Sound Transmission Class
(or STC) is an integer rating of how well a building
partition attenuates airborne sound. In the USA, it is
widely used to rate interior partitions, ceilings/floors,
doors, windows and exterior wall configurations (see ASTM
International Classification E413 and E90). Outside the USA,
the Sound Reduction Index (SRI) ISO standard is used.
The STC number is derived
from sound attenuation values tested at sixteen standard
frequencies from 125 Hz to 4000 Hz. These transmission-loss
values are then plotted on a sound pressure level graph and
the resulting curve is compared to a standard reference
contour.
Acoustical engineers fit these values to the
appropriate TL Curve (or Transmission Loss)
to determine an STC rating. The measurement is accurate for
speech sounds but less so for amplified music, mechanical
equipment noise, transportation noise or any sound with
substantial low-frequency energy below 125 Hz.
Outdoor-Indoor Transmission Class (OITC) is a
standard used for indicating the rate of transmission of
sound between outdoor and indoor spaces in a structure that
considers frequencies down to 80 Hz (Aircraft/Rail/Truck
traffic) and is weighted more to lower frequencies.
STC is roughly the decibel
reduction in noise a partition can provide, abbreviated
'dB'. The dB scale is a logarithmic one and the human ear
perceives a 10dB reduction in sound as roughly halving the
volume - a 40 dB noise subjectively seems half as loud as a
50 dB one.
If an 80dB sound on one side of a
wall/floor/ceiling is reduced to 50dB on the other side,
that partition is said to have an STC of 30.
This number
does not apply across the range of frequencies, since the STC value is derived from a curve-fit of many datapoints.
Any partition will have less TL at lower frequencies. For
example, a wall with an STC of 30 may provide over 40dB of
attenuation at 3000 Hz but only 10dB of attenuation at
125 Hz.
Typical interior walls in
homes (2 sheets of 1/2" drywall on a wood stud frame) have
an STC of about 33. When asked to rate their acoustical
performance, people often describe these walls as "paper
thin". They offer little in the way of privacy. Adding
absorptive insulation (i.e. fiberglass batts) in the wall
cavity increases the STC to 36-39, depending on stud and
screw spacing. Doubling up the drywall in addition to
insulation can yield STC 41-45, provided the wall gaps and
penetrations are sealed properly.
Note that doubling the mass
of a partition does not double the STC. Doubling the mass
(going from two total sheets of drywall to four, for
instance) typically adds 5-6 points to the STC. Breaking the
vibration paths by decoupling the panels from each other
will increase transmission loss much more effectively than
simply adding more and more mass to a monolithic
wall/floor/ceiling assembly.
Structurally decoupling the
drywall panels from each other (by using resilient channels,
steel studs, a staggered-stud wall, or a double stud wall)
can yield an STC as high as 63 or more for a double stud
wall (see table below), with good low-frequency transmission
loss as well. Compared to the baseline wall of STC 33, an
STC 63 wall will transmit only 1/1000 as much sound energy,
seem 88 percent quieter and will render most frequencies
inaudible.
To get to
STC 68 it is necessary to have essentially two walls
back-to-back with an air gap between them, fiberglass
insulation between the studs and a double-layer of
5/8-inch sheet rock on each face.
Due to their high density,
concrete and concrete block walls have good TL values (STC's
in the 40s and 50s for 4-8" thickness) but their weight,
added complexity of construction and poor thermal insulation
tend to be limitations.
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It must be noted that
acoustical performance values such as STC are measured in
specially constructed acoustical chambers and field
conditions such as lack of adequate sealing, outlet boxes,
back-to-back electrical boxes, medicine cabinets, flanking
paths and structure-borne sound can diminish acoustical
performance. The as-built 'field-STC' (FSTC) is usually
lower than the laboratory-measured STC.
Section 1207 of
International Building Code 2006 states that separation
between dwelling units and between dwelling units and public
and service areas must achieve STC 50 (STC 45 if field
tested) for both airborne and structure borne. However, not
all jurisdictions use the IBC 2006 for their building or
municipal code.
In jurisdictions where IBC 2006 is used,
this requirement may not apply to all dwelling units. For
example, a building conversion may not need to meet this
rating for all walls.
In serious cases (for
instance, a bedroom adjacent to a home theater room, and an
inconsiderate nocturnal neighbor, to boot) a partition to
reduce sounds from high-powered home theater or stereo
should ideally be STC 70 or greater, and show good
attenuation at low frequencies. An STC 70 wall can require
detailed design and construction and can be easily
compromised by 'flanking noise', sound traveling around the
partition through the contiguous frame of the structure,
thus reducing the STC significantly. STC 65 to 70 walls are
often designed into luxury multifamily units, dedicated home
theaters, and high end hotels.
The demanding THX reference
standard (a guideline for high-quality audio in movie
soundtracks) requires partitions to achieve 50dB of
attenuation at 63 Hz. Few walls can meet that, as that
requires a wall with an STC of 80 or higher. For all
practical purposes, no sound will be heard on the other side
of the wall with this level of construction.
