(JBED) - Summer 2006 - The Whole Building Design Guide

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Feature Architectural Glazing for Sound Isolation (an Acoustician’s Perspective) By Jeffrey L. Fullerton, Acentech, Inc. ABSTRACT For most buildings, glazing is selected for its thermal or optical performance. However, there are numerous buildings where exterior noise impacts are a factor in whether the interior space will function properly. Often, the most important element for reducing intrusive noise is the architectural glazing. This paper discusses the acoustical tests and ratings of glazing systems, various upgrades to architectural glazing and several case studies where improved sound isolation from exterior noise was achieved using architectural glazing. INTRODUCTION: ACOUSTICS 101 Sounds propagate through the air as pressure waves. Normal human ears are sensitive to a wide range of varying sound pressure. To simplify the representation of these acoustical pressure waves, the pressure levels are represented in a logarithmic ratio expressed in decibels, or dB. Human ears are more sensitive to midand higher- pitched sounds (the “treble” component), compared to lower frequency sounds (the “bass” component). This is represented by the so-called A-weighting filter for measuring sound, noted as decibels, A-weighted, or dBA. Since sound levels are reported in logarithmic decibels, they are inherently non-linear. As a result, comparing the loudness of various sounds may not be as simple as it appears. For example, studies of human responses to sound levels have shown that changes of 3 dB are considered a “just noticeable difference” and changes of 5 dB are considered to be “significant”. Changes of approximately 10 dB are perceived as a “doubling” of the sound level. A 20 dB change is perceived as being “four times louder”. To provide some perspective on various sound levels, the following environments or sound sources are described below. Rural Neighborhood: 30 to 40 dBA Urban Neighborhood: 40 to 50 dBA Speech (at 3ft): 55 to 80 dBA Traffic (at 100ft): 65 to 80 dBA Aircraft Overflights: 65 to 110 dBA ACOUSTICAL PERFORMANCE TESTING AND RATINGS The two primary types of acoustical tests for exterior glazing are either conducted in 1) a laboratory; or 2) in an installed condition, referred to as a field test. Based on these two types of tests, there are two ratings for quantifying the acoustical performance of glazing systems. 1) Laboratory testing Laboratory testing is intended to provide a standardized and repeatable test of a building construction relating to its performance for sound transmission. The tests are standardized by ASTM E90-04, where all of the details regarding the laboratory, the test specimen, the test protocol, test conditions and measurement procedures are specified. The thoroughness of the standards is intended to pro- 26 Journal of Building Enclosure Design
vide test results that are comparable and repeatable in any laboratory that meets the ASTM standards. A list of laboratories accredited by the National Voluntary Laboratory Accreditation Program is available at the National Institute of Standards and Technology website: www.nist.gov. There are a number of strengths and potential weaknesses to the results gathered from laboratory testing. It is important to understand that laboratory tests are generally conducted on a particular element of a building construction, rather than an entire composite construction. For example, manufacturers often test individual elements such as windows or doors in order to demonstrate their product’s specific sound transmission performance. While it is often useful to know the relative performance of one product versus another, it is often more important to relate that performance to the other elements of the façade in order to understand the performance of the composite construction. Also, keep in mind that laboratory tests are conducted under ideal conditions. Accordingly, the results from laboratory testing are generally considered to be the ideal performance of that product, and may not be as achievable when installed by a typical contracting crew. These considerations should be considered when applying laboratory test data to a specific project. 2) Field testing Field-testing provides a means of conducting a test of an actual, installed construction. In most cases, these tests measure the performance of the entire façade rather than a particular element in the façade, though this is also addressed in the field test standard methodology. The field tests are standardized by ASTM E966-04, where all of the details regarding the installation, sound source, test protocol, test conditions and measurements are specified. The standard is intended to provide a methodology for comparing the test results to laboratory results or other field tests that are conducted to the same standard. The most significant strength to field testing is how the installed product performed. Depending on the quality of installation of the test specimen, the performance may closely represent what might be expected for a typical installation of that particular composite construction. ACOUSTICAL PERFORMANCE RATINGS The results from the sound transmission tests can be reduced to several single number ratings. The most commonly used rating is the Sound Transmission Class (STC), determined by ASTM E413-04. This rating is determined by comparing the sound transmission loss data from the test results to a weighted transmission loss spectrum that represents the STC contour. The one drawback of the STC rating is that it assumes a sound source with a spectrum similar to human speech. For this reason the STC rating is often a poor predictor of sound isolation for low frequency sources such as music, mechanical system or transportation noise (see below). An example of test data compared to the STC contour is shown in Figure 1. More recently, another rating was developed which is called the Outdoor–Indoor Transmission Class (OITC). The methodology of this rating is defined in ASTM E1332. This rating is determined by calculating the difference between the product’s sound transmission loss performance and a standardized exterior source spectrum. The standardized spectrum is an average of three transportation spectra: a freeway, a railroad passby and an aircraft takeoff. The resulting number is intended to relate to the perceived sound levels in the interior receiving space as the result of an impact from a transportation source. At this time, this rating is not as widely used as the STC rating, however, it can often be calculated provided that sound transmission loss data for a product is available. There are several publicly available acoustical performance data sets for glazing. The two most common data sets are provided by Solutia (a Monsanto company, formerly named Saflex 1 ) and Viracon. These data sets provide the performance of only the glazing systems, without a window framing system. As a result, these data may somewhat overestimate the performance of a window product with a frame, particularly when the STC ratings of the glazing exceed STC 40. Figure 1 Ratings of glazing systems in this range and higher may be adversely impacted by the incorporation of a window framing system, which often degrades the performance. ACOUSTICAL UPGRADES FOR ARCHITECTURAL GLAZING SYSTEMS Architectural glazing can vary dramatically in sound isolation performance. The following sections describe a variety of different window glazing options with different acoustical performance. 1-inch thick insulated glazing unit: In many areas of the country, a common glazing assembly that is used on commercial projects is a 1-inch thick insulated glazing unit, consisting of 1/4-inch thick lites separated by a 1/2-inch thick airspace. This glazing generally provides a laboratory rating of about STC 35 and OITC 30, though depending on the frame it is installed in, the ratings may decrease by a few points. This glazing provides acceptable sound isolation from exterior noises in rural or quieter areas, or when a moderate amount of intrusive noise is acceptable. Insulated glazing unit with glass panes of different thicknesses: An alternate 1-inch thick insulated glazing unit Summer 2006 27
Page 1: JBED Summer 2006 Journal of Buildin
Page 6: Message from NIBS David A. Harris,
Page 10 and 11: Feature Dynamic, Integrated Façade
Page 12 and 13: 1. COMMERCIALLY AVAILABLE SOLUTIONS
Page 14 and 15: procure them for the final building
Page 16 and 17: obstruction and/or daylight reflect
Page 19 and 20: Feature All That Glass Is there an
Page 21 and 22: façade at the street corner. It is
Page 23 and 24: cess to the perimeter, concrete str
Page 28 and 29: could consist of lites that have di
Page 30: esidents. Another concern from the
Page 33 and 34: from the California Central Valley
Page 37 and 38: Feature Window Comfort & Energy Cod
Page 39 and 40: By Valerie L. Block and Tammy Amos,
Page 41 and 42: Feature How Does Fenestration Fit I
Page 43: Council (ICC), one might expect tha
Page 46 and 47: Industry Update By James C. Benney,
Page 48 and 49: BEC Corner HOUSTON-BEC By Andy MacP
Page 50: Buyer’s Guide AIR BARRIERS Dupont

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