Airborne sound is transmitted between adjoining spaces directly through the separating wall, floor and ceiling assemblies and via the junctions between these separating assemblies and the flanking assemblies.
The Sound Transmission Class (STC) rating describes the performance of the separating wall or floor/ceiling assembly, whereas the Apparent Sound Transmission Class (ASTC) takes into consideration the performance of the separating element as well as the flanking transmission paths. Therefore, from the occupants’ point of view, the best indicator of noise protection between the two spaces is the ASTC rating.
As a key principle, it is important to follow a “whole system” approach when designing or constructing assemblies that separate dwelling units because the overall sound performance of walls and floors is also influenced by fire protection measures and the structural design of the assemblies. Likewise, changes to the construction of assemblies to meet sound transmission requirements may have fire and structural implications. Another key principle is that enhancing the performance of the separating element does not automatically enhance the system’s performance.
For horizontally adjoining spaces, the separating assembly is the intervening wall and the pertinent flanking surfaces include those of the floor, ceiling, and side wall assemblies that have junctions with the separating wall assembly, normally at its four edges. For each of these junctions, there are a set of sound transmission paths. Figure A-9.11.-A illustrates the horizontal sound transmission paths at the junction of a separating wall with flanking floor assemblies.
Horizontal Sound Transmission Paths Floor/Wall Junction
For vertically adjoining spaces, the separating assembly is the intervening floor/ceiling and the pertinent flanking surfaces include those of the side wall assemblies in the upper and lower rooms that have junctions with the separating floor/ceiling assembly at its edges, of which there are normally four. For each of these junctions, there is a set of sound transmission paths. Figure A-9.11.-B illustrates the vertical sound transmission paths at the junction of a separating floor/ceiling assembly with two flanking wall assemblies.
Vertical Sound Transmission Paths Floor/Wall Junction
The metrics used to characterize the sound transmission performance of assemblies separating dwelling units do not account for the adverse effects of air leaks in those assemblies, which can transfer sound. Sound leaks can occur where a wall meets another wall, the floor, or the ceiling. They can also occur where the wall finish is cut for the installation of equipment or services. The following are examples of measures for controlling sound leaks:
- Avoid back-to-back electrical outlets or medicine cabinets;
- Carefully seal cracks or openings so structures are effectively airtight;
- Apply sealant below the plates in stud walls, between the bottom of gypsum board sheets and the structure behind, around all penetrations for services and, in general, wherever there is a crack, a hole or the possibility of one developing;
- Include sound-absorbing material inside the wall if not already required.
The reduction of air leakage is also addressed to some extent by the smoke tightness requirements in the Code.
The NRC report entitled “Best Practice Guide on Fire Stops and Fire Blocks and their Impact on Sound Transmission,” provides additional information regarding the possible impacts of fire protection measures on sound transmission.
The calculation of and laboratory testing for STC and ASTC ratings are performed on intact assemblies having no penetrations or doors. When measuring ASTC ratings in the field, openings can be blocked with insulation and drywall.
To verify that the required acoustical performance is being achieved, a field test can be done at an early stage in the construction; ASTM E336, “Standard Test Method for Measurement of Airborne Sound Attenuation Between Rooms in Buildings” gives a complete measurement. A simpler and less expensive method is presented in ASTM E597, “Practice for Determining a Single Number Rating of Airborne Sound Insulation for Use in Multi-Unit Building Specifications”.
The rating derived from this test is usually within 2 points of the STC obtained from ASTM E336. It is useful for verifying performance and finding problems during construction. Alterations can then be made prior to project completion. Section 9.11. has no requirements for control of impact noise transmission. Footstep and other impacts can cause severe annoyance in multi-family residences. Builders concerned about quality and reducing occupant complaints will ensure that floors are designed to minimize impact transmission. A recommended criterion is that bare floors (tested without a carpet) should achieve an impact insulation class (IIC) of 55. Some lightweight floors that satisfy this requirement may still cause complaints about low frequency impact noise transmission. Adding carpet to a floor will always increase the IIC rating but will not necessarily reduce low frequency noise transmission. Good footstep noise rejection requires fairly heavy floor slabs or floating floors.
Most frequently used methods of test for impact noise are ASTM E492, “Standard Test Method for Laboratory Measurement of Impact Sound Transmission Through Floor-Ceiling Assemblies Using The Tapping Machine”, or ASTM E1007, “Standard Test Method for Field Measurement of Tapping Machine Impact Sound Transmission Through Floor-Ceiling Assemblies and Associated Support Structures”. Elevators, garbage chutes, plumbing, fans, and heat pumps are common sources of noise in buildings. To reduce annoyance from these, they should be placed as far as possible from sensitive areas. Vibrating parts should be isolated from the building structure using resilient materials such as neoprene or rubber.