The ability of a fenestration product to attenuate the transmission
of sound is measured in several ways: Transmission Loss (TL), Sound Transmission
Class (STC) and Outdoor-Indoor Transmission Class (OITC). In all cases,
the higher the number, the more the intruding sound is blocked.
TL is a standardized measure of noise reduction in decibels for specific
frequency ranges. Noise is composed of multiple sound frequencies, so
the STC, a single numerical rating, is the most commonly used metric for
indoor partitions and acoustic wall treatments. STC is based on sounds
in the frequency range typical of human speech. Although this is generally
a satisfactory way to rank the ability of building envelope components
to reduce noise from sources such as neighbors’ conversations, it does
not properly rate insulation against noise from the typical outdoor urban
To cover this deficiency, the OITC was devised to represent the attenuation
of noise from exterior sources more accurately. It is based on a spectrum
weighted more to lower frequencies typical of the noise produced by cars,
motorcycles, trucks, elevated trains and air traffic.
How much the intrusive outdoor sound is blocked depends on a window’s
shape, its size relative to the wavelength of the sound, the glass-to-frame
ratio and the way in which the glass is mounted in the sash or frame.
Improved sound control tends to be an extra benefit of energy-efficient
window design, as the features that reduce thermal conductivity and air
infiltration also tend to reduce sound transmission.
These features include (but are not limited to):
• Double glazing (insulating glass units). Generally,
the wider the air space between the inner and outer glazing, the greater
the sound attenuation. However, there is a point of diminishing returns.
Despite the intuitive belief that adding another layer of glass must be
beneficial, triple glazing provides little additional improvement over
double glazing, unless the interlayer separation is very large.
• Laminated glass. The plastic interlayer dampens
vibrational energy; however, the degree of attenuation is somewhat temperature
dependent; cold winter weather can diminish this benefit.
• Sealants and weatherstripping. Sound transmission
through operating joints around windows may drastically reduce their sound
attenuating capability, especially at higher frequencies.
• Robust framing. Strong, stiff frames resist vibration.
Those with engineered profiles consisting of open spaces help reduce noise
transmission in much the same way as double glazing.
• Glass thickness. Though of limited value, the use
of different glass thicknesses in insulating glazing units has proven
to provide greater sound control than using the same thickness for both
lites, due to the different frequency response. Thicker glass on the outdoor
side is said to work best.
Industry Standards for Sound Control
As with any performance parameter, a reliable methodology must be employed
to obtain uniform and comparable measurements that enable evaluation of
different products in a fair and consistent manner. To accomplish this,
AAMA recently updated its Voluntary Specification for the Acoustical Rating
of Windows, Doors and Glazed Wall Sections (AAMA 1801-13), which describes
sound transmission loss measurement procedures for fenestration products.
AAMA TIR A1, Sound Control for Fenestration Products, offers additional
information on the characteristics of sound, how it is transmitted, how
it is measured and how its transmission can be controlled and quantified
using STC and OITC indices.
Dean Lewis is the educational and technical information
manager for the American Architectural Manufacturers Association in Schaumburg,
Architects' Guide to Glass & Metal
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