Sustainable design has encouraged building in densely populated urban environments. Yet without the suburbs’ lavish space between sites to act as a buffer, working and living in the city can be noisy. Hearing a neighbor’s heavy footsteps every morning or the adjacent office’s happy hour every week can become bothersome. One solution for designers and their clients is acoustical separation.
Sound is garnering greater importance and attention in architectural design, says Jeff Fullerton, a supervisory consultant in acoustics at Acentech in Cambridge, Mass. From a passing airplane overhead to rush-hour traffic on the street below to a coworker’s telephone conversation one cube over, good acoustical design can help ensure the comfort of the occupant. Money also plays a role, he says. “Successful acoustics make for the perception of higher quality [in residences], greater productivity [in work spaces and classrooms], and improved enjoyment [in restaurants and entertainment venues].”
Unwanted sound—or noise—not only distracts building occupants, but also may cause physical damage. Measured in decibels (dB), loudness represents the amplitude of sound waves. The sound of a pin dropping registers at 10 dB, a normal conversation at about 60 dB, and a motorcycle at 100 dB. At 125 dB, sound starts to become painful for a human being.
Hearing is also affected by the duration of exposure. For example, OSHA varies the allowable noise level exposures based on time. A sound level of 90 dB is permitted for 8 hours per day.
To control noise levels, architects must understand the basics of acoustics. Strategies for controlling sound depend on whether it is moving through air or through a solid mass. The ability of a material or construction assembly to reduce noise traveling through air is indicated by its sound transmission loss (STL) value, which is determined in a controlled laboratory environment.
The STL values are then converted to a single number rating called the sound transmission class (STC). As designers develop their projects, they can use STC ratings to select appropriate construction configurations for spaces that require acoustical separation from surrounding spaces. A standard minimum STC rating for a residential partition wall is 50. Hollow, 10-inch-thick masonry walls or grout-filled, 6-inch-thick masonry walls could achieve this rating, as could a double-stud wall with two layers of 5/8-inch gypsum sheathing filled with batt insulation.
For sound traveling through a solid, the ability of a material to reduce sound caused by impact (such as footsteps or dropped objects) is based on an impact insulation class (IIC) rating.
Where an STC or IIC rating measures an assembly’s ability to transmit sound, a sound absorption coefficient and noise-reduction coefficient (NRC) measures how well a material can change acoustic energy, or sound, into another form of energy, usually heat. NRC values vary between zero and 1, with zero indicating that a material absorbs no sound, and 1 indicating that a material absorbs all sound. Materials with an NRC value greater than 0.35 may typically be classified as a sound absorber. Glass ranges from 0.05 to 0.10, 1-inch-thick open-cell polyurethane foam has an NRC of about 0.30, and 1 inch of sprayed cellulose fibers on concrete can range from 0.50 to 0.75.
Fullerton says that an architect must determine whether to improve the sound insulation of the construction system, improve the acoustics within the room, or both. Design strategies and the use of acoustical products vary, and the success of each depends on the project objective. Acoustic insulation can be useful for improving sound isolation or increasing sound absorption, depending on how it is applied. It is available in a variety of materials, including glass fiber, mineral fiber, recycled cotton, spray-in cellulose, and spray-in, open-cell polyurethane foam.
When used in the cavity of a demising wall, these acoustical insulation materials will reduce sound transfer by 3 dB to 5 dB, which is a threshold that most people can perceive. The interconnected air pockets are the key to the insulation’s sound-absorption and thermal-insulation properties.
Batt insulation—and particularly those types made from glass, mineral fiber, and recycled cotton—are easy to handle and install in wall, floor, and ceiling cavities. It typically doesn’t have an NRC value because it is not exposed directly to sound, but is instead used as a component in a wall assembly where it can increase the STC rating by four to 10 points. Batt insulation is best suited for new construction when wall and floor/ceiling cavities are open.
In terms of eco-friendliness, cotton insulation is largely made of recycled content—for example, old denim jeans—but it doesn’t expand readily due to the way it is packaged. Since air pockets are crucial to insulation’s sound absorption capabilities, it may not perform as rated.
Cellulose and open-cell foam spray-on insulations may be more cost effective in retrofit applications where they can fill an existing wall, floor, or ceiling cavity, or serve as an exposed acoustical finish on a wall or ceiling. Their disadvantages include a heavier weight than batt products and the need to wait for the insulation to dry prior to closing the building cavity. Environmental concerns may surround the chemicals that make up the insulation composition.
Acoustic insulation applied to a wall surface may increase the room’s sound absorption average, measured by percentage of sound absorbed, but it may, however, not increase a construction system’s sound-insulating performance.
When used as a finish material, acoustic insulation is often covered with a sound-transparent, or transondent, material such as a breathable fabric or scrim, a perforated metal, or wood. Fullerton recommends that “these acoustic insulations and their transondent facings should achieve a sound absorption average no less than 0.70 to be effective for most applications.”
The panoply of measurements and materials means that architects must understand how sound behaves in order to implement effective and appropriate acoustical separations. While acoustical insulations help reduce sound transmission through a building assembly, some designers mistakenly use it as a sound-absorbing surface finish to improve sound isolation, Fullerton says. “While such an installation will help reduce the buildup of sound in the space … it may not provide any significant reduction to improving the sound isolation of a construction.”
For example, to mitigate noise from that neighbor with the heavy footsteps, a rug on their floor could improve the IIC of the floor/ceiling assembly, but leave the STC unchanged. Meanwhile, adding insulation in the joist cavity could increase the STC rating and also the IIC rating. In this case, to control both airborne and structureborne sound, the floor/ceiling assembly should have an IIC rating equal to its STC rating.