By Steve Haas
To begin the first article in the acoustic series,
I had originally thought that I would dive right into the basics
of room sizing, shape and location for home theaters and other
media rooms. However, my trip to the recent CEDIA Expo has
shown me how beneficial it might be to many people to try to
first define all the acoustic lingo and acronyms floating around
so that I can easily refer to these terms in later articles
without someone trying to figure out what makes an NC different
from an STC & NRC. At the same time, I would like to dispel
a few myths and misconceptions about acoustics that have long
been plaguing the industry. So here goes . . .
ACRONYMS
NC (Noise Criteria) (Whole Number >0.
e.g., NC-25)
The NC level of a room is a number rating of
the noise level of an interior space. The NC number is associated
with a series of sound energy level-versus-frequency curves
known as Noise Criterion curves. For new construction, a Noise
Criteria is established based on the room's type and its intended
function, and is used as a goal in the design of sound isolation
construction and the attenuation of mechanical systems noise.
To determine the NC rating of an existing space, octave-band
frequency noise levels are measured and plotted against the
series of NC curve spectra.
In a Home Theater or other similar space, it
is important to have a very low background noise, or NC level,
since this directly affects the dynamic range of the audio.
If your noise level is high, you will have to turn up your
system to hear the quietest passages. Unfortunately, this will
also make your loudest passages even louder, which may be uncomfortable
to your ears and require additional amplifier power and sound
isolation from other rooms.
STC (Sound Transmission
Class) (Whole number >0. e.g., STC-56)
The STC is a single number method of rating the
sound isolation performance of a wall, floor, ceiling, door
or window. Like the NC, the STC number actually refers to an
entire spectrum sound level, in this case, divided into one-third
octave band values. The higher the STC number, the better a
partition isolates sound overall.
STC numbers should be used only as a broad comparison
between two or more partitions. One reason for this is because
the same sound source played through an STC 50 drywall partition
and an STC 50 concrete block partition will sound quite different
on the other side because of the way each type of wall isolates
different frequencies of sound. The one-third octave band levels
(known as Transmission Loss (TL)) are essentially the difference
in level between a sound measured on the same side of a partition
and the levels measured on the opposite side. For a thorough
sound isolation design, the TL numbers should be evaluated
based on the frequency and level content of the source noise
and the specific NC level required in an adjacent space.
NRC (Noise Reduction Coefficient)
0.00 <= NRC <=1.00* e.g., NRC = 0.85)
The NRC value is a single number method of rating
the sound absospecific frequency band if it reflects virtually
all sound at that band. Concrete and brick are examples of
materials with low sound absorption coefficients at all frequencies.
Materials such as thick (4" or greater) fiberglass insulation
and some foams have high absorption coefficients near 1.00
at most frequencies. The NRC rating is defined as the arithmetic
average of the material's measured sound absorption coefficients
at the 250Hz, 500Hz, 1000Hz and 2000 Hz octave bands. These
frequency bands represent the range of sound most associated
with speech.
If the material is required to absorb very low
or high frequencies of sound, the individual sound absorption
coefficients should be used for comparison, rather than the
NRC value. This is very important to note in a Home Theater
where the audio contains a lot of low-frequency sound energy.
You may be tempted to use an acoustical wall panel or ceiling
tile with a high-NRC rating throughout your Theater space.
However, since the NRC doesn't tell you how or if the material
absorbs low-end sound, you could end up with a very bass-heavy
room unless you try to achieve similar amounts of sound absorption
at all frequencies of interest.
* Note that data published for some acoustical
materials may show absorption coefficients greater than 1.00
at one or more frequencies. This is because the effective absorbing
surface area in a thick or shaped material is greater than
the material's face area used to determine the absorption coefficient.
Definitions
dead room: A room containing
a large amount of sound-absorbing material.
diffusion material is a material in a room that causes sound waves impacting
on its surface to be scattered in multiple directions. Examples of diffusive
shapes include convex or splayed walls and ceilings, coffers, columns, pilasters
and very ornate architectural surfaces. Hard furniture and sound-absorbing
panels spaced at intervals along a reflective boundary surface will also add
some diffusion to a room.
flanking path: a path between
adjacent spaces other than through a common partition that
sound or vibration is transferred
flutter echo: a rapid series
of reflections usually created when a sound is played between
two hard and parallel room surfaces. Flutter echo is often
perceived as a "buzzing" or "ringing" sound
and can be detrimental to the clarity or intelligibility of
a sound. Simple solutions for eliminating this occurrence
include: creating an offsetting
angle of at least 5° between the two surfaces, adding sound
absorptive materials to one or both surfaces or adding diffusive
shaping to the surfaces.
live room: a room containing
very little sound absorbing materials.
room modes: Fluctuations in
the energy level of sound dependent on frequency, source position
and listener position in the room. Modes are produced at frequencies
relative to the room's primary dimensions and are caused by
the reinforcement and cancellation of multiple sound waves.
They are often referred to as standing waves, since a sound
produced at the fundamental modal frequency appears to be stagnant
in the peak energy position.
reverberation time: The amount
of time at a specific frequency that a sound in an enclosed
space takes to decrease 60 decibels (basically to inaudibility)
in level after the source sound has stopped. The reverberation
time gives a listener the sense of the size, liveness and warmth
of a room. Reverberation time increases proportionally with
the cubic volume of the room and decreases proportionally with
the quantity of sound-absorbing surfaces in the room. So, unless
a Home Theater is unusually large (which it shouldn't be) or
highly-reflective (which it shouldn't be) there will be very
little sound buildup or decay. Thus, reverberation time is
not a very useful measure for the acoustic quality of a Home
Theater.
source and receiving room: terms
used in sound and vibration isolation analysis to designate
the room containing the sound or vibration producing source
(source room) and an adjacent (receiving) room requiring that
the source noise be attenuated by the intervening partitions
to a specified noise level.
Myths and Misconceptions
M&M #1: Fiberglass or foam
placed on a wall will prevent sound from going through it.
Reality #1: These materials only absorb sound and do not provide a barrier
to it. Heavier building materials and resilient attachments to structure are
the best methods for isolating sound.
M&M #2: Carpet on a floor
will reduce sound transmission to a room below.
Reality #2: Carpet is a sound absorbing material mainly at high frequencies,
and has very little airborne sound isolation properties. Carpet does, however,
reduce the amount of impact sound from footfall or things dropped that is transmitted
to the space below.
M&M #3: Egg cartons on the
wall improve the sound of the space.
Reality #3: While egg cartons do have some sound-absorbing and diffusing properties,
they are concentrated in a relatively narrow frequency band and do not positively
effect the quality of speech or music to any significant degree. They also
have negligible sound isolation properties.
M&M #4: Adding insulation
to a sheetrock wall will keep all sound from going through
it.
Reality #4: Insulation between stud cavities in a sheetrock partition does
improve the sound isolation value of a partition and should be used whenever
possible. The improvement, however, is too small to bring about an appreciable
difference in the degree of isolation, and the insulation should only be thought
of as a partial solution to upgrading the isolation of a partition.
M&M #5: SOUNDPROOFING!!!!
Reality #5: This word is the catch-all phrase used by many for improving anything
that has to do with acoustics. "Soundproofing" implies building
a room that will keep all possible sounds outside the space from transferring
in, and all sounds generated in the space from transferring out. Building
construction can be designed to attenuate a fixed degree of sound, but cannot
theoretically prevent all possible sounds from passing through the boundaries
of the room, except in extremely rare (and expensive!) situations. Better
terminology to use when describing a client's acoustical needs may perhaps
be "Noise Reduction" (for sound isolation) and "Sound Enhancement" (for
room acoustics).
Steven Haas is a member of CEDIA and president
of SH! Acoustics, an independent acoustical consulting firm
specializing in custom and "high-end" residential
acoustics. His experience stems from nearly a decade of designing
over one-hundred concert halls, theaters, recording facilities,
museums, educational arts facilities and corporate offices
throughout the world. Steven can be reached by telephone at
(203) 258-9577, fax at (203) 377-1227 and E-mail at SHAcoustic@aol.com.
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