The International Building Code (2018 Edition) regulates sound transmission in Section 1206, which is one of the primary sections regulated in Chapter 12, Interior Environment.
Section 1206 requires that common interior walls, partitions, and floor/ceiling assemblies that are designed and/or constructed between adjacent dwelling/sleeping units and between such units and public areas (including but not limited to hallways, corridors, stairways, or service areas) be afforded a minimum degree of acoustic separation. Of note, these rules found in Section 1206 only apply to dwelling units and sleeping units, and are therefore primarily residential in nature. Therefore, acoustic separation should be a consideration when designing or building such separations. The code requires two types of sound control: control of airborne sound, and control of structure-borne sound.
Airborne sound is measured by an assembly’s STC rating, that is, its Sound Transmission Class. An assembly’s STC rating notes how effective an assembly is in limiting airborne sound from transferring through the system. The higher the STC rating, the better the assembly is at limiting airborne sound transmission. Airborne sound includes things like music, talking, laughing, TV, etc., where the sound travels through the air and is to be dampened by the assembly.
Structure-borne sound is measured by an assembly’s IIC rating, that is, its Impact Insulation Class. An assembly’s IIC rating notes how effective an assembly is in limiting structure-borne sound from transferring through the system. The higher the IIC rating, the better the assembly is at limiting structure-borne sound transmission. Structure-borne sound includes things like stomping your feet, jumping and landing on the floor, dropping something on the floor, moving furniture, etc., where the sound comes from an impact on the assembly.
Per IBC 1206.2, airborne sound must be controlled by assemblies meeting a minimum lab-tested STC rating of 50 per ASTM E90, or 45 if field-tested. ASTM E90 is the standard test method for laboratory measurement of airborne sound transmission loss of building partitions and elements. While the code establishes a 50 as the minimum lab rating, it affords essentially a 10-percent tolerance for field testing, since testing in the field cannot be done in as controlled a setting as laboratory testing. However, it is vitally important to understand that the required STC rating of 50 really is a minimum standard, and should not always be the targeted goal for every design and construction project out there. The International Code Council (ICC) publishes an accessory standard to the IBC, published as the G2-2010 document, Guideline for Acoustics. This guideline recognizes the fact that code minimum levels of acoustic performance are not always sufficient to meet occupant needs, and thus provides some additional guidance on acoustic design. Table 2 of this ICC guideline provides two additional criteria for STC-rated assemblies: STC 55 for “Acceptable” or “Grade B” performance, and STC 60 for “Preferred” or “Grade A” performance. For designers, it is important to keep in mind the expectations for any given project and to consider if the code minimum STC 50 is sufficient, or if the project requires higher levels of sound performance. For example, a luxury or higher-end multi-family building may demand STC 60 for the level of sound performance expected by the clients or end-users, and therefore a reasonably competent designer should incorporate this information into the design documents.
Per IBC 1206.3, structure-borne sound must be controlled by assemblies meeting a minimum lab-tested IIC rating of 50 per ASTM E492, or 45 if field-tested. The logic behind the minimum lab ratings versus field ratings for IIC ratings is the same as for STC ratings. ASTM E492 is the standard test method for laboratory measurement of impact sound transmission through floor-ceiling assemblies using a tapping machine, which is a machine that intermittently impacts the floor with small hammers at controlled force levels and time intervals. These taps are then measured by microphones in the space under the assembly being tested to establish how much sound is moving through the system due to the tapping. Similar to the STC requirements, it is equally as important to understand that the required IIC rating of 50 is the minimum standard, with the same ICC guideline for acoustics recommending IIC 55 for “Acceptable” or “Grade B” performance, with IIC 60 meeting “Preferred” or “Grade A” performance.
As architects and designers, acoustic control and separation should not be dismissed, but should be met with properly designed and rated assemblies that will meet the expectations of the clients and ultimately the end users/occupants of the spaces, where acoustical privacy is necessary and/or expected.