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It All Started with Carpet Pad: A residential sound control perspective

It All Started with Carpet Pad: A residential sound control perspective


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by Dean A. Hacker
When the baby boomers—the largest generation in U.S. history—began to set up households in the 1960 and ’70s, design trends were very different than they are today. Thick carpets with accompanying pads covered the floor in most living spaces.

Thick and relatively soft vinyls and early linoleum covered most hard surfaces. Despite the aesthetic limitations of these building materials, they absorbed impact sound extremely effectively. As a result, the earliest efforts at multi-family noise control primarily addressed airborne noise.

As baby boomers matured, so did the floorcoverings. Hard flooring surfaces became incorporated into apartment buildings and airborne noise complaints surfaced. However, since apartment-dwellers were often short-term and resident turnover could be high, builders and property managers felt little urgency to address the problem. To this day, uncontrolled airborne noise remains a challenge in multi-family living. No one wants to hear a neighbor’s television programs, music from the outside, or any other auditory nuisance.

Airborne sound is known by its sound transmission class (STC). STC is mitigated by mass—the more mass between the airborne noise source and the listener, the less audible it is. A linear relationship exists between increased mass and improved STC noise control.

The current acoustical standard dates back to 1963. The Federal Housing Administration/U.S. Department of Housing and Urban Development (FHA/HUD) publication, “Noise Control in Multi-Family Housing,” suggested three levels of acoustical isolation:

  • entry level;
  • market rate; and
  • luxury housing.

The table above suggests separations between different units in a project, based on the acoustical expectation of the three different multi-family housing types. The International Building Code (IBC) adopted the Entry Level standard as minimum code in 2010.

Pourable, lightweight underlayments have been used since the 1950s to assist in creating fire-rated floor/ceiling assemblies. They are also an ideal solution to airborne sound. Thicker and denser underlayment leads directly to improvements in STC sound as shown in the chart below:

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There is a second type of noise to be controlled in multi-family construction: structure-borne sound, with examples including footfalls, clicking high heels, and washing machines. Measured by impact insulation class (IIC), such structure-borne sounds were rarely noticed by the renters of the 1970s, thanks to carpeting, pads, and thick vinyls. As that changed in the decade to follow, this type of sound was seen as a problem.

In response, underlayment installers moved the carpet pad from above the underlayment slab to beneath it. It was a natural choice and, in the short run, a good solution. The re-tasked carpet pad created a spring that absorbed and muffled the vibrational energy on the top side of the joist. The addition of resilient channels isolated the bottom side. Together, that system generally achieved a 50 IIC sound reduction with the floorcoverings of the day.

Unfortunately, the carpet pad would degrade over time due to the additional weight of the underlayment slab. These earliest sound mats, not designed for extended widespread loads, compressed—as a result, their acoustical value was limited.

picture1The next phase saw geotextiles converted into sound control mats in the 1990s. The first attempts used nylon core products that were glue-bonded to non-woven fabrics. When used in conjunction with gypsum-based concretes that bonded robustly to the backing fabric, these products achieved a ‘Residential’ grade under ceramic tile for ASTM C627, Standard Test Method for Evaluating Ceramic Floor Tile Installation Systems Using the Robinson-Type Floor Tester. While highly resilient nylon cores performed extremely well acoustically, the glue bond limited the robustness of the overall assembly. Developers also balked at applying ‘residentially’ rated products in multi-family structures that experienced frequent move-outs with the attendant wear on the flooring system.

With modifications to the building code, the need for noise reduction in multi-family structures dramatically increased. Polypropylene-core sound-control mat development proved pivotal. These mats could be heat-bonded to a non-woven backing—this process is more resilient than the traditional glue-bonding, so the products now met the ‘Light Commercial’ standards that were more appropriate for most multi-family dwellings. These mats are more dimensionally stable when exposed to heat and water. Further, they permitted installers to avoid nailing sound control matting to the wood substrate, enabling them to engage in best practices all around.

However, the polypropylene mat products also have limitations. Outside their local manufacturing area, for example, they are not able to help a project earn credits toward the Leadership in Energy and Environmental Design (LEED) rating program. Additionally, single-source polypropylene is more expensive than the types of recycled fillers commonly used in grocery bags and other disposable items. To achieve recycled material credits and reduce costs, some sound attenuation mat suppliers chose to incorporate up to 40 percent recycled fillers. The resulting mats proved far too brittle and, over time, held up no better than the 1970s carpet pads that had been displaced more than three decades prior. This proved to be a major step backward in the development of sound control mats.

This led Hacker Industries, Inc. to enter the sound control business after partnering with third-party vendors for over three decades. The company removed the recycled fillers to restore the resiliency that made the sound control mat a key component of a well-designed sound attenuation system. Initial tests of the resiliency of FIRM-FILL® SCM products showed they returned to more than 95 percent of their initial thickness after applying a 227-kg (500-lb) load. In terms of durability, this meant FIRM-FILL® SCM resembled—as closely as possible—the oriented strandboard (OSB) substrates on which it was placed. Having tackled the resiliency challenge, Hacker Industries, Inc. conducted extensive laboratory and field acoustical testing.


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Laboratory and field sound tests carry inherent strengths and weaknesses, which designers must accurately navigate. Data can be compared across labs with a fair degree of confidence by an experienced acoustical engineer, but inter-lab biases exist. It is important to note laboratory tests are generally performed on an assembly that has been carefully constructed for the purpose of testing. Lab tests provide a baseline for comparison on how an assembly should perform when the framing is done properly, the ceiling penetrations are isolated, and the sound control elements are correctly installed.

The scale is similar to the richter scale, an improvement of three points is 30 times more effective/quieter.
1 and 2 Minimum Code for AIIC is 45.

In contrast, field tests are reflective only of the conditions within the building in which they are installed. Different regions have differing methods of accounting for penetrations, furnishings, and other elements that can adjust the assembly’s performance. Depending on building conditions and the judgment of the sound engineer performing the test, field data can either closely mirror, or widely vary, from lab performance. Both types of data should be considered whenever possible in making material selections.

To prove raw polypropylene was superior to recycled, a series of field acoustical tests compared FIRM-FILL® SCM with a competitor’s sound mat (that contained 40 percent recycled material) of the same thickness. Hacker Industries, Inc. performed this series of testing twice—FIRM-FILL® SCM exceeded the competitor’s mat on both occasions (see table at right).

picture9With this proof-of-concept work completed, Hacker Industries, Inc. created systems addressing more of the building community’s needs. This involved careful consideration of how contractors and the trades function in the field. Spotting a 3-mm (1/8-in.) difference in the thickness of a sound mat is difficult on a typical jobsite. Therefore, the company chose to color-code its FIRM-FILL® SCM products:

  • blue (3 mm [1/8 in.]);
  • red (6 mm [1/4 in.]);
  • purple (10-mm [3/8-in.]); and
  • green (19-mm [¾-in.]).

The mats also correspond to color coding used to differentiate the compressive strength grades of FIRM-FILL® Brand Gypsum Concretes. Hacker Industries, Inc.’s laboratory testing indicated higher-psi gypsum underlayments need a thicker resilient sound mat to achieve the same acoustical performance as lower-psi gypsum underlayments.

This stands to reason, as higher-psi underlayments are denser and harder. Just as harder floorcoverings are less forgiving of impact noise, so are harder underlayments. A 2500-psi slab may only need 6 mm (1/4-in.) resilient sound mat to meet the IBC, while achieving the same performance in a 3500-psi slab or a 1500-psi slab might require 10 mm (3/8 in.) and 3 mm (1/8 in.), respectively. However, to maintain the same ASTM C627 performance, the thickness of those slabs must increase. The numbers in the table below illustrate the minimums for sound control mat thicknesses.

*400-mm (16-in.) joist span.

Gypsum-based underlayments bond easily to most substrates—this is a major advantage because they enable thinner, more cost-effective sound control systems and create a very durable connection between the slab and the backing of the sound mat. The previous generation of Portland cement-based systems were unbonded, or very weakly bonded with a primer coat, and needed thicknesses of 38 mm (1 ½ in.) to resist cracking. Per ASTM C109, Standard Test Method for Compressive Strength of Hydraulic Cement Mortars, lightweight Portland slabs require reinforcing with expanded metal lathe at transition areas even at those thicknesses. Best practices extend that reinforcing to sound mat areas. Gypsum-based concretes may be installed at 25 mm (1 in.) over a 6-mm (1/4-in.) sound mat per ASTM F2419, Standard Practice for Installation of Thick Poured Gypsum Concrete Underlayments and Preparation of the Surface to Receive Resilient Flooring.

Interestingly, gypsum-based concretes develop flexural strength rapidly until they reach about 2000 psi in compressive strength as measured by ASTM C472, Standard Test Methods for Physical Testing of Gypsum, Gypsum Plasters and Gypsum Concrete. Beyond 2000 psi, however, the additional compressive strength provides considerably less additional flexural strength. While reinforcing with metal lathe, nylon mesh or fiberglass may prevent a resulting crack from spreading, it cannot prevent it from forming. Therefore, the best course for high-end finishes is to place reinforcing material between both a thicker pour of FIRM-FILL® Gypsum Concrete and a thicker sound control mat.

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missing-chartThe chart at right shows the relationship between compressive and flexural strength.

The density of the underlayment slab can now begin to work against the designer. Gypsum underlayment above 2500 psi has a resonant frequency that limits the assembly’s aggregate IIC performance. This may be compensated for by increasing the void space under the gypsum concrete slab.

This is referred to as a ‘mass-spring-mass system,’ wherein a spring is placed between two masses to control both STC and IIC sound. It can be used in three places in a typical floor-ceiling assembly.

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Products placed under hard surfaces—such as closed cell foam or rubber pads—can be used between the floorcovering and the gypsum underlayment slab. Care must be taken to select a product that will retain its resiliency over time. A floor will be placed under load that is released countless times over its life. A mass-spring-mass assembly becomes less efficient when the spring fails to return to its original shape.

The second place a mass-spring-mass system can be included is between the gypsum concrete slab and the structural subfloor.

The third location is under the floor joist and above the gypsum board. There are several types of resilient channels that can provide isolation in this area. While analysis of resilient channels is beyond this article’s scope, it warrants mentioning proper installation of the channel is essential for both the effectiveness of that product and indeed the entire system. Assemblies for limiting structural-borne sound are highly interdependent—problems in one area can cause issues in others. Simply purchasing a sound-control product does not ensure success—proper design and installation of all components is essential.

Therefore, to achieve minimum sound code (“Entry Level”), a designer should expect to isolate in two of the three areas described above and conduct careful inspections to ensure the materials are installed properly. To achieve the “Market Rate Sound Level,” the designer should expect to isolate all three areas and take care to ensure the materials are installed properly. To attain the “Luxury Sound Level,” all three areas will need to be addressed, as well as taking further steps to reduce joist spacing to maximize the overall assembly’s structural stiffness.

Residential sound control has come a long way since thick shag and carpet pad. Through increasing builder and consumer demands since the 1970s, the sound control market has emerged. This emergent niche includes building material manufacturers, installers, acoustical engineers, and laboratories around the world.

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Remarkably, the minimum sound-control code standards established in the 1980s have yet to be revised to keep pace with materials, science, and testing advancements. Many industry professionals continue to pursue the science of sound control to produce better compressive strengths, harder surfaces, and smoother finishes capable of controlling IIC and STC sound more effectively. These innovations are already being incorporated into today’s luxury stacked flats, urban condos, and millennial micro-residences.

To transform the acoustic industry further, the key is collaboration: between acoustical trades, engineers, builders, developers, and laboratories. Many advocate to incorporate sound control experts into the project team earlier, during design, to ward off and save thousands of dollars later in the construction cycle. Collaborative research and development among trades will generate further advancements, promote healthy competition, and catapult acoustical achievement that will not be achievable through any one firm’s measures. Hacker Industries, Inc. regularly participates in floor assembly R&D with fellow innovators. For those looking to be part of the industry’s innovations, inquiries are welcomed and encouraged.

All information listed in this section was submitted by Hacker Industries, Inc.
Kenilworth Media Inc. cannot assume responsibility for errors of relevance,
fact or omission. The publisher does not endorse any products featured in this article.