Sound isolation code requirements

Figure 2: Calculating airborne sound transmission class (STC) and impact isolation class (IIC) ratings.
Figure 2: Calculating airborne sound transmission class (STC) and impact isolation class (IIC) ratings.

The International Building Code (IBC) establishes minimum requirements for airborne and impact performance of multifamily buildings. The minimum code requirement is STC 50 and IIC 50. Since many factors can affect the transmission of sound in the field, including non-standardized source and receiver rooms as well as construction tolerances, a field measurement (ASTC or AIIC) of three to five points below the lab measurement is acceptable to meet code requirements.

As the understanding increased of how STC and IIC ratings correlate with occupant comfort, the International Code Council (ICC) issued ICC G2-2010, “Guideline for Acoustics,” which established two additional levels of acoustical performance:

  • acceptable, defined as STC 55 and IIC 55; and
  • preferred amount of isolation as STC 60 and IIC 60
    (Figure 3).

Understanding what affects sound transmission can help designers select appropriate floor/ceiling assemblies. Assembly weight, incorporating airspace into the assembly, and the use of a vibration break, are important variables in determining how much sound is transmitted across a partition.

Figure 3: International Code Council’s (ICC’s) guideline for acoustics.
Figure 3: International Code Council’s (ICC’s) guideline for acoustics.

Massive materials block more sound than lighter products, and heavier partitions are needed to block low frequencies (e.g. subwoofer on a 5.1 surround sound system) than higher frequencies (people talking).

Additionally, sound isolation can be increased by adding airspace into a partition. A mass-airspace-mass assembly by using a gypsum concrete topping slab (mass 1) above a ceiling plenum (the airspace) of 305-mm (12-in.) or greater with a gypsum board ceiling (mass 2) and insulation in the cavity is an effective sound-blocking partition assembly.

However, when all the partition elements are connected rigidly, sound and impacts are effectively transmitted through the structure. To achieve high levels of sound isolation (above STC 55) and adequate amount of impact isolation (more than IIC 50), a vibration break, provided by an entangled mesh sound mat or resilient ceiling channels or clips, must be incorporated into the design (Figure 4).

While using a carpet and pad instead of a hard surface is effective at achieving high IIC values (above 70), it is also important to think about the long-term use of the building. As hard floor finishes become more popular, relying on carpet to achieve the minimum IIC level limits the renovation palette for future owners. Incorporating a vibration break in the form of an entangled mesh sound mat means the floor finish can be changed without regard to the acoustic performance while maintaining the impact isolation performance.

Emerging trends – mass timber construction

Mass timber construction is the generic term used to describe the use of large solid wood panels to build loadbearing structures including walls, floors, and roofs. Several types of mass timber exist, including cross-laminated timber (CLT), nail-laminated timber (NLT), dowel-laminated timber (DLT), and mass plywood panels (MPP). Interest in mass timber construction is growing rapidly throughout North America. In addition to its visual impact, mass timber offers significant sustainability benefits, can speed construction, and reduce costs.

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