When numbers lie: Interpreting IIC ratings for flooring underlayment performance

Figure 2: Rigid mats have the ability to decouple the surfaces with a thinner profile while providing higher compressive strength.
Figure 2: Rigid mats have the ability to decouple the surfaces with a thinner profile while providing higher compressive strength.

IIC is a measurement of an assembly’s ability to mitigate impacts through physical contact, such as footfalls or dropped furniture. IIC tests can be accomplished using a tapping machine that provides a series of calibrated impacts to the specified assembly. The SPL of these impacts is measured in the room with the machine and in the space below using calibrated meters. STC ratings are obtained by measuring the difference in SPL over 16 frequencies between the test room and the unit below.

Each of these tests attempts to cover a broad range of the most relevant data available for each sound class. While the tests provide useful date when considering the capabilities of a sound attenuating mat, they have some limitations. Since the human ear can detect frequencies outside of the range of these tests, they are unable to present a complete picture encompassing real-world performance.

While many people assume the resulting ratings for each of these refer to an actual drop in decibels, it is more complicated than that, and this article acknowledges its risk of oversimplifying the issues. For both STC and IIC test results, a higher rating often shows improved performance. However, since the rating is essentially an average over the 16 frequency points tested, a look at the actual test data may reveal significant deficiencies in a particular frequency range. In fact, a product can perform exceptionally well in one range, poorly in another, and still end up with a better IIC rating than a competing material that is that may have performed better in a frequency range more relevant to the project’s requirements.

Properly installed sound mat covered with a layer of gypsum underlayment provides long-term benefits for property owners and residents.
Properly installed sound mat covered with a layer of gypsum underlayment provides long-term benefits for property owners and residents.

The products in Figures 1 and 2 have both achieved an IIC rating of 52. While performance may seem similar at first glance, Figure 3 reveals the assembly using a rigid mat is performing consistently better at almost every frequency tested. The frequencies below 250 Hz are even more telling as these frequencies are testing almost 5 dB of pressure lower than the sound mat tested in Figure 4. This result clearly illustrates what the authors believe to be the primary shortcoming of current IIC test standards. In their paper “A dual-rating method for evaluating impact noise isolation of floor-ceiling assemblies,” John Loverde and Wayland Dong discuss the need to create an IIC testing model to address two separate frequency ranges, a solution that would allow for clearer pictures of actual test results. The authors of this article agree with this assessment as it would do much to clarify the expected performance of products for specifiers.

What is not shown in either graph, but can also significantly impact the results, are the full details of the assembly utilized to test each sound mat. Companies are quick to share the subfloor, truss assembly, channel, and gypsum board employed during testing but may not be sharing other methods used to increase the ratings. These can also include the use of non-standard insulation between the subfloor and ceiling, a high-performance pad under soft floorcoverings, or a vinyl plank floor with an attached underlayment. While these system modifications improve the performance of acoustical treatments, one now needs to specify identical products on a project in the hope of achieving the manufacturers’ published ratings.

As a best practice, all of these factors are usually considered when comparing products. It is advisable to sort through the misleading ratings by gaining access to the actual sound test data from the manufacturers showing the IIC contour line on the transmission loss graph.

The performance in relation to the IIC contour line reveals the consistency of the product over the most common frequencies as well as any deficiencies. Interestingly, standards in IIC testing change frequently, and tests performed before 1999 may not produce identical results if conducted today, so it is always a good practice to confirm the dates as well. Accurate data is derived from testing performed to ASTM E 90-09, Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements, for STC and E 492-09, Standard Test Method for Laboratory Measurement of Impact Sound Transmission Through Floor-ceiling Assemblies Using the Tapping Machine, for IIC.

When combined, these two ratings provide a snapshot of the acoustical performance of the tested assembly in a controlled environment. While these tests provide a standardized method to evaluate performance, the results alone do not tell the whole story. Understanding how to read the test data reveals the whole picture, allowing one to make an informed decision with a clearer picture of real-world performance.

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