Revised November 1, 2005
Sound Rated Floors
What is Sound Control?
There are 2 types of ratings used for sound control: STC (Sound Transmission Class), which is airborne sound, such as speaking, music, etc.; and IIC (Impact Insulation Class), which is sound from people walking, moving chairs, dropping objects, etc. Presenting the greatest challenge for the STC component of sound control is high frequency sound energy, such as that which is produced by a blow-horn or a whistle. The greatest challenge for sound control in regard to IIC is low frequency sound energy, such as that which occurs when a basketball is dropped on the floor.
The term STC refers to the single figure of evaluation used to quantify the transmission of airborne sound through building elements, such as walls or floor systems. These types of sounds would be the equivalent of voices, radio, or television in the context of a multi-family building. The term STC translates to Sound Transmission Class and is measured and stated in accordance with ASTM Standard number C634 and tested via the test methodology of ASTM Test Methods E90, E336, and E596. When done in an accredited test laboratory, these values are stated as an exact number to the right of the initials STC. When done in the field, using ASTM Method E336, the values are designated by the initials FSTC to the left of the numeric value. STC values are in a large part influenced by the solid mass of the structure, but are also dependent on isolation and resilience within the structure.
The term IIC refers to the statistical measurement standards used to quantify the transmission of impact sound energy through a floor/ceiling assembly system. These types of sounds would be the equivalent of foot traffic, dropped articles, or furniture moving in the context of a multi-family building. The term IIC translates to Impact Isolation Class and is measured and stated in accordance with ASTM Standard numbers C634 and E989 and tested via the test methodology of ASTM Test Method E492. In addition, a new test protocol for concrete subfloors has been introduced under ASTM E2179-03e. When done in an accredited test laboratory, these values are stated as an exact number to the right of the initials IIC. When done in the field, using ASTM Method E1007, the values are designated by the initials FIIC to the left of the numeric value.
IIC values are not heavily influenced by the presence of solid mass in the structure. IIC values are usually dependent on the presence of a resilient material somewhere in the assembly to isolate and absorb the sonic energy created by impacts.
STC and IIC Ratings and Building Codes
In multi-family construction, in most jurisdictions, there are minimum IIC and STC values that the floor/ceiling assembly must achieve in order to meet the building code standards. Most common are the ICC/BOCA U.B.C. Uniform Building Code and I.B.C. International Building Code, which call for a minimum 50 IIC and 50 STC value. The higher the IIC or STC, the better the sound attenuation, with 50 considered the minimum for multifamily dwellings. Some states, municipalities, and counties have different building code standards, but the U.B.C. and I.B.C. codes are by far the most common. Consult your local Building Department for the exact code standards applicable in your area.
In addition to the building code standards, some condominium developers and condo homeowners associations have their own minimum standards written into their Covenants, Conditions, and Restrictions (CC&R’s), which are often more stringent than the building code in that given jurisdiction. It is wise to consult your condo association in regard to their CC&R standards for the required IIC and STC values before installing hard surface flooring materials in your unit or project.
A number of different materials are promoted and used for sound control in floors. Each material is only one component of a complete system in which each component is an essential part of the total assembly. Elimination of any component of the assembly can seriously deplete the sound rating desired. Most of the data available to the market relates to tests of floor/ceiling systems that are comprised of concrete slabs with gypsum wallboard hung on resilient furring channels (suspended or sound-rated ceiling system) with a layer of mineral wool or fiberglass insulation in the cavity. Unfortunately, most of the construction detail in the field does not have this type of ceiling treatment.
In addition, a new test protocol for concrete subfloors has been introduced under ASTM E2179-03e. When done in an accredited test laboratory, these values are stated as an exact number to the right of the initials IIC and actually give a separate IIC contribution value number of the product assembly by itself. This number provides an excellent way to evaluate different products, and to determine if additional key components (e.g., a suspended or sound-rated ceiling assembly) are needed. Products that have only Field Test reports (FIIC) and no laboratory testing should be considered with caution. Field tests are project specific; comparing different field tests done in different buildings is not an accurate way to make a proper assessment of product performance.
Many materials that are promoted for sound control in hard surface flooring may not be suitable for direct bond tile and stone installations, as they are too compressible and do not provide proper structural support for these installations. In many cases, it is necessary to use additional reinforcing materials, such as mortar beds, poured in place overlays, backer boards, epoxy mortars and grouts, thicker tiles, and other methods to provide a structurally sound installation.
It is recommended that all products/systems to be used for sound control be subjected to ASTM C627, commonly referred to as the Robinson Floor Test, and that such products/systems meet a minimum “Residential Rating.” All components of the test assembly should be divulged to determine that the product in question is able to achieve the performance rating desired, for the given installation design.
Concrete Slab Subfloors
Concrete slabs come in a variety of thicknesses and compositions (e.g., hollow core, post-tensioned, prestressed). The most commonly cited are 6" and 8" concrete slabs with or without a suspended ceiling assembly. Test results for these two thicknesses vary when comparing field tests versus laboratory tests. The field tests result in a much higher range of values than those conducted in the labs. The following table demonstrates the wide range of numbers, particularly those within the field reports:
Concrete Slab Thickness No Sound Rated Ceiling Sound Rated In Lab IIC** In Field IIC
6" X 26 to 30 24 to 32
8" X 28 to 32 25 to 35
6" X 45 to 52 33 to 48
* Suspended Sound Rated ceiling composed of: 7" plenum, 3" of insulation, resilient channels, 5/8" Type X gypsum wallboard panels.
** Tests were conducted in several different labs. Hence, the range of values for each slab thickness shows the variance between labs, not a variance in the test results within a single lab.
If a suspended ceiling assembly is not possible, the most effective method to establish an improved IIC rating is to install a “floating” floor system. This can be done by utilizing a layer of some type of acoustically rated resilient material, followed by a layer of lightweight concrete, mortar bed, or gypsum concrete (typically ¾” or more, depending on material), onto which the tile or stone is installed.
Wood Joist Floors
For aesthetics and design purposes, wood frame construction will normally have a gypsum wallboard ceiling assembly. To achieve a solid base of sound control, the use of resilient metal channels supporting the gypsum wallboard and sound-absorbing insulating batts in the cavity is recommended. An assembly of this nature with a single layer subfloor has an approximate IIC rating of 45. Adhering a tile or stone surface will actually lower the IIC to around 40. The reason for this is that the hard surface increases noise levels at the higher frequencies, thereby reducing the overall IIC rating. If the sub-floor thickness is doubled and the ceiling wallboard panels are doubled as well, the net effect will increase the STC rating, as well as the IIC rating.
Key Points to Remember
· It is not good practice to select materials or systems based solely on field tests. Lab tests are a more accurate model for predicting performance in a range of different construction types, field tests are accurate only for the site where the test were performed.
· If field tests are being conducted, require both before and after installation testing.
· Testing labs should be NVLAP certified and meet the criteria established in ASTM guidelines E548, E597 and in ASTM Standard E717.
· Sound abatement materials are often quite resilient, so a Robinson Floor Test (ASTM C627) is strongly recommended.
· The thickness of the material used in the Robinson Floor Test should be the same as that shown for the sound control rating desired.
· If a suspended sound rated ceiling is not being used, then a relatively thick assembly on top of the slab may be required (e.g. full mortar bed) to achieve a 50 IIC.