Practice Tool: Acoustics in Multi-Family Residential Design

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Cherokee Mixed-Use Lofts – Architect Pugh + Scarpa


Each day complaints are received by property managers at home owners association meetings regarding noise from the neighbors above. Some of these complaints involve airborne noise sources, such as talking, music or plumbing, but many of the complaints are regarding noise from footfalls. In this article we will discuss the descriptors and building standards associated with sound impact insulation and provide some useful tips to solve this design issue in your next project.

To better understand controlling sound within our design it is important to have a basic knowledge of sound properties. When controlling sound it is important to remember it has two components loudness and tone. As designers we use the decibel and frequency. As humans we hear sound as low as 1 decibel and sense pain when it rises above 120 decibels. The typical human voice at conversation level measures 55 decibels at a distance of 5 feet. We also have a tonal range from 20 Hertz to 20,000 Hertz with most of our information communicated to us between 200 Hertz and 8000 Hertz. Humans are more sensitive to high-frequency sound then low-frequency sound. As designers, descriptors used to describe sound within the human environment have taken into consideration both our sensitivities to loudness and tone. The two most commonly used descriptors in multi-family residential construction for the control of noise are the STC and the IIC. Often we find that these two descriptors are misunderstood or interchanged.

There are multiple web sites devoted to unhappy apartment and condominium dwellers sharing their sorrow over their noisy neighbors, they play loud music, sing in the slower, walk around in the middle of the night, and have a loud answering machine. The walls and floors that separate units should be designed to prevent intrusive noise. For airborne sound transmission the descriptor that quantifies this performance is the Sound Transmission Class (STC) rating.

The STC rating is a single number used to describe a materials ability to block sound transmission between two spaces. It can be used to quantify the performance of a wall, window, door or floor. The value is derived from the measured transmission loss (TL) values between 125 Hz and 4000 Hz. This is the typical range of noise generated by people. Though our ability to hear is as high as 20,000 Hz, a wall that successfully blocks sound at 4,000 Hz will block higher frequencies. The American Society for Testing and Materials (ASTM) establishes procedures for STC testing under ASTM E336-97, Standard Test Methods for Measurement of Airborne Sound Insulation in Buildings. Measurements are made using a loud, broad-band 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. They are then plotted on a graph with transmission loss (in decibels) 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 decibels and that no single value is less than 8 decibels. The single value for the STC is chosen as the decibel value at 500 Hz when the curve has been positioned.

By selecting a wall with a sufficiently high STC rating, and by ensuring it is installed properly, intrusive noise should be completed. Building Codes establish standards for minimum performances for partitions separating residences. For a floor/ceiling assembly between units, an STC 50 is considered the minimum acceptable performance under code in the United States.

In recognizing the need for guidelines that provide higher levels of sound insulation, the Department of Housing and Urban Development created A Guide to Airborne, Impact, and Structureborne Noise Control in Multifamily Dwellings. This comprehensive guide establishes three grades of performance that can be regionally tied to the cost of construction. Under their guidelines a Grade III provides a basic level or “minimum” that equates to local building code. A Grade II performance provides for a significant improvement of 3 – 5 rating points higher than building code, depending on the condition. The highest level, Grade I, is prepared for units associated with “luxury” and improves on a Grade II by an additional 2 – 4 points. This guideline goes further by establishing criteria that is specific to the types of spaces that are being separated. For instance, the rating for a Grade II partition between two bedrooms is an STC 52, while the Grade II rating for separating a kitchen from a bedroom is an STC 55. This adjusted performance addresses the additional noise generated in a kitchen that should be addressed when adjacent to a space used for sleeping.

Typically it is necessary to achieve an STC 52 for the majority of residences to perceive adequate sound isolation.

In reviewing construction documents, efforts can be seen to control airborne noise in the selection of partitions using an STC rating. However, structure borne noise, generated by footfalls, is quite often overlooked. Footfalls can generate noise levels that are highly intrusive. Levels above 50 decibels can be generated to units below from walking on a wood or concrete system. Sound is efficiently transmitted through the structure as vibration and re-radiated as noise using walls and the ceilings as the airborne component.

Impact insulation is needed to control noise that is radiated in the structure from footfalls on floors. Without this insulation, impacts are transmitted directly to the structure, as if we were “knocking on a door”. The vibration is passed into the floor system and down the walls in the space below, effectively creating multiple new sources of sound to try to block below. A common misconception is that increasing the STC performance of the ceiling below can control impact noise. Unfortunately, this method only treats one of the paths and can provide only a limited amount of improvement. In a typical wood frame flooring system with hard surface flooring above, the addition of a layer of drywall will only provide a 3-decibel improvement. Because improving the transmission loss of the assembly does not directly control impact noise, a different descriptor is used, Impact

The IIC rating is a single number used to describe a floors ability to limit noise when excited by impactive sources. Measurements are made using a small “armadillo” like machine that has 5 feet on a single camshaft. When operated, the machine raises and drops a .5 kilogram mass 4 centimeters 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 size and reflectivity of the space. The procedure for conducting IIC testing is established under ASTM E989-89 and E1007-97. The measurement is conducted over the 16 octave bands from 100 Hz to 3150 Hz.

Insulation Class (IIC). The IIC rating is used in building code to provide means to limit noise from footfalls between residential units.

This rating allows the comparison of different assemblies to the same reference source. The minimum performance allowed by building codes is an IIC 50. As with an STC rating this level of insulation is not intended to provide comfort for all occupants. It is the minimum and should be used with care. The HUD Guidelines shown in Table 1 provide for different grades of performance. These can be selected based on the use of the source and receiver space to develop a tailored approach to controlling impacts. Isolating a kitchen over a bedroom requires more insulation than a kitchen over a living room.
The Performance of Various Standard Assemblies

To develop solutions for sound and impact insulation to work together, it is a good idea to start with the basic assemblies and their STC and IIC performances for treatment. Table 2 presents a list of standard assemblies showing their rated performances.
Table 2. Acoustical Performances of Standards Floor/Ceiling Assemblies

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. The most commonly used material is carpet and pad. A typical wood frame floor/ceiling assembly can achieve above an IIC 60 using carpet and pad. For a hard surface flooring finish, a floating floor is used to improve the IIC performance. There are two types of floating floor systems, a locally reactive or a resonantly reactive. With a locally reactive floor, the surface material and the intermediate elastic material damp the impact force. An example is ceramic tile over cork. In a resonantly reactive system, 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 within the mass as cylindrical waves spread from the impact. This system is commonly constructed using gypsum topping over an isolation pad. Each of these systems can be used in all forms of construction. However, there are clear advantages for one over the other based on the finish floor surface and the grade of impact insulation desired. This should be coordinated further with the desired sound isolation. For efficiency, one should support the other. Here are some basic rules to follow:

  • For a standard 10” floor joist system, always us 3-1/2” un-faced glass fiber insulation, or thicker, in the joist cavity. This will provide a 5-point improvement over an assembly without.
  • To achieve an STC 50 or better, the use of resilient channel will provide as much as a 10-point improvement over a system without. To achieve a 4 point improvement in the IIC rating use two layers of gypsum wall board on resilient channel vs. one layer.
  • The total mass of the sub-floor and ceiling layer combined should be greater than 5 lb/SF to achieve an STC 52 or better.

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 1-1/2 inches to prevent cracking, while a ¾-inch layer is required for a 1-hour rated assembly when used directly over the sub-flooring. This added mass of the 1-1/2 inch topping works to improve the STC rating at the same time. For a concrete slab floor, 6-inches is sufficient to achieve an STC 52 without a ceiling system below. Ideally we would prefer an impact system that does not utilize gypsum topping in a concrete floor system considering the STC has been satisfied. For this reason it is important to select the right system and product for the conditions at your project. For impact insulation, one product does not serve all conditions. Solutions vary depending on the performance, type of construction, and finish flooring.

Hard surface flooring areas are the conditions that require the of an impact insulation layer. In wood frame, 10-inch construction where the finish flooring is going to be a mixed-hard surface system throughout, (tile and engineered wood flooring) it is best to use the 1-1/2 inch gypsum topping over an insulation pad and a single layer of wallboard mounted on resilient channel below with glass fiber insulation. This will achieve both a Grade II Standard for both STC and IIC in a stacked floor plan. An additional layer of wallboard below can further improve this performance. Products that have tested in the field and laboratory to achieve this are:

  • Cork Sheeting 3/8-inch
  • Colbond Enkasonic 9110

This method can also be employed in individual rooms by using the isolator under the gypsum topping under the hard surface area, such as in the kitchen or hall. If the isolation pad is placed in these areas with a dam, to allow the gypsum topping to float without being ground by the adjacent space, the isolation pad can be left out in other area of the housing unit. This will allow the use of a single layer of 5/8”gypsum wallboard mounted on resilient channel below. This method does require additional labor and cost.

In a 10-inch wood frame assembly with ¾-inch gypsum topping on the sub floor, where the finishes are going to be mixed between carpet and hard surfaces a less costly alternative to using 1-1/2 inch topping is available. Using a thin, locally reactive system can achieve a Grade II performance. Each type of flooring finish uses a different impact insulator:

  • Ceramic Tile or Marble – Cork Sheeting 3/8-inch
  • Sheet Vinyl or VCT – Jumpax
  • Engineered Wood Flooring – Sound Muffler or Silent Guardian

In each of these systems it is necessary to use 2 layers of 5/8” gypsum wallboard on the ceiling below with resilient channel to achieve a Grade II performance. The benefits to this type of system are:

  • A thinner system for ceiling heights
  • Lower overall material and labor costs
  • Installation of isolator after kitchen cabinets have been installed
  • Transitions heights between different finish surfaces are minimized.

For a tongue and groove wood flooring system in a wood frame assembly, it is necessary to provide mass, as discussed earlier, 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 that allow the assembly to be isolated from the structure. Without fabricating a system in the field, we have found only one product designed for this purpose:

  • Soundeater – wood strips with fibrous wood product sheets for insulation

Concrete construction solves half of the problem by having sufficient mass in a 6-inch 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 4-inch airspace, glass fiber insulation, and a single layer of gypsum wallboard the assembly will achieve an STC 58. For impact insulation, the follow options are available:

  • Ceramic Tile/Marble – 3/8-inch Cork
  • Engineered Wood Flooring – ThinsuLayment, Rudpax, ½-inch shredded rubber
  • Sheet Vinyl/VCT – Jumpax
  • ¾-inch T & G Flooring – Soundeater

Summary
Solving impact insulation within multi-family residential design is a factor of performance, type of construction, and flooring finishes. For each there is a method that will provide the necessary isolation to achieve minimum Building Code and higher. The Project Architect or an Acoustical Engineer can develop solutions for sound and impact insulation. Acoustical Engineers focus on these details by developing system recommendations or field-testing assemblies for performance. Typically the services of a good consultant will more than pay for themselves by providing correct solutions that are evaluated for being cost effective. Once a system has been developed and tested, many Architects are able to incorporate the solutions in future projects. Recommendations to control sound and impact insulation should contain details for lighting penetrations, use of resilient channel, resilient caulking, and the control of noise through flanking paths in framing. 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, taking care of noise issues within your project should be addressed before they become issues to further occupants.

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  1. avatar
    John C. Keiffer

    This is a nice summary of acoustics in Multi-family design. Glad to see it.

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