Which wall assemblies offer the best residential sound isolation?

A new study reveals key findings

A study by AIA partner PABCO Gypsum examines the impact of constrained layer damping on sound transmission and sound isolation, critical details as homes transform into offices and entertainment spaces.

Toy house with giant yellow headphones around it

The importance of sound control in single-family residences is growing. Spaces in homes are increasingly serving as offices, exercise rooms, and theaters, resulting in greater importance of quiet and privacy. This has increased the need for sound isolation in the home.

There are several ways to improve sound isolation in the home, including mass, absorption, decoupling, and damping. Historically, little sound isolation testing has been completed to evaluate how these various sound control options perform in single-family residence assembly designs. PABCO Gypsum executed an extensive research study to quantify this information for the benefit of residential building construction professionals.

Improving sound isolation in homes

Sound control treatments commonly used in homes are categorized into four general types: mass, absorption, decoupling, and damping.

  • Mass: Adding gypsum layers to increase the overall mass of the partition
  • Absorption: Adding insulation to the wall cavity such as fiberglass or stone wool insulation
  • Decoupling: Acoustically isolating the gypsum wallboard from the building structure by adding fiberboard as an intermediary layer
  • Damping: Minimizing sound vibration through the wall through specialty panel products such as QuietRock Constrained Layer Damped (CLD) gypsum panels, multilayer composite panels comprised of gypsum wallboard and viscoelastic polymers

Fundamentals of building sound isolation

Sound transmission loss (STL) is currently the most widely known and in-depth measurement of sound isolation in buildings throughout North America. STL is measured in a laboratory by creating sound with loudspeakers in one room (source room), measuring this noise with microphones, and then measuring the sound transmitted through a partition sample in an adjoining room (receiving room), also with microphones.

The measured receiving room sound levels are essentially subtracted from the measured source room sound levels. The resulting dB STL values are then provided in 1/3-octave band frequencies from 125 Hz to 4,000 Hz. These STL values can be used to calculate the single-number STC (sound transmission class) rating using a contour curve-fit method.

Sound transmission line chart

Calculation of sound transmission class (STC) from sound transmission loss (STL) data. The measurement frequencies in Hertz in 1/3-octave bands are shown on the horizontal axis, and the STL values in decibels are shown on the vertical axis.

Test methodology

STL testing in this study was completed in accordance with the ASTM E90-09 (2016) Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements. North Orbit Acoustical Laboratory, a third-party accredited laboratory located in Dyersville, Iowa, conducted the test in May 2020.

All tests were conducted on nominal 2×4 single wood framing with studs spaced 16” on center, the wood framing assembly commonly used in the construction of single-family homes. The testing looked at nine different wall assembly configurations (see Figure 2). In addition, three cavity types were installed in each assembly: no insulation, 3½” R-13 fiberglass insulation, and 3” stone wool unfaced insulation.

diagram of various tested wall assemblies

The nine wall assemblies used for this study. Key: GWB = Gypsum Wallboard, FB = Fiberboard, CLD = Constrained Layer Damped panel.

Test results

  • Mass: Adding a second and third layer of wallboard onto the base assembly (one layer of gypsum wallboard on both sides) resulted in a 4-STC-point increase in performance and a 3- to 8-dB increase in STL over the frequency range reported by the laboratory.
  • Absorption: Adding fiberglass or stone wool insulation to the wall cavity resulted in a 3-STC-point increase—slightly less than the addition of mass. The effect of the insulation was different across the measured frequencies—little to no improvement in STL at low frequency, a large improvement in the lower mid-frequency range of 10 dB, and then a steady 5-dB improvement throughout the higher frequencies measured. No statistically significant difference was found between the fiberglass and stone wool insulation types.
  • Decoupling: Installing the gypsum wallboard over a decoupling fiberboard panel increased the STC rating by 6 points. The STL performed consistently 5 dB better than the baseline GWB until 1,000 Hz, at which point the STL difference gradually increased to 15 dB at the high-frequency range.
  • Damping: The study tested two different configurations of damping—retrofit (where the constrained layer damped panel was installed over the existing GWB and direct-attached, as would be the case for new construction. The retrofit damped configurations resulted in a 7- to 12-STC point increase versus the baseline GWB performance. The STL difference varied with the largest improvement in the high-frequency range, upwards of 25+ dB difference. The direct-attached CLD configurations resulted in a similar increase in both STC and dB STL difference from the baseline assembly.


The difference in STC rating between the baseline GWB partition without insulation and every other treatment—mass, absorption, decoupling, and damping—is shown in Figure 3. The most marked improvements in performance occurred when the QuietRock Constrained Layer Damped Panels were added to the assembly.

Bar chart about the treated wall assembly

Difference in STC rating between mass, absorption, decoupling, and damping as compared to the single-layer gypsum wallboard assembly with no insulation. The test configurations are shown on the horizontal axis and the ∆STC values are shown on the vertical axis. Solid bars indicate the presence of insulation and dashed bars indicate no insulation present.

Note: The test report numbers used in the study included NOAL 20-0501, 20-0506, 20-0514, 20-0502, 20-0503, 20-0530, 20-0509, 20-0510, 20-0520, 20-0521, and 20-0522.

Hear for yourself the difference QuietRock Constrained Layer Damped panels can make when controlling sound in residential construction retrofit applications and new construction applications. Visit www.quietrock.com for more information on how constrained layer damping can help you achieve the STC results you need using less space, less material, and less labor than conventional alternatives.

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