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Let’s Talk About NRC

Published On: April 2, 2021Tags: , , ,

Large Room Acoustics

This week we take a step out of our relatively small listening rooms and studios and think about the treatment of larger spaces. Many of us work or spend time in “big” rooms like conference centers, restaurants, auditoriums, or houses of worship (or we will again soon, hopefully!). It’s worthwhile to understand a thing or two about how to make those rooms sound good–who knows, maybe you will be recruited to help make decisions about one of them!

And of course, for the big jobs in which armchair acoustician work just won’t cut it, give ASC a call.

Enjoy Art Noxon’s original article in its entirety here!

What is NRC?

NRC stands for noise reduction coefficient. Specifiers and architects like to keep things fairly simple and use a single number to express the complicated properties of various sound absorbers. But, first things first: how do we quantify sound absorption?

We use units of sabins, which are the sonic equivalent of a 12″ x 12″ open window through which sound leaves and does not return. The “absorption coefficient” indicates the fraction of sound energy absorbed by a surface, with a coefficient of 1.0 acting just like an open window, and a coefficient of 0.01 absorbing almost no sound energy.

To determine the NRC, an average value is calculated from the absorption coefficients at 250, 500, 1000, and 2000 Hz. One sample absorption data set* is shown below, with the associated NRC calculation.

For the El-800 (2″ thick fiberglass), the NRC is found by adding 0.64 + 1.08 + 1.13 + 1.06 = 3.9. Then divide by 4, and round to 1.00. Voila!

*no, values higher than 1.0 are not erroneous!

Tonal Limitations of NRC

An NRC rating does not take into account the highest or lowest octaves of speech. Human speech covers a wide range of frequencies, starting on the low end with humming tones and reaching the highest frequencies with clicking sounds. Voiced vowel sounds and fricative hissing sounds cover the range in between, and are the most important for intelligibility, and therefore communication.

Much of the musical range is outside of this. While important to realize, that is not the topic at hand today; we are talking about speech intelligibility in large spaces.

Using NRC to Estimate the Amount of Acoustics Needed

In a large room, with many people speaking at the same time, or even with just one amplified voice emanating from multiple loudspeakers, the sound can quickly devolve into a loud, murky hubbub of reverberation that renders communication extremely difficult. Common sense may indicate that sound absorption is needed to improve intelligibility.

But how much of what material? To answer, let’s look at an example and do some easy math.

A typical large room might be 80 feet square and 40 feet tall, for about 25,000 square feet of surface area and a volume of 256,000 cubic feet. The painted block walls may have an NRC of 0.05, and the floor might be carpeted with an NRC of 0.3. The painted drywall ceiling also has an NRC of about 0.05.

  • Walls = 12,800 square feet * 0.05 = 640 sabins
  • Floor = 6,400 square feet * 0.3 = 1920 sabins
  • Ceiling = 6,400 square feet * 0.05 = 320 sabins
    • Total sabins in room from surface absorption = 2,880

Now let’s say 200 soft chairs are in the room, providing 3 sabins each, along with 200 people providing 4 sabins each. That gives us another 1,400 sabins. Don’t underestimate the effect of occupants!

Total sabins in room (prior to acoustic treatment) = 2,880 + 1,400 = 4,280

In a room this size, about 30 sabins per person will deliver a good ratio (about 10dB) of direct signal to reverberant noise—exactly what we are looking for to render intelligibility usable. With an expected occupancy of 200 people, a total of 6,000 sabins are required.

So, we have a deficit of about 1,700 sabins. In order to provide this, we could install 1,700 square feet of the El-800 fiberglass used in the example above. Where does it all go? You’ll have to wait for another installment for that tip! (hint: distribute it!)

The ASC SoundPlank: Your Building Block for Large Room Acoustic Control

The standard ASC SoundPlank provides 3.8 sabins each with a frontal surface area of 2.5 square feet. The performance improvement from “edge absorption” can be significant! About 450 of these units within the example room discussed would create a very intelligible environment.

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