Ensuring acoustic performance of windows and curtain walls

Outdoor/indoor transmission class
The OITC rating was devised to more accurately quantify the amount of a reference exterior sound signal attenuated by a given partition. The reference sound approximates typical noise sources commonly occurring in urban areas. The OITC rating (referenced in ASTM E1332, Standard Classification for Determination of Outdoor/Indoor Transmission Class) is calculated from the following formula, using a logarithmic summation of individual third OB TL measurements:

OITC = 100.14 − 10log10åf10((Lf-TLf+Af)/10)

Lf = reference source spectrum level at each third octave band frequency;
Af = A-weighting adjustment at each third octave band frequency; and
TLf = specimen TL at each third octave band frequency.

San Francisco International Airport and the Federal Aviation Administration’s (FAA) first-of-its-kind airport traffic control tower and integrated facility building relies on high-performance curtain wall and window systems to meet acoustic requirements near an active runway. The new $80-million, 67-m (221-ft), torch-shaped tower’s seismic design allows the structure to withstand a Richter magnitude 8 earthquake. The team included design-builder Hensel Phelps Construction Co., with Fentress Architects, T2 Partners, and HNTB Corporation.
Photo courtesy Wausau Window and Wall Systems

There is no way to convert from OITC to STC (or vice versa), without access to the underlying TL data. OITC is predominantly a measure of low-frequency attenuation, as low-frequency sounds are generally the more prevalent in typical urban environments. In one typical test on high-performance laminated insulating glass with a makeup of 8-mm (5/16-in.) exterior, 25-mm (1-in.) air space, and 11-mm (7/16-in.) laminated inboard, 85 percent of the total sound pressure transmitted was less than or equal to 200 Hz, as derived from OITC calculations per ASTM E1332. (See Element Materials Technology’s test report, ESP017522P-5.)

Test repeatability and performance
Test-to-test and lab-to-lab results for STC and OITC of similar specimens can vary by as much as three dBA, due to inconsistent installation practices, varying temperature and humidity, size differences, and aspect ratio changes. Attachments and sealants used around the perimeter can provide unrepresentative damping. Both the specimen size and the glass-to-frame ratio also affect the sound transmission loss. Assessment of incremental improvements must be made in the context of test-to-test variation, which often ‘masks’ small changes in TL resulting from design changes. (Field test procedures for sound transmission also exist, and the field results can vary from the laboratory results by as much as five points due to installation accessories, sound flanking though adjacent building elements, and interior noise sources. Most project specifications allow for a difference between the lab and field test results. ‘Flanking’ refers to sound transmission through areas outside the glazed area being tested, and can be caused by excess air infiltration, or lower performance of spandrel areas or adjacent wall construction.)

Many laminated glass fabricators publish acoustic data on ‘glass-only’ prototypes, based on testing loosely supported lites without framing. While somewhat useful for glass-to-glass comparison purposes, it is not recommended to use such results in project specifications, as any rigidly framed system tests significantly lower.

Comparable glass-only acoustic data was available for 26 of the 80 acoustic test reports found in one manufacturer’s library of acoustic test reports.

In only a handful of cases were the glass-only results the same as the whole-window results. In typical cases, where glass-only results were better than whole window results, the difference in STC ranged from 1 to 4 dBA. The difference in OITC ranged from 1 to 5 dBA. There was no discernible pattern to the differences when comparing higher-and lower-performing systems.

The significant variance associated with this data means whole-window STC or OITC cannot be systematically calculated or predicted from published glass-only data by the use of an adjustment factor to account for the framing. Subsequent test results could vary by 5 dBA or more, resulting in the need for significantly more expensive glass to meet requirements, or the perception of costly over-design.

There are a number of relatively cost-effective means of improving acoustic performance of conventional window and curtain walls. Specifiers should identify the specific OBs needing improvement before deciding on a design strategy.

Bigger is not better
Large glass panels can vibrate at higher amplitude than their smaller counterparts, causing a dip in the TL at the natural frequency of the glass. Square lites with an aspect ratio (the ratio of height-to-width) close to 1.0 are more prone to resonance than rectangular lites with aspect ratios of 1.5 or greater. Results of previously tested glass-frame combinations should be reviewed in that context. Additional framing members added to reduce glass surface area may be sufficient to improve acoustic performance to targeted levels. The effects of size and aspect ratio are less pronounced when laminated glass is used.

Most standard product acoustic tests are run at the standard sizes cited in ASTM E1425 Standard Practice for Determining the Acoustical Performance of Windows, Doors, Skylight, and Glazed Wall Systems, which calls for:

  • window test specimens neither less than 1.9 m2 (20 sf) nor more than 2.2 m2 (24 sf), with neither dimension less than 1070 mm (42 in.);
  • single-door test specimens neither less than 1.8 m2 (19 sf) nor more than 2 m2 (22 sf), with neither dimension being less than 910 mm (36 in.); and
  • double-door specimens neither less than 3.5 m2 (38 sf) nor more than 4 m (44 sf).

There is no category in ASTM E1425 for window wall, storefront, or curtain wall systems.

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