June 1, 2003
by John Bradley
A recent study looking at the role of sound insulation in mitigating aircraft noise has resulted in a new, more accurate design tool that allows users to compare the effectiveness of different building constructions in providing sound attenuation.
Researchers at the National Research Council’s Institute for Research in Construction (IRC) made extensive laboratory measurements of the sound insulation of building envelope components, including various wall and roof constructions, as well as windows and doors. Further experiments in a test house at Ottawa airport explored the complexities of sound propagation into a building exposed to aircraft noise.
The effective sound insulation of a building envelope component, such as a window, depends on its area and on its sound transmission loss (the attenuation of the transmitted sound energy measured in decibels). Selecting components with high transmission loss values or with a small area improves sound insulation. The effectiveness of the design of a complete building, however, depends on both the transmission loss, which varies with frequency, and the areas of each component of the building envelope.
Transmission loss is measured in the laboratory over a wide range of frequencies. The measurements are usually combined into a single-number rating, such as Sound Transmission Class (STC), to make it possible to rank products. Although STC is widely used to rate internal partitions, it is not appropriate for rating sound insulation used to combat outdoor noise because of the strong and usually dominant low-frequency sounds. Instead, the Outdoor-Indoor Transmission Class or OITC (ASTM E1332) is used to rank sound insulation ratings of building envelope components according to their effectiveness as judged by occupants. A simple wood stud wall with lightweight surfaces (gypsum board and Oriented Strand Board) would have an OITC of about 25 decibels (dB) while the same wall with brick added to the exterior would have an OITC of about 40 dB–sound insulation values ranging from mediocre to superior.
All building envelope components are not equal when it comes to keeping out noise, with windows being the weak link in many designs. In addition, one must also consider the specific characteristics of each component, as one window (or wall or roof) can vary greatly from another.
Typical windows with double glazing (two layers of three millimetre glass with a 13 mm air space) have an OITC rating of about 22 dB. The greatest improvement in the sound insulation of windows can be achieved by using thicker glass and a larger air space. Laminated glass and special gases in the cavity can provide small improvements at higher frequencies.
Standard double-glazed windows can be improved inexpensively by installing a conventional storm window with a large air space between the storm and the regular window. The addition of a storm window with a 76 mm airspace to a double-glazed aluminum casement window can increase the OITC rating from 23 to 30 dB, a substantial and clearly noticeable improvement.
Modern practice requires well-vented attic spaces to prevent moisture build-up, but vents allow sound to leak through. However, when the attic space is well insulated thermally, the negative effects of adding roof vents are relatively small. In attic spaces with 264 mm of fibrous insulation (corresponding to R40), adding roof vents reduces the OITC ratings by only one or two dB, a barely noticeable change.
Wood stud walls
In Canada, wood stud construction is commonly used in low-rise apartments and single-family homes. As is the case for other panels and partitions, wood stud walls are least effective when used as barriers to low frequency sound. Low frequency resonances in the walls result in the highest indoor sound levels. Outdoor noise can be dealt with and significant improvements achieved in the following ways: mounting the gypsum board on resilient channels; using staggered-stud construction; and increasing the mass of the surface layers (for example adding double layers of gypsum board and heavy exterior cladding).
Because the roof is often the largest area exposed to aircraft noise, it is important to ensure that a particular construction is capable of providing an effective sound barrier.
A sloping roof construction (raised heel wood truss) with asphalt shingles on the exterior, two layers of 13 mm gypsum board mounted on resilient channels for the interior surface, with roof vents and R40 thermal insulation provides an OITC of 43 dB. Flat roofs with similar details but based on 356 mm wood trusses or 235 mm wood joists provide only slightly less sound insulation. However, replacing the asphalt shingles on OSB sheathing with steel sheeting directly attached to the wood trusses reduces the OITC rating by four dB, a small but noticeable degradation.
Limits to optimal sound insulation
Calculating the total indoor sound level as the sum of the contributions of each component of the building envelope works well for conventional construction types. But when extra measures are taken to provide increased sound insulation, it may be necessary to consider other more complex effects. For example, when gypsum board is mounted on resilient channels to improve the sound insulation of an exterior wall, a flanking path (an indirect path for sound energy) through another building component, such as the floor, may negate the effect of increased sound insulation. To obtain maximum reductions in intrusive outdoor sounds, it is important to consider the complete building design, paying particular attention to the indirect paths traveled by sound energy that can affect indoor noise levels.
The new IBANA-Calc software makes it easy to calculate the combined contributions of various building envelope components for all frequencies from 50 to 5,000 Hz. The user simply selects components from the program’s database (which contains over 100 wall, roof and window constructions) to create various designs that can be compared in terms of their ability to reduce indoor sound levels. These comparisons can be made in two different ways: graphically, by plotting the relationship between indoor sound levels and frequency, and aurally, by simulating the sound of aircraft fly-overs to create the indoor sound levels achieved with a particular construction. The new software provides highly accurate sound insulation calculations–without the tedium of performing the calculation.
Dr. John Bradley is a senior researcher in acoustics at the National Research Council’s Institute for Research in Construction. He can be reached by e-mail at [email protected]