What’s the deal with acoustic fabrics? Chances are you encounter them more frequently than you realize, and they likely make a big impact on your day-to-day experiences. Whether you’re in an office, classroom, performing arts center, or an arena, the success of the space depends on your ability to hear clearly. But not all fabrics perform the same acoustically. Henderson Engineers recently tested a variety of acoustic fabrics to see how they stack up.

NOISE REDUCTION COEFFICIENT (NRC) Most fabric manufacturers market acoustics as one of the primary benefits of their product. In doing so, they sometimes reference the fabric’s Noise Reduction Coefficient (NRC) rating. NRC is a single number rating which indicates a material’s ability to absorb the sound energy that strikes upon it. The NRC rating ranges from 0.0 to 1.0, with the number representing the percentage of sound energy the material absorbs (i.e., NRC-0.0 indicates 0% sound absorption, while NRC-1.0 is 100% sound absorption). The rating is an arithmetic average of the material’s sound absorption coefficients in the four, center-octave band frequencies from 250 Hz – 2,000 Hz. This is the range in which human speech occurs, but overall it is a very small portion of the full range of human hearing, which is normally between 20 Hz – 20,000 Hz.
We have noticed a recent uptick in the selection of fabrics comprised of polyethylene. This type of plastic can be beneficial for non-acoustical considerations such as color, design pattern, light reflectance, durability, and cleanability, and at first glance its NRC rating indicates it to be a perfectly acceptable fabric to use in acoustical wall panel or loudspeaker covering applications. Below is detailed one-third octave band test data for a polyethylene fabric applied over 1” thick fiberglass insulation (a typical acoustical wall panel). As shown by the bar graph, the sound absorption coefficient of the fabric is good from approximately 200 Hz – 800 Hz, but begins to significantly decrease starting at 1,000 Hz, and becomes essentially reflective in the higher frequencies. However, based on the NRC testing standard, this fabric achieves a rating of NRC-0.9 and is marketed as “acoustic.” In reality, applying these panels in real-world applications would likely result in a room with very poor speech intelligibility since there would be a lack of sufficient sound absorption above 1,000 Hz. If this material was used as a loudspeaker cover, the sound system would have to be heavily equalized which would hinder sound system power capabilities. Because of this, NRC is not a particularly good indicator of a fabric’s acoustical performance.

ACOUSTICAL TRANSPARENCY
A much better assessment is to gauge the fabric’s acoustical transparency. When fabric is used in acoustical design applications, it is much more important for the fabric to be acoustically “invisible” so sound energy can either be absorbed by the acoustical material it’s covering (such as the fiberglass core in an acoustical wall panel), or so that loudspeaker energy can pass through the fabric with little to no degradation in fidelity. Recently, we tested a polyethylene fabric in our test lab to assess its acoustical transparency.
The Y-axis indicates decibels (dB) and the X-axis shows one-third octave band frequencies. Using pink noise as a test signal, data showing 0.0 dB indicates sound energy is passing through the material at 100% transparency. The negative values in the high-frequencies indicate significant sound degradation in the polyethylene fabric. Two important items can be drawn from this test:
1. If this fabric was to be used as a loudspeaker cover, there would be essentially no sound passing through from 2,000 Hz onwards (note the 23 dB loss at 5,000 Hz!).
2. If the fabric were used as an acoustical wall panel, the sound absorbing panel would actually become a sound reflecting panel in the high-frequencies.

CONCLUSION
When designing spaces where acoustics and sound is a concern, there is much more to selecting the right type of fabric than a single-number NRC rating. We always recommend requesting the laboratory test report for any acoustic material so it can be reviewed by the project’s acoustical consultant. If a test report does not exist, or if the report indicates nothing more than an NRC rating, then it is strongly recommended to obtain a product sample for its own technical assessment by the design team. Please contact Henderson with any questions when designing spaces where optimum acoustical performance is a necessity for the overall success of the project.