Most designers are familiar with the NRC (Noise Reduction Coefficient) standard. Most equate materials with high NRC values with good acoustic design. Surprisingly most acoustical absorbers, even those with high NRC values, are not good for music environments. In fact, some products can leave a room sounding unbalanced and worse than if no acoustic treatment was used at all.
Non music rated panels over absorb in higher frequencies eliminating the brightness that makes a music room or venue perform properly. Music Rated panels work in symphony, requiring the right mixture to better the acoustics of the space.
While sufficient for frequencies associated with conversation, NRC ratings by themselves do not provide the data needed when designing for music spaces.
The NRC standard collects the absorption coefficient from four frequencies and averages them. Coefficients can be considered a percentage of the sound that is absorbed at that frequency written in decimal form.
Very Narrow Frequencies
Humans can hear between 20 and 20,000 Hz. A standard piano ranges from 27.5 to 4,186 Hz. NRC is the average of just four frequencies in a very narrow band- specifically 250, 500, 1000 and 2,000 Hz.
Interesting Averaging The NRC rating is calculated by averaging the absorption coefficients. Due to some oddities in the standard and testing rooms, results can return absorption coefficients that are above 1.00
While the NRC numbers can be quite different, the actual performance of the panels at the NRC frequencies are identical.
Apples-To-Apples Mounting The ASTM standard for NRC allows for a wide array of testing setups from basic wall panels to hanging baffles. Always check the test results to verify how the panels were tested. Anything other than an “A” mount can return unusually high results.
Can “low” NRC numbers be better than high numbers? Yes! For example, products that are tuned for low frequency or even absorption will not test as well as non-music rated panels. This can be a very good thing if the room requires it.
T60 or RT60 is the amount of time it takes sound to decay 60 decibels. Widely available reference materials offer a single T60 time for most venues such as classrooms, performing art centers, and movie theaters For music spaces, the T60 times of a wide range of frequencies should be considered, a single number doesn’t help design for the type of music or space. Symphony halls should be designed differently than rock venues.
Building materials such as concrete block, steel decking, and drywall offer a small amount of absorption across a wide frequency range. Room finishes such as carpet, curtains, open weave upholstery, and even people absorb a significant amount of high frequency sound. Adding standard fiberglass, PET, or foam absorbers to these spaces will remove all the easily-treated high frequencies out of the room while leaving behind booming low frequencies. In this way, adding acoustic treatment will make a room sound worse!
To properly design for music the full frequency of sound needs to be considered. Start with the building materials, finished goods, and people using the space to create a baseline for the type of frequencies that need to be treated. Most rooms will require a mix of different type of music-rated absorbers such as our TAD, VersaTune, and VTLF panels. Even wood absorbers offer great treatment for music spaces. Lastly, the T60 time (see above) will determine the proper amount of square footage of treatment.