Volume 8, Issue 8 - September 2007

AAMA Analysis

Assessing the Performance of Acoustic Windows
by John Lewis

The health effects on the well-being of the general population due to exposure to lower-level but persistent “background noise” (i.e., noise pollution) has recently become the subject of concern for building designers, especially those allied with the green-building movement. In addition to homes, commercial and institutional buildings, those most affected by noise problems include schools, hospitals, hotels and office buildings.

Even “safe” sound levels can cause non-auditory health problems because noise is a psychosocial stressor that activates the sympathetic and endocrine systems. Unrelenting street noise, jet planes or rail traffic can trigger enough stress in humans to elevate blood pressure and cause muscles to contract. Sustained stress can lead to chronic hypertension, ulcers, indigestion and insomnia.

“The idea that people get used to noise is a myth,” reports the Environmental Protection Association (EPA). “Even when we think we have become accustomed to noise, biological changes still take place inside us.”

Measuring the Noise
Design professionals seeking to shut out the din must consider the noise-reducing features of the building envelope. This naturally focuses on fenestration, where a design adjustment can make a noticeable difference.

But how do we tell if one product is a better noise-insulator than another?

The ability of a fenestration product to attenuate, or reduce, 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.

Sound TL is a standardized measure of the noise reduction in decibels for specific frequency ranges.

But noise is composed of multiple sound frequencies. So, the STC, a single numerical rating of sound transmission, is the most commonly used benchmark. STC ratings are a logarithmic scale similar to the earthquake Richter scale; an increase in STC from 28 to 38 equates to a 90-percent reduction in noise.

STC is determined based on sounds of the frequency range typical of human speech. However, the primary outdoor noise sources—cars, motorcycles, trucks, elevated trains and air traffic—have strong low-frequency content.

The OITC was devised to represent the attenuation of these lower frequencies more accurately.

For the sake of illustration, the table below  indicates typical STC and OITC ratings for commonly-configured double-hung windows.

Building an Acoustic Window
It turns out that improved sound control tends to be an extra benefit of energy-efficient windows, 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). The wider the air space, the greater the sound attenuation. 

  • Glass thickness. This is of limited use as the stiffness of glass limits the improvement. In insulating glass units, using different thicknesses of glass for the layers of double-glazing gives greater noise reduction than using the same thicknesses for both lites.

  • Laminated glass. The plastic interlayer dampens vibrational energy. 

  • Soft resilient gaskets. Sound transmission through cracks around operable windows may reduce their sound attenuating capability drastically; therefore, this type of weather-stripping can improve transmission loss.

An Industry Standard for Sound Transmission Rating
As with any performance parameter, a uniform measurement methodology must be employed to enable evaluation of different products in a fair and uniform manner. To accomplish this, AAMA has just updated its Voluntary Specification for the Acoustical Rating of Windows, Doors and Glazed Wall Sections (AAMA 1801-07), which describes sound transmission loss measurement procedures for fenestration products. The procedure includes the use of sound transmission loss test data obtained per ASTM E1425-91 (1999), Standard Practice for Determining the Acoustical Performance of Exterior Windows and Doors, to calculate Sound Transmission Class (STC) and Outdoor-Indoor Transmission Class (OITC) ratings. Recognizing that air infiltration, operating force and latching force are integral elements of the acoustical performance of the tested unit, AAMA 1801-07 also requires concurrent testing of these parameters.

Windows may be tested optionally and labeled as conforming to AAMA 1801-07 through an acoustic certification program operated by Architectural Testing Inc. in York, Pa. 

AAMA 1801, Voluntary Specification for the Acoustical Rating of Windows, Doors and Glazed Wall Sections
AAMA TIR-A1, Sound Control for Fenestration Products
ASTM E 1425, Standard Practice for Determining the Acoustical Performance of Exterior Windows and Doors 
ASTM E 1408, Standard Test Method for Laboratory Measurement of the Sound Transmission Loss of Door Panels and Door Systems
ASTM E 1332, Standard Classification for Determination of Outdoor-Indoor Transmission Class

Typical STC and OITC Ratings for 
Commonly-configured Double-hung Windows
Configuration  STC*  OITC*
Single-glazed window 24-26 16-20
Double-glazed window 31-33 21-27
Laminated double-glazed  35-38   25-30
Dual window   39-50 26-40
*The above data is representative and for illustrative purposes only. Performance of individual products will vary.

John Lewis is the technical director for the American Architectural Manufacturers Association in Schaumburg, Ill. He may be reached at jlewis@aamanet.org. Mr. Lewis’s opinions are solely his own and not necessarily those of this magazine.


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