Acoustical Performance

The decision on what facade concept to employ for building construction is usually dictated by the aesthetic and thermal requirements of the building program.  However, with the recent environmental noise pollution concerns and the growth in construction of high-rise residential condominiums in noisy urban areas, prescribed acoustic requirements have become more prevalent in the design of building envelopes.  Furthermore, the availability of new computational methods developed for use in the automotive, aircraft and entertainment electronics industries present an opportunity to advance acoustic analysis and performance considerations to a new level in the construction industry.

Classification

Acoustical performance evaluation of architectural products has been the subject of continuous interest in the past 40 years. The two main classification indices used are Sound Transmission Class and Outdoor-Indoor Transmission Class. The current practice is almost always experimental, and the sound transmission classification is not itself a measure of comfort or sound dissipation, but rather a measure of relative performance. Furthermore, due to the nature of sound perception and normalization methods, the results are not always consistent and intuitive, leaving room for interpretation and subjective assessment.

The two classifications are defined as:

  • Sound Transmission Class (STC): A single number rating that is calculated using the ASTM E413 classification for the rating of sound insulation characteristics of interior walls and floor partitions exposed to typical office building noise (e.g., speech, radio, television, etc.). An STC contour curve is applied to the actual measured transmission loss data, and the transmission loss value on the contour curve at 500 hertz is the STC single number rating.
  • Outdoor-Indoor Transmission Class (OITC): A single number rating that is used to classify wall partitions, doors and windows which are exposed to lower frequency noise sources such as cars, trains and aircraft. The ASTM E1332 test method specifies the transportation spectrum and logarithmic summation that is applied to the transmission loss data to obtain the OITC rating.

Performance Analysis

Enclos has developed analytical and numerical tools for the acoustical analysis and evaluation of curtainwall systems as a predictor of performance. Our ongoing, in-house testing programs have built a database of captured performance data from completed projects. This database, in conjunction with best-practice guidelines, is used for system evaluation during the schematic design phase of a project.

Typical insulated glass has a STC rating of 34 to 38. This rating will be compromised by the aluminum framing in the curtain wall on an average by 2 to 4 points. Performance can be improved by the use of laminated glass.

Double-Skin Facades

Double-skin facades were originally conceived as means of mitigating outside noise in office buildings. To this end they achieve a significant increase in the transmission loss of the building skin. On average, a 10 dB gain can be realized by the proper utilization of a double-skin facade.

To compute the overall acoustical performance of double-skin facades, the individual components need to be accurately tested in laboratory conditions. Then the composite effect resulting from the coupling of the various components can be simulated using advanced techniques, such as statistical energy analysis (SEA). The main concept in SEA is that a structure (i.e., a curtainwall system) is partitioned into coupled “subsystems” and the stored and exchanged energies are then analyzed.

The transmission of sound via the cavity of a double-skin facade is an important parameter when considering the acoustical insulation between the rooms located on the facade side. This can be an issue especially when the inner glass facade is operable. In double-skin facade applications, the relative performance seems more pronounced in floor-to-floor conditions verses room-to-room. This is due to a traditional office building’s room-to-room transmission losses (STC of 35 to 45) being considerably lower than floor systems (STC of 54 for a 6 inch concrete floor).

The cavity depth is the prime parameter in the acoustical performance of double-skin facades, although there is an asymptotic limit to the depth efficiency of dual-wall systems. The increase in STC values becomes negligible for cavity depths greater than 24 inches. Furthermore, the composition of the glass panel (laminated vs. monolithic) has a lesser effect in larger depth cavities.