The Need for Sound Control in Multi-Family Dwellings

by Bill Stewart
For many property managers of multi-family dwelling structures, complaints from homeowners about noisy neighbors is a constant issue. Some of these grievances involve airborne sound sources (i.e., talking, music, plumbing), but many are due to the reverberation of footfall-a structure-borne noise transmission that differs from its airborne counterpart. Designing a quiet condominium complex entails an understanding of not only acoustics, but also impact insulation, along with the respective building codes and standards.

Criteria for human exposure: The spectrum of noise for humans ranges from 0 decibels (dB) to 120 dB-from the first whispers of soft sounds to the level where pain begins. At conversation level, the typical voice measures 55 dB at a distance of approximately 1.5 m (5 ft). The range of human hearing (or tonal range) is from 20 Hz to 20,000 Hz, with most information communicated between 200 Hz and 8000 Hz.
To understand the effects of noise and to develop regulatory guidelines and standards, noise is measured and compared to human response. The sound descriptors used in designing towards acceptable noise levels consider sensitivities to both loudness and tone. The most commonly used descriptors in multi-family residential construction are the sound transmission class (STC) and the impact insulation class (IIC). These describe different types of noise, but are often interchanged or misunderstood. Understanding STC codes and standards Walls, floors, and doors separating units can be designed to prevent intrusive noise. The descriptor quantifying this performance for airborne noise is the STC rating. By selecting a wall with a sufficiently high STC rating and ensuring its proper installation, architects/engineers (A/Es) can help eliminate airborne noise complaints.
 
 ASTM International establishes procedures for STC testing under ASTM E 336, Standard Test Methods for Measurement of Airborne Sound Insulation in Buildings. Measurements are made using a loud, broadband noise source on one side of an assembly to derive the remaining noise level on the other side. These 'receiver room' values are corrected for the room size and reflectivity. Values from these measurements are recorded in 1/3 octave-band values between 125 Hz and 4000 Hz, and then plotted on a graph with transmission loss (TL, measured in dB) on the vertical axis and frequency (in 1/3 octave bands) on the horizontal axis. A fixed curve is then moved vertically to a point where the sum of the values below the curve is less than 32 dB and no single value is less than 8 dB. The STC's single value is chosen as the decibel value at 500 Hz when the curve has been positioned. 

Building codes have established standards for minimum STC performances of partitions between residences. For a multi-family floor/ceiling assembly, STC 50 is considered the lowest acceptable performance in the United States. However, the Department of Housing and Urban Development (HUD) has released its own guidelines for providing higher levels of sound insulation (Table 1).

This comprehensive resource establishes three grades of performance regionally tied to the cost of construction:

Grade III: basic level (or minimum equating to local building code);
Grade II: significant improvement of three to five STC/IIC rating points higher than the code, depending on the condition;
Grade I: improves on Grade II by an additional two to four points (often associated with luxury units).

The HUD guideline goes further than building codes because it establishes criteria specific to the types of spaces being separated. For example, the rating for a Grade II partition between two bedrooms is STC 52, while the Grade II rating for separating a kitchen from a bedroom is STC 55. This criteria addresses noisier areas (i.e., kitchens, bathrooms, living rooms) in relation to the quieter ones.

Feeling the impact:  Efforts to control airborne noise are often seen in construction documents through the specification of partitions using STC ratings as a guideline. However, structure-borne noise generated by footsteps is a problem often overlooked by many A/Es. Footfall on a wood/concrete system can generate highly intrusive noise levels (above 50 dB) to units below. Sound is efficiently transmitted through the structure as vibration, and is re-radiated as noise, using the walls and ceilings as the airborne component.

Impact insulation is needed to control the sound of footsteps from being radiated in the structure. Without this insulation, impacts are transmitted directly to the structure, an effect similar to knocking on a door. Vibration passes into the floor system and down the walls in the space below, effectively creating multiple new sources of sound. Controlling impact noise by increasing the STC performance of a ceiling below simply does not work-this method only treats one of the paths, and therefore provides only limited improvement. For example, adding a layer of gypsum wallboard to a typical wood frame flooring system (with hard surface flooring above) only provides a 3-dB improvement.

Since improving the assembly's transmission loss does not directly control impact noise, IIC is used as a descriptor, rather than STC. The impact insulation class rating is used in building codes to describe a floor's ability to limit noise when excited by impactive sources. As with STC, the lowest code-allowed performance (IIC 50) is not intended to provide comfort for all occupants-it is the bare minimum and should be used with care. Again, HUD guidelines (Table 1) provide different grades of performance based on the use of the source and receiver space to develop a tailored approach to controlling impact. For example, isolating a kitchen over a bedroom requires more insulation than a kitchen over a living room.

IIC measurements are made using a small 'armadillo-like' machine with five hammers on a single camshaft. When operated, the machine raises and drops a 0.5-kg (1.1-lb) mass 40 mm (1.5 in.) to the floor in successive, uniform impacts. The noise generated from this process is then measured from the room below, with the resulting spectrum corrected for the space's size and reflectivity.

The procedure for conducting IIC testing is established under ASTM E 989, Determination of Impact Insulation Class, and ASTM E 1007, Standard Test Method for Field Measurement of Tapping Machine Impact Sound Transmission Through Floor-Ceiling Assemblies and Associated Support Structures. The measurement is conducted over the 16 octave bands from 100 Hz to 3150 Hz.

Floating floors and other standard assemblies
To develop complementing solutions for sound/impact insulation, it is important to begin with the basic assemblies and their STC and IIC performances. Impact insulation is best accomplished by preventing the vibration energy from getting into the structure. This method is appropriate in wood frame, concrete metal deck, or reinforced concrete flooring, with the most commonly used material being a simple carpet and pad combination. A typical wood frame floor/ceiling assembly can achieve above an IIC 60 using this method. For a hard surface flooring finish, a floating floor is used to improve the IIC performance. There are two types of these systems: locally and resonantly reactive.

Locally reacting floating floor

This floor allows impact force to be transmitted to the structure through the immediate area of the excitation point, and controls the impact using the spring isolation pad deflection as the isolator. With a locally reactive floor, the surface material and the intermediate elastic material dampen the impact force. An example is ceramic tile over cork.

Resonantly reacting floating floor

This floor includes a rigid layer above the spring isolation pad to dissipate vibration energy through stiffness and dampening. The mass in the floated layer plays an important role. The floating slab is thick and stiff, allowing much of the energy to be dissipated in the upper layer before it can be transmitted to the lower layer as cylindrical waves spread from the impact. This system is commonly constructed using gypsum topping over an isolation pad. Each of these floating floor systems can be used in all forms of construction, but the finish floor along with the desired grade of impact insulation and sound isolation determine which is most appropriate. Some basic rules for A/Es to follow include:

1. For a standard 254-mm (10-in.) floor joist system, 89-mm (3.5-in.) un-faced glass fiber insulation (or thicker) should be used in the joist cavity for a five-point STC/IIC improvement.
2. For achieving STC 50 or better, the use of resilient channel provides as much as a 10-point improvement.
3. Using two layers of gypsum wallboard on resilient channel achieves a four-point improvement in the IIC rating over those relying only on one layer.
4. The total combined mass of the sub-floor and ceiling layer should be greater than 239 Pa
(5 psf) to achieve an STC 52 or better.

Impact-insulation product applications

Impact insulation can be applied below gypsum topping or on the surface depending on the desired rating and finished floor types. A typical topping thickness for a resonantly reactive system is 38 mm (1.5 in.) for crack prevention while a 19-mm (0.75-in.) layer is required for a 1-hour rated assembly when used directly over the sub-flooring. The topping's added mass also works to improve the STC rating. For a concrete slab floor, 152 mm (6 in.) is sufficient to achieve an STC 52 without a ceiling system below.

An impact system without gypsum topping is preferable in a concrete floor system as the material's thickness already satisfies the STC requirements on its own. For this reason it is important to select the right system for the project conditions. One product does not serve all conditions in regard to impact insulation-solutions vary depending on the performance, construction type, and finish flooring.

Wood frame
Hard surface flooring areas require the addition of an impact insulation layer. In 254-mm (10-in.) wood frame construction where the finish flooring is going to be a mixed-hard surface system throughout (i.e. tile and engineered wood flooring), it is best to use the 38-mm (1.5-in) gypsum topping over an insulation pad on the floor system, un-faced glass fiber insulation in the joist cavities, and a single layer of wallboard, mounted on resilient channel on the ceiling below with glass fiber insulation. This achieves a Grade II standard for both STC and IIC in a stacked floor plan. (An additional wallboard layer below can further improve this performance.) Products field/laboratory-tested to achieve this include 10-mm (0.4-in.) cork sheeting and modified drainage mats.

This method can also be employed in individual rooms by using the isolator under the gypsum topping below the hard surface area, such as in the kitchen or hall. When the isolation pad is placed in these areas with a dam (for allowing the gypsum topping to float without being grounded by the adjacent space), the isolation pad can be left out in other areas of the housing unit.

This allows the use of a single layer of 16-mm (0.6-in.) gypsum wallboard mounted on resilient channel below, but requires additional labor and cost. For the 254-mm (10-in.) wood frame assembly, if there is 19-mm (0.75-in.) gypsum topping on the sub-floor and the finishes are going to be mixed between carpet and hard surfaces, a less costly alternative is to use a thin, locally reactive system. Each type of flooring finish uses a different impact insulator: Ceramic tile or marble: 10-mm (0.4-in.) cork sheeting Sheet vinyl or vinyl composite tile (VCT): hardboard two-sheet system Engineered wood flooring: felt pad

In each of these systems it is necessary to use two layers of 16-mm (0.6-in.) gypsum wallboard on the ceiling below with resilient channel to achieve a Grade II performance. The benefits include lower overall material/labor costs and minimized transition heights between different finish surfaces.

Tongue and groove
For a tongue-and-groove wood flooring system in a wood frame assembly, it is necessary to provide mass using plywood, gypsum wallboard, or gypsum topping. To achieve a Grade II performance for an IIC rating, it should be floated on a system of sleepers allowing the assembly to be isolated from the structure. Without fabricating a system in the field, the author knows of only one product-type designed for this purpose: the isolation board with wood nailers (wood strips with fibrous wood product sheets for insulation).

Concrete
As previously mentioned, concrete construction solves half the problem by having sufficient mass in a 152-mm (6-in.) thickness to achieve an STC 52. With this as a start it is relatively easy to achieve a Grade II or even a Grade I standard for STC and IIC performances. By adding a 102-mm (4-in.) airspace, glass fiber insulation, and a single layer of gypsum wallboard, the assembly achieves an STC 58. For impact insulation, options include: Ceramic tile or marble: 10-mm (0.4-in.) cork sheeting Engineered wood flooring: fiberboard Sheet vinyl or VCT: hardboard two-sheet system 19-mm (0.75-in.) tongue and groove flooring: isolation board with wood nailers.

Closing thoughts
Documented recommendations and specifications for sound/impact insulation should contain details for lighting penetrations, use of resilient channel, resilient caulking, and the control of noise through flanking paths in framing.
For example, a specification for the use of sealants would resemble the following:
Sealant:
1.Install acoustical sealant around perimeter of all acoustically insulated partitions; two (2) continuous beads at top and bottom metal stud tracks, and other framing members where they interface with substrate.
2.Seal all penetrations, through acoustical assemblies, except for penetrations in fire rated construction to receive fire stopping. This includes penetrations including but not limited to electrical receptacle boxes, light fixtures, sprinkler heads, ductwork, pipe, etc.
It is also important to consider the transitions between these different finish treatments and the necessary insulation early in the project to deal with heights between corridors and units, and within the unit between carpet and hard surfaces. Either way, noise issues within a project should be addressed early on before they become issues to future occupants.


Additional Information
Author
William Stewart is a managing partner of SSA Acoustics LLP. A senior acoustic consultant, he is a member of the American Society of Acoustics (ASA), and the Noise and Vibration Committee 2.6 with the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). Stewart has an undergraduate degree in architecture and a graduate degree in engineering acoustics and has been practicing acoustics since 1991. He has served as a lead designer for over 100 condominiums, apartments, hotels, and resorts. Stewart can be contacted at (206) 839-0819.