Room Acoustic Ratings

For interior room acoustics, key metrics are reverberation time and absorption.

Reverberation. When a person talks in a room, the sound of their voice travels through air. A portion of that sound strikes the room surfaces and is reflected back as reverberation. A long reverberation time and many reflections contribute to degraded speech intelligibility.

The time it takes for the sound to drop in energy, or “decay” to a specific sound level, is a key design factor for building spaces. Auditoriums, gymnasiums, lecture halls, theaters, and other performance areas are all designed with specific reverberation times to meet the expectations for the activities that will be performed there. Designers need to understand that with an increase in demand for certain materials, such as tile, marble, and stone, even small spaces may have intense reverberation that must be acoustically managed.

RT60. The standard measure for reverberation time is RT60—the time required for sound to decay 60 dB. Although reverberation time is frequently stated as a single value, it is typically measured in bands of different frequencies. The human ear is sensitive to frequencies from 20 Hertz (Hz) to 20,000 Hz. This broad range is typically broken into octave bands, each doubling the previous one. In the range of human hearing, the octave bands are broken down with these center frequencies: 31.5, 63, 125, 250, 500, 1,000, 2,000, 4,000, 8,000 and 16,000 Hz. Designers typically focus on a specific range of bands, depending on the performance requirements.

Sound absorption. When a sound wave strikes a surface, the amount of sound absorbed into that surface material will depend on its composition. The absorbed sound causes the particles of the absorbing material to vibrate, and this vibration creates tiny amounts of heat due to the friction. This conversion to heat energy dissipates the sound energy. Fibrous materials are efficient absorbers. The sound absorbing characteristics of any finish material varies significantly with frequency. In general, high-frequency sounds, which have small wavelengths, are more easily absorbed than low-frequency sounds, because the wavelength is larger than the absorptive material.

Additionally, the thicker a material is, the better it will generally be at absorbing sound. The acoustical performance of a porous surface material can often be enhanced by incorporating an air space behind the surface to improve the absorption of low-frequency sounds.

NRC. How absorptive a material is can be rated using a standard test method described in ASTM C423. This test defines the Noise Reduction Coefficient (NRC) for a given material, which is an average of results demonstrating the change in the amount of sound energy recorded at different frequencies during the test. An NRC of 0 indicates perfect reflection; an NRC of 1 indicates perfect absorption.

ASTM C423 uses a reverberation room test method, in which sensitive recording equipment is used to measure the decay rates in a room before and after the specimen is installed. The final result of the calculations is reported as sound absorption coefficients (“alphas”) at octave band center frequencies from 125 Hz to 5,000 Hz. For a convenient, single-number rating, the alphas in different ranges may be averaged to give a Sound Absorption Average (SAA) or Noise Reduction Coefficient (NRC). The higher the number, the more sound is absorbed.

Sound Transmission Ratings

To travel, sound needs a source, a path, and a receptor. The path is the only factor a building designer can control. Sound, like water, seeks the path of least resistance. Airborne sound between rooms travels most easily through gaps, doors, windows, and solid walls—in that order. Most barriers contain all of these sound paths.

Sound Transmission Class (STC). An STC rating is a single-number rating of a partition's ability to resist airborne sound transfer in one-third octave bands with counter frequencies from 100 to 5,000 Hz. The higher the STC rating, the more sound a partition will block. When evaluating the transmission of an airborne sound, such as the voice of a neighbor or their television, an STC rating provides a useful way to rate the effectiveness of a sound barrier. (See Table 2 for a general comparison of different STC ratings.) An STC rating is not suitable for evaluating acoustical performance in all applications, however. Because the frequency range is limited, different acoustical analyses should be used when designing mechanical equipment rooms, large conference rooms, recording studios, and music practice and performance spaces.

Note: This table provides a general indication of what can be heard on the other side of a partition wall. It is assumed that there it is a quiet 30 dB of background noise on the “listening” side.

An STC rating is derived by a laboratory test and calculation (per ASTM E90 and E413). This test involves a standardized sound source in the source room and a microphone system on the other side of the partition to be tested in the receiving room. Measurements are taken in specified frequency bands in the receiving room. Those measurements are then used to calculate a single number rating. The higher the number, the higher the resistance the building assembly has to airborne sound transmission.

In addition, an STC rating may be derived for a specific building partition using a field test (FSTC per ASTM E336 and E413). This field test uses the same general testing methods in the actual building after construction is completed.

There are two key things to understand about STC ratings: they only refer to airborne sound, and they only apply to the tested partition, not an individual element of the tested assembly.

Also keep in mind that while a partition might have a high STC rating, there are many ways airborne sound can travel. Through the building assembly is only one path. However, there are numerous flanking paths, including the air path over walls and through ductwork, under doors, through outlet penetrations (especially back-to-back outlets in a wall), open vents, louvered doors, and wall/floor connections to name a few. Certainly, one of the best ways to reduce sound transmission is by filling open gaps in building assemblies.

Impact Insulation Class (IIC). While STC ratings evaluate the transfer of airborne sound, IIC ratings evaluate the transfer of structure-borne sounds through floor-ceiling assemblies. These sounds result from impacts, such as someone walking in high heels, dropping items on a floor, or sliding heavy objects. As with an STC rating, the higher the IIC value of a floor-ceiling assembly, the better its ability to control impact sound transmission.

Like an STC rating, IIC results are derived from standardized tests: ASTM E492 details the lab test; ASTM E1007 for the field test and the single number ratings are calculated per E989. These tests are similar to STC testing. The difference is that the sound source in the source room or upper chamber is a tapping machine that impacts the floor with several hammers.

Here is an example of the effect different materials can have on the IIC rating of an assembly: A floor constructed with an 8-inch concrete slab attains an IIC rating of 32. Adding an acoustic underlayment and finished floor (hardwood) may improve the rating to 54.

Here, too, the important thing to remember about an IIC rating is it only describes the entire tested building assembly, not a single material.

ΔIIC. The “Delta IIC” test is another test that can be used to compare the impact insulation class characteristics of individual materials. This test is detailed in ASTM E2179 and consists of two IIC tests conducted over the same concrete slab that is 6-inches (150mm) thick. One test is over the bare concrete subfloor (without any flooring materials) to define a base value for the assembly. This actual assembly should be close to an IIC 28, the reference floor defined in the ASTM standard. Then, another test is conducted over the concrete subfloor with a small sample of the floor covering materials and any proposed underlayments. The ΔIIC, or improvement of impact sound insulation, is obtained by subtracting 28 (the reference floor) from the adjusted IIC of the whole assembly.

The key thing to remember is that ΔIIC measures the effectiveness of the floor covering in reducing impact sound transmission through 6-inch concrete floors and only tests a small specimen, not an entire floor. As with any IIC test, the results pertain only to the assembly. It can be used as a basis for comparison when evaluating one material over another, but should not be used beyond that.

Detailed description of assembly required. When evaluating an STC or IIC report, designers need to pay attention to how the report is detailed. Remember that IIC and STC tests (both lab and field) are not tests of single components, but evaluations of entire floor-ceiling assemblies. A report for any of these sound tests should include a detailed description of the floor-ceiling assembly used in the test. Without accounting for the entire assembly, the results are meaningless. Using IIC and STC results to represent the sound deadening ability of an underlayment without describing, in detail, the whole floor-ceiling assembly causes confusion at all levels.

A 5-mm-thick recycled rubber underlayment with 19 dB—more than most flooring underlayment materials. Recycled rubber provides excellent resilience and does not age or harden. Photo courtesy of ECOSilence