require
compromise at perhaps 1.25 seconds for a
traditional performance while electronic music
allows speech to be clear by using a dry room
with artificial ambiance added to taste for the
music.
People count
Acoustical taming of a
performance space is most often achieved by
adding
sound absorption to room surfaces, usually
on walls and ceiling. Some added sound control
can be accomplished with padded seats and carpet
on the floors. In general, the thicker the
material, the more absorption is achieved
provided it is fluffy and porous. When adding
material, it should be noted that the difference
in sound absorption may not equal the
specifications of the product, depending upon
the surface being covered. The full rating of an
acoustical material could be “as advertised”
over rigid gypsum drywall but may only equal the
difference in absorption between an existing
acoustical surface and the added acoustical
panels, for example, if the existing walls are
porous or covered with materials already
providing some acoustical properties. People
count in this cover-up as well. Padded seating
will add to the acoustical absorption of an
empty room and allow the space to be similar in
sound with a full or partial audience. However,
if the seating is hard, the acoustics will
change as the seats are filled with people. The
ambiance will dry up drastically in winter, when
those attending wear heavy, fluffy coats.
Failure to account for this has produced
acoustical difficulties in some well known
venues (which will remain nameless in this
article).
Form, function, fire
and finish
While symmetry of acoustics
left-to-right is desirable, even multiples in
the stepped increments of a theater’s rising
listener area may not. The ancient outdoor
amphitheaters were not spaced on even multiples.
At least one recent outdoor “bowl” has uniform
math and equally uniform bad sound due to the
commonality of wavelengths, reinforcing or
canceling the same frequencies, rather than
disbursing them over the full range of hearing.
Good looks can have, but do not necessarily
produce, good acoustics.
This leads into the topic of “form
function, fire and finish”. If form is the
principal consideration, good looks can lead to
trouble and are almost always the path to
greater expense, if acoustical function is not
an equal consideration. Both may be achieved if
given equal weight at the start. The sound
absorption function alone, can be achieved with
a bale of hay, but there are allergy and fire
considerations. Once fire specs are met, the
final consideration driving price will be
finish. A $20 panel can meet these minimum
requirements and perform as well as the panel
costing six times as much.
Last is
first
Sound absorption placed anywhere open to the
air in a room will reduce reverberation time.
Acoustical panels have been mounted under
seating in places where there is not enough open
wall area. However, absorption lowering
reverberation alone will not solve all problems.
Once reverb is reduced, annoying direct
reflections are more easily heard. The most
troublesome of these reflections are those
bouncing back from the rear wall that are out of
time sequence with the musicians and cause the
person speaking to repeat himself involuntarily,
reducing intelligibility and increasing
distraction from the subject of the
presentation. Following second is treating the
stage wall, if there are floor monitors aimed
toward it, and the sound reflects to interfere
with the main source. This can confuse the
performers and be more annoying to the audience
listening.
Science to art
Beyond controlling and
containing sound is the shaping of it. This is
achieved by spreading sound uniformly with
diffusion that lowers intensity without
eliminating the sound by absorption. Boom can be
controlled with
bass trapping that requires size and depth
for low frequency control. Once the sound is
tamed, it can be trained by these methods, but
that is an art requiring a more in-depth
discussion exceeding the space of this article.
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