April 27, 2007

Opinion

Masonry construction - sounds like the right choice

One important reason for building with masonry is that it provides superior sound control. Whether it’s concrete block, clay brick, or stone, masonry possesses an inherent resistance to the passage of airborne sound, which makes it a superior performer in attenuating low-frequency, airborne noise caused by building mechanical systems, elevators, amplified music, traffic and aircraft. Although some alternative systems may perform as well as masonry for frequencies in the speech range, these lower mass systems have difficulty insulating against low-frequency noise.

They rely on a combination of sound control techniques that are more difficult to construct, and less forgiving to construction errors and omissions. Low frequency sounds are a major cause of complaints in multi-family buildings.

As cities become more densely populated and as increasing numbers of people look to converted living spaces, apartments and condos, effective sound control has become a critical aspect of urban quality of life. Urban environments are inherently noisy and are likely to become more so. While noise can be controlled at its source, this is usually a very complicated undertaking. More than engineering is involved. The most effective solution is by sound insulation, that is, by reducing noise along its path from its source to the listener.

Sound insulation is an important aspect of comfort and health. Researchers are discovering that noise is an important factor in people’s well being. Studies have found that noise may increase blood pressure or be a risk factor for coronary heart disease. It is known to cause annoyance, stress, sleep interference, speech interference, task interference, and to affect the body’s physical reactions, and our relations with other people. Aside from the physiological and biological costs, depreciation of real-estate values has been found to be in the range of 0.5 to 1.0% per dB of noise increase.

SOUND ADVICE

Gary Sturgeon

Noise insulation is measured in the laboratory using standard testing procedures. The result - Sound Transmission Class (STC), expressed in decibels (dB) – is a single number rating that provides a measure of the sound insulating properties of interior walls. The higher the STC rating, the better the assembly can block or reduce the transmission of sound across it. STC rating is intended for use with typical indoor noises such as speech and office sounds. Lightweight wall systems having the same or similar STC rating as masonry will not perform as well as masonry when subjected to low frequency noise.

According to research by the National Research Council of Canada (NRC): “Noise from neighbours in multi-unit buildings is a serious problem that degrades the quality of life of the residents. It is only when sound insulation is approximately STC 55 or greater, that sound insulation is effective in minimizing the negative effects on residents. STC 55 is therefore recommended as a realistic goal for acceptable sound insulation.”

The Ontario Building Code requires an STC rating of 50 as a minimum acceptable value, and STC 55 in specific areas. Many builders prefer to design for STC 55 or more if end users are demanding and willing to pay for a higher quality of accommodation.

Work undertaken by NRC shows that masonry walls can easily meet this requirement. Depending on the density and thickness of concrete block units, concrete block masonry walls can provide STC ratings of well above 70. For example, an STC 70 assembly would consist of a “core” of 190 mm normal weight concrete block, and 38 mm x 64 mm wood strapping, absorptive material between the studs and 12.7 mm gypsum board on both sides. By using concrete block “it is possible to construct high-quality walls that can meet the most acoustically demanding situations.”

Growing attention also is being paid to the sound insulation properties of exterior walls. Here, the measure is the Outdoor-Indoor Transmission Class (OITC), expressed in decibels. OITC is a single number rating, and the higher the rating, the better the assembly. OITC has been developed specifically for exterior facades intended to insulate against outdoor noises.

A major study undertaken at the National Research Council of Canada investigated sound transmission loss of over 100 constructions representing building facade components with wood frame. The study found that wood stud walls had OITC ratings ranging from about 25 for the base wall (11 mm oriented strand board on 140 mm studs with a 406 mm spacing, 152 mm glass fibre thermal insulation in the cavity and 13 mm gypsum board as the other surface layer) to 40 for a masonry veneer (base wall 90 mm brick cladding). An additional 5-8 points could be gained by increasing the stud spacing to 610 mm, adding resilient channels, or using a staggered stud configuration. Of the five different types of exterior cladding tested, the masonry cladding provided the highest OITC rating, and is considered by the research to represent the high-end of OITC for an exterior wall system, “Brick walls are often thought of as a performance standard desirable of other walls.”

These findings confirm that masonry wall construction offers a significant advantage in reducing noise transmission through exterior and interior walls. Masonry construction is the sound choice for designers, builders and consumers.

Gary Sturgeon is a technical services engineer with the National Concrete Masonry Association.

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