Defined as the movement of sound, acoustics are a critical component of any building design.  The importance of acoustics is demonstrated by the fact that 65 percent of office workers surveyed said that workplace noise impacted their ability to complete their work accurately and efficiently.¹ Further, 44 percent stated that noise in their workplace negatively impacted their overall well-being, and more than 40 percent said the noise made them feel stressed. This was one of many surveys investigating the effects of poor acoustics in assorted business types and settings, and it demonstrated the importance of selecting the right materials, products, and systems, as well as well-sealed building enclosures, to optimize acoustics, which is growing in importance.

This article addresses noise reduction in the interior environment through the use of materials and assemblies that reduce the transmission of exterior noise through fenestration in the building enclosure.

AMAA standard

A source of guidance and education for specification professionals, architects, fenestration designers, and building owners, the Fenestration Glazing Industry Alliance (FGIA) is currently updating its AAMA TIR-A1, Acoustical Performance of Exterior Fenestration Products.² The new document is anticipated in 2026. This report walks professionals through key sound and noise definitions and values, explains how sound travels through and around windows, skylights, and other exterior glazed assemblies, and how it is impacted by materials, products, exposure, and building layout. The goal is to familiarize professionals with key concepts for understanding and measuring sound so they can speak intelligently about the topic.

Ultimately, the guideline is intended to support the specification of higher-performing acoustical products for glazing, windows, curtain wall, and storefront systems.

STC versus OITC

Key to this discussion of acoustics is clarifying and differentiating between the two methods of product classification, Sound Transmission Class (STC) and Outdoor Indoor Transmission Class (OITC). Most architects are familiar with STC, which measures the performance of building construction by how sound is transmitted through interior walls and floors. This article discusses the importance of OITC as a measure of the performance of the building enclosure in terms of sound transmission through exterior walls and roofs.

The technical STC and OITC definitions per AAMA TIR-A1 are as follows:

• STC: “A single number rating, that is calculated using the ASTM³ E413 Classification for Rating the Sound Insulation characteristics of interior wall and floor partitions that are exposed to noise typical of offices and buildings (e.g., speech, radio, television, etc.). An STC contour curve is applied to the actual measured one-third octave band transmission loss data at frequencies from 125 to 4000 Hertz (Hz [cycles per second]). The transmission loss value on the contour curve at 500 Hz is the STC single number rating.”
• OITC: “A single number rating calculated using ASTM E1332 [Standard Classification for Rating Outdoor-Indoor Sound Attenuation] to evaluate the transmission loss of facade element, when they are exposed to transportation noise (planes, trains, and automobiles). To obtain the OITC rating, a transportation spectrum and logarithmic summation is applied to the transmission loss data in the one-third-octave band frequencies from 80 to 5000 Hz.”

STC is typically used to evaluate interior partitions, including interior doors and windows, as well as floor/ceiling assemblies. However, when specifying exterior fenestration products such as windows, doors, curtain wall, framing, and glazing, OITC is the relevant metric, as it defines the fenestration’s ability to block exterior noise from entering a space.

It is important to note that the Noise Reduction Coefficient (NRC) is a separate acoustical measure that evaluates the ability of construction materials and assemblies to reflect or absorb sound that originates within an interior space, whereas STC and OITC measure how much external noise is transmitted through its enclosure. For example, office design teams use NRC when recommending and specifying furniture for its sound-absorbing characteristics. As such, NRC also has an impact on the noise experienced within interior spaces, but it is not discussed in this article.

Ratings and calculations

AAMA TIR-A1 Section 2 is where specifiers will find details on how to understand and calculate acoustic values.

STC ratings

Since the Sound Transmission Class (STC) remains the most widely recognized and commonly referenced acoustic rating in North America, despite being frequently misapplied to exterior assemblies, it is useful to establish its basis and limitations before comparing it to other metrics, such as OITC.

Breaking down the origins of the number rating systems, the STC classification system used in North America was first introduced in 1970 by ASTM E413, Classification for Rating Sound Insulation. The metric is based on the amount of attenuation required to reduce each octave-based level of a somewhat arbitrary “standard household noise” spectrum, which includes noise emanating from sources, such as speech, radio, television, a vacuum cleaner, or air conditioner.

The number classification system makes it easy for users to evaluate and compare the acoustical performance with higher numbers denoting better acoustics.

As explained in ASTM E413:

These single-number ratings correlate in a general way with subjective impressions of sound transmission for speech, radio, television, and similar sources of noise in offices and buildings. This classification method is not appropriate for sound sources with spectra significantly different from those sources listed above. Such sources include machinery, industrial processes, bowling allies [sic], power transformers, musical instruments, many music systems, and transportation noises such as motor vehicles, aircraft and trains. For these sources, accurate assessment of sound transmission requires a detailed analysis in frequency bands. A single-number sound transmission rating for building facade elements is given in Classification E1332.

STC classification is considered adequate for assigning numbers to products with acceptable accuracy when the incident sound is broadband and dominated by mid- and high-frequency sound energy, i.e. 500 Hz and greater. STC numbers for products also work well with broadband sounds with somewhat lower-frequency characteristics, such as those of automobiles, trucks, and aircraft, if the Transmission Loss (TL) performance of the product is free of low-frequency “notches.” A “notch” is defined as a significant local weakness in any portion of the TL spectrum.

The STC rating will usually provide the same ranking as the Weighted Sound Reduction Index (Rw) if there are no significant notches in the TL spectrum. The Rw is used for building facades that are exposed to transportation and other airborne noise and covers the frequency range of 100 to 3,150 Hz.

Cases where the STC classification system can be misleading occur when the incident sound is dominated by low-frequency energy—125 Hz and below—as is the case with railway, airport, and highway noise.

Of note, ASTM E413 specifically states that the STC calculation should not be used to measure the acoustics of partitions exposed to outdoor machinery, industrial, and transportation noise, such as motor vehicles, aircraft, and trains.

AAMA TIR-A1 Section 2 continues with the procedure of converting STL data to an STC single rating number as described in ASTM E413.

To determine the STC for an acoustical barrier, the STL is recorded for a series of 16 frequency bands. Each band encompasses one-third of an octave over the range of 125 to 4,000 Hz per ASTM E90, Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements.

As noted, STC is a well-known value and more commonly referenced in acoustical literature, building codes, and many governmental regulations.

Enter OITC

To address the different criteria for sound transmission through exterior enclosures, a different metric is needed to measure the level of noise entering from the outside. ASTM developed the single-number OITC rating to evaluate the noise reduction of exterior partitions such as walls, windows, and doors exposed to transportation noise. In other words, the OITC value quantifies a building enclosure’s ability to reduce the perceived loudness of ground and air transportation noise transmitted into buildings.

As stated in AAMA TIR-A1 Section 2.2:

With the majority of windows installed in exterior walls, the OITC rating is the logical single number rating used to rate their acoustical performance. As acoustical data has been made available and the acoustical community (architects, consultants, etc.) becomes more aware of its existence, OITC has replaced STC as the preferred single number rating for acoustical performance of exterior windows, doors and wall systems.

OITC testing evaluates low-frequency incident sounds and is suitable for comparing the sound isolation performance of building facades. When low-frequency sounds typically associated with outdoor environments, such as vehicular, aircraft, and railway noise, are present to a lesser extent, the advantages of the OITC method over STC are less significant.

For cases where the incident sound band has a narrow range, i.e. tone in place of a broadband noise, neither the OITC nor the STC methods are appropriate. Further, when incident sound is dominated by frequency energy below 63 Hz, OITC is not suitable.

Since the calculation methodologies used to compute STC and OITC values differ, the resulting numbers are not comparable. The OITC rating is calculated for the frequency of 80 to 4,000 Hz, whereas the STC rating only covers 125 to 4,000 Hz.

Further, the OITC rating was developed to address the sound isolation of exterior walls, windows, and doors, while the STC rating targets the sound isolation of interior partitions.

If specifiers are seeking a basis of comparison between the OITC and STC numbers, additional evaluation of the individual one-third octave TL results can be performed by an acoustical professional. This type of comparison may be necessary when available test data is reported as STC, but project requirements specify OITC, due to the longer history and familiarity of STC. The ease of use and applicability of the single-number classification system are often preferred. The more technical computations of the one-third octave band TL data typically will be done on a specific case basis.

Glass selection

When designing glazed facades for acoustic performance, the characteristics of the glazing will have a large influence on sound block levels.

As explained in AAMA TIR-A1 Section 3, the following factors should be evaluated:

• Glass thickness—By increasing the thickness of an insulated glass unit (IGU), the STL will increase, assuming other variables are held constant. IGUs with asymmetrical glass thicknesses also enhance acoustical performance by reducing resonance effects.
• Non-laminated versus laminated—In general, glass has very low inherent damping. With laminated glass, the interlayer provides constrained-layer damping, which improves the TL in a way that can only be achieved through significant increases in glass thickness. Along these lines, special plastic interlayers have been developed to increase glass damping and further enhance TL.
• Cavity space—While average cavity spaces do not affect TL performance, if the space is significantly increased, for example, from 12 to 100 mm (0.5 to 4 in.), this will boost TL ratings by several points.
• Gas filling—While the gas used inside an IGU may affect acoustical performance, the differences between air-filled and argon-filled units is negligible.
• IGU spacer systems—Softer spacer systems, like foam, typically outperform more rigid spacer systems, like metal.
• Size—The larger the glass lite, the more flexible it might be. And the more flexible, the more the glass will vibrate when exposed to a sounds source if resonant frequencies and mass-air-mass coupling are not considered. Consequently, it is possible to consider testing results of different-sized glass modules to estimate the acoustical performance of smaller vs. larger glass lites for a project.
• Vacuum Insulating Glass (VIG)—VIG tends to have excellent acoustic performance, producing higher STC and OITC ratings than monolithic glazing or an equivalent glass-thickness IGU. VIG incorporates a vacuum layer between the glass lites, which acts as a sound barrier by eliminating the medium for sound wave propagation, thereby reducing noise transmission. VIG excels in the low-frequency range, which helps reduce common urban noise, such as traffic and construction. Of note, there are variations in VIG manufacturing techniques and materials, so acoustical testing of the finished fenestration product incorporating VIG is recommended.

Windows and curtain wall

In addition to glazing, the main factors to consider with windows and curtain wall systems are the acoustical characteristics of the framing and the airtightness of the glazed assembly.

Higher air leakage levels may lead to lower acoustical performance, so well-sealed systems are important for enhancing acoustic performance.

Mass and air space can impact the design of framing members because a portion of the incident sound field will strike these areas. Adding cavities or sound-deadening materials between the innermost and outermost frame surfaces can be a strategy for improving acoustical performance of the frame at certain frequencies.

OITC and STC values will vary based on glazing thickness and air space. As a broad generalization, residential windows using relatively small glass lites exhibit an OITC of 19 and up, and an STC of 24 and up.

For example, a smaller residential IGU unit with two 3 mm (0.125 in.) glazed lites separated by 12 mm (0.5 in.) of air space will produce an OITC range of 22 to 26 and an STC range of 26 to 30.

With larger windows, curtain wall, and storefronts using larger lites, exhibit an OITC of 25 or higher and an STC of 29 or higher.

For instance, with an 8 mm (0.3125 in.) and a 6 mm (0.25 in.) glazing lite, sandwiched with a 12 mm (0.5 in.) air space, the OITC range will be between 29 and 34, and the STC range, 32 to 40.

To achieve the highest performance ratings, a dual-glazed window configuration with two sets of sashes, or a prime window with an exterior or interior storm panel, is generally required. The air space between the primary and secondary windows must be at least 25 mm (1 in.).

The cladding design of the non-glazed portions of an exterior wall is one final factor that weighs into the extent to which the building enclosure can keep out sound. If the exterior wall system is poorly designed, in regards to acoustical performance, even the highest quality acoustical window will not keep noise from entering the building. In other words, the composite TL of an exterior facade is based on the TL of the individual aspects of the facade, including walls, curtain wall, storefront, windows, and doors.

It is important to understand this component of building design well before ground is broken to avoid dissatisfaction of building inhabitants. AAMA TIR-A1, Acoustical Performance of Exterior Fenestration Products, in the FGIA online store, supports specification and design professionals with a shared understanding toward achieving their project goals.

Notes

¹ Refer to HR News, “Noise at work is having a negative impact on health and productivity, new survey reveals.”

² See AAMA TIR-A1, Acoustical Performance of Exterior Fenestration Products.

³ Refer to ASTM E413, Classification for Rating Sound Insulation, ASTM E1332, Standard Classification for Rating Outdoor-Indoor Sound Attenuation, and ASTM E90, Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements.

Author

Rich Rinka is the technical manager for fenestration standards and U.S. industry affairs for the Fenestration and Glazing Industry Alliance (FGIA). He began his career with the association in 2012. He oversees the development of fenestration standards and represents FGIA at other industry organizations’ meetings. Rinka has a B.S. in chemistry from the University of Wisconsin, River Falls and an MBA from the Keller Graduate School of Management. He previously served as the association’s certification manager. He can be reached at rrinka@FGIAonline.org.