Welcome to part
two of our
look at audio
control room acoustics
and the reasons we expend
so much effort to
build great sounding
rooms. In part one we
looked at how low frequencies and reflections
behave in both large and small rooms.
This time we’re going to take a high-level
look at the topic of isolation and see that
it is the foundation upon which audio control
rooms are built.
Isolation is primarily concerned with
making rooms quiet through noise control
and it depends mostly on mass. It’s also the
area of control room design that gets the
most scrutiny when a project is proposed
because proper isolation of some spaces
can be costly. The noisier the environment,
the costlier and more invasive it will be
to properly isolate the room, but without
proper isolation the mix engineer may not be able to tell whether what he/she is hearing
is a part of the mix or part of the environment.
There are certain situations, such as television
mobile units, where there is little
that can be done because the environment
is always going to be noisy. Similarly, some
live audio mix rooms will have high levels of acoustic noise, not entirely
from the equipment, but
from the communications
that are necessary during
a live event. Mix engineers
for live sporting events are
seemingly bombarded with
noise from every direction,
but they still need the ability
to turn it all down, tune people out, and focus on listening if audio
issues crop up.
External noise sources include not just
sounds made acoustically, but also those
transmitted through floors, walls, ceilings
and mechanical systems. To properly isolate
the audio control room, noise from all of
these external sources must be minimized or eliminated. However, isolation isn’t just
about keeping noise out of the control
room; it’s also about minimizing leakage
into adjoining spaces. This is especially important
when other production spaces are
adjacent or nearby because the sound leaking
out of the audio control room could interfere
with work in other rooms.
High-end audio control rooms are designed
with isolation in mind and are perfect
studies in noise control. These rooms
are actually built as a room within a room,
with the outer room’s floor and thick walls
connected to the building, and an inner
room that is separated by an air cavity. This
room-within-a-room design is excellent for
isolation and noise control, but requires
more physical space, so they are rare in today’s
television production facilities where
space is always at a premium.
There are also hefty construction requirements.
The inner room sits on a
floating floor designed to eliminate noise
transmission into the room through the
building’s structure and requires pouring
two floors, one on top of the other, with a
layer of isolating materials between them.
The solid ceilings of inner rooms are suspended
from the outer ceiling by isolators and inner room walls are attached only to
the floor and ceiling of the inner room.
|Floating floor levels
This method of construction isolates
the outer and inner room from each other
because most surfaces are physically separated,
and some form of isolator separates
surfaces that must meet. Of course cable
troughs and pipes do penetrate the space,
but they include insulators to help avoid
bridging the rooms.
The ultimate goal of this design is to
achieve minimal leakage of sound through
the control room’s structure, and an acoustically
quiet space with a low-noise floor
(around 30 dBA for a quiet control room).
Construction documents call for walls in
audio spaces to target a sound transmission
class (STC) rating, which rates sound transmission
loss through walls, doors and windows.
STC ratings are based on frequencies
concentrated in the range of human speech
(125 Hz to 4 kHz) rather than the full spectrum
of frequencies that will be heard in
the control room. Architects and construction
foremen are familiar with it, and can
purchase materials and components with
this rating, so it’s a good reference to work
Windows and doors in the control room
need to have as close to the same STC rating
as the walls, which means doors will
be heavy and come with special seals and
windows will have multiple panes of glass.
This allows everything to function as a unit,
preventing a similar amount of sound transmission
from the entire room.
Requiring just as much forethought as
the room design is the design of the HVAC
system. The systems that contractors install
every day usually do not work well in audio
control rooms because they tend to have
small rectangular ducts with sharp right angle
bends, a design that creates turbulence
and noise in the system. We want to avoid
either acoustical or mechanical noise from
the HVAC system since we’ve gone through
so much trouble to make the room quiet.
Some of this noise can be reduced by
locating the mechanical components far
away from the control room, installing vibration
isolators where the unit mounts
to the building, or placing the mechanical
components in an acoustical enclosure. The
system should be designed to be high volume,
low velocity to minimize turbulence.
Ducting should be large in diameter, flexible,
round if possible, and some can be
lined with acoustic material if additional
soundproofing is required.
It is also critical, when designing the
control room HVAC system, to make sure
it does not bridge into a noisy space since
there could be transmission from that
space through the ducting.
That wraps up our quick look at isolation.
There’s not nearly enough space in
this magazine to go into it in depth, but I
find exercises like this to be a helpful reminder
of why we expend so much effort
when building audio rooms. In the third
and final installment of this series, we’ll
look at the importance of room size and finish
up with practical tips for working with
small spaces and tiny budgets.
Jay Yeary is not an acoustician, but has
spent many years building audio rooms,
trying to make sure they give mix engineers
the accurate listening environment
they need. He can be reached through TV
Technology or via Twitter at @TVTechJay.