The upscale Omni Hotel specified a sound reduction system for the comfort of its guests. Projects of this size and scope are common in this specialized area. Photo courtesy of Maxxon.

There are a multitude of systems available and their performance and cost varies widely. Systems may be as simple as gluing down the appropriate membrane where a finished ceiling is below or as demonstrated here, a membrane followed by a coat of lightweight cement. Photo courtesy of Maxxon.

Sound control is a flooring option being demanded and sought out increasingly by building owners and tenants throughout the United States. Any floor system can be designed to reduce noise caused by normal everyday living activities. With the growing use of hard surface flooring, this has become especially important in multifamily homes. Big concrete structures make excellent echo chambers and in multi-unit buildings, the sounds generated from hard surface flooring can be annoying to say the least.

Traditionally sound proofing has often been a code or homeowners association requirement in specific geographical areas. These units were typically hi-rise or condominium unit construction in coastal areas. These areas are also traditionally the highest use areas of ceramic tile. With the growing use of tile, the benefits of adding sound reduction are being embraced by some homeowners and demanded by others. Growing product awareness of sound control products is causing a dramatic growth in this specialized area. For tile distributors and installers alike this means additional sales dollars at typically a higher margin. Questions have risen with increasing regularity on the complex subject of sound reduction. In this month’s TILEissue, we will attempt to demystify some of the basics and terminology for our readers.

This photo shows an example of tile being installed over a sound-reduction sheet membrane. Photo courtesy of MAPEI Corporation.

What is Sound Control?

There are two types of ratings used in sound control:

STC (Sound Transmission Class):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-unit building. The term STC translates to Sound Transmission Class and is measured and stated in accordance with ASTM (American Society for Testing of Materials) 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 such as STC 35. When done in the field, using ASTM Method E336, the values are designated by the initials FSTC to the left of the numeric value such as FSTC 35. 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.

IIC (Impact Insulation Class):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. As previously indicated, 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.

This photo shows mortar being applied over a sound-reduction membrane. Photo courtesy of MAPEI Corporation.

STC and IIC Ratings and Building Codes

In multi-family construction there are many jurisdictions that must meet minimum IIC and STC values in floor/ceiling assembly to meet the building code standards. Most common are the ICC (International Code Council) 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. You should always 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) 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 or larger 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 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.

*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.

Concrete Slab Subfloors

Concrete slabs come in a variety of thicknesses and compositions, e.g., hollow core, post-tensioned, pre-stressed. 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:

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 3/4” 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.

Effective sound reduction while easily achieved in most instances requires very diligent adherence to the specific product manufacturer’s recommendations. When STC and IIC values are provided by a manufacturer they typically come as a result of a very specific assembly. As previously mentioned, it would not be wise to install a compressive material under ceramic tile (yes, this includes porcelain) unless the product has been performance tested under ASTM C-627, the Robinson Floor Test. You should also inquire about the tile size, setting material, and grout recommendations. All may vary with each different sound reduction product.