Classroom acoustics: It’s more complex than NRC

If you’re an architect designing educational facilities, you’ll know that reducing noise, both the background noise and the reverberation noise, is a key issue in the design of lecture halls and classrooms.

But achieving the best acoustic environment is a complex field that requires professional planning, and it involves more than simply choosing products with the highest sound absorption ratings.

Poor acoustics and a noisy environment in classrooms can lead to a wide array of problems. For the learner it can affect their listening comprehension, higher order cognitive function and speech intelligibility, language development and development of English as a second language. 

A noisy classroom is harmful to teachers too and can lead to a variety of health concerns.¹

(Read more in our blog 4 Health Risks of teaching in a noisy classroom.)

To add to the problem, open-plan, collaborative learning spaces (often referred to as Innovative Learning Environments) are on the rise in Australian schools, and while this design trend has advantages for students’ social development and teamwork, open-plan classrooms tend to be noisy.

Read more in our blog Open plan classrooms are popular, but what about the noise?

What are the standards?

The Building Code of Australia (BCA) details the minimum sound insulation requirements of walls and ceilings between neighbours, but it does not provide acoustic standards for educational facilities.

According to the Association of Australian Acoustical Consultants (AAAC) there are no Australia-wide regulations or standards that encompass all aspects of the acoustical qualities of educational and training facilities. The Australian/ New Zealand standard AS/NZS 2107 provides recommendations regarding design sound levels and reverberation times inside classrooms.

The AAAC has released a document, the AAAC Guidelines for Educational Facilities to improve the acoustic environment in educational facilities with particular emphasis on classroom acoustics. Room acoustics is now regarded internationally as one of the prime considerations in the design of new classroom facilities.

Calculating acoustics

Securing good indoor acoustics is a complex field that requires multiple acoustic descriptors and calculations. They include Noise Reduction Coefficient (NRC), Weighted Sound Absorption Coefficient (Alpha W or α_w) and reverberation time (RT).

One common way to calculate acoustics is to take an average of how a material absorbs and reflects different sound waves (under laboratory conditions).  This average is known as the Noise Reduction Coefficient (NRC).

In simple terms, an NRC is a number that rates how effectively a material absorbs sound. It is calculated by averaging out a material’s sound absorption coefficients at four typical octave band frequencies ranging from 250 Hz to 2000 Hz.

NRC is measured on a scale from from 0 to 1. An NRC of 0 means that the product absorbs no sound or reflects all sound. An NRC of 1 means that it absorbs all sound or reflects no sound.

The higher the NRC, the better the material is at absorbing (or not reflecting) sound in a room. An acoustic product with a 0.95 NRC rating means that 95% of sound in the space is absorbed, while the other 5% is reflected.

NRC is replaced by the Sound Absorption Average (SAA) in the American Standard ASTM C423 - 2017, which represents more measurement values. SAA is calculated by averaging a material’s sound absorption coefficients at twelve one-third octave band frequencies ranging from 200 Hz to 2500 Hz.

A more sophisticated way to measure acoustic performance is to calculate what is called a weighted sound absorption coefficient (α_w).

Like the NRC or SAA, it is also calculated as a single number, but is considered to be more representative of how the human ear interprets sound. That’s because it is calculated by comparing sound absorption coefficients to a standard curve.

While it’s a more complicated calculation, it gives a better picture of a material’s performance across all of the important frequencies ranging from 200 Hz to 5000 Hz. Unlike arithmetic average ratings like NRC or SAA, α_w is governed by the lowest performing value in the frequency range of concern.

The higher the α_w figure, the more evenly a material absorbs sound across all of the important frequencies. It has become the preferred European unit for making comparisons of sound absorption performance.

Reverberation time (RT)

In an enclosed space, sound gets reflected many times from hard and smooth surfaces and creates a reverberation. In other words, the sound persists long after its source ceased generating it. Sounds reflected from multiple surfaces also add up to increase the noise (decibel) level in the room.

In an enclosed space, for a sound of a given frequency or frequency band, the time that would be required for the reverberantly decaying sound pressure level in the space to decrease by 60 decibels is called reverberation time, represented by RT (or RT₆₀) in seconds.

Also read our blog Quick and easy way to estimate reverberation time.

Achieving good acoustics

Achieving good acoustics is not as simple as satisfying one or even a combination of these (and other) descriptors. Simply choosing products with the highest NRC rating, for example, might not achieve the desired effect if other factors, such as the best placement of products, don’t form part of the planning.

The size of the room, the diffusion of sound and the location of sound absorbing panels are also important when planning acoustical treatment. The room occupancy rate during use is another variable that has to be considered.

German study

In a recent comprehensive study ¹ undertaken in Germany together with the Fraunhofer IBP in Stuttgart, measurements were performed with different surface treatments in a model classroom.

The results of the tests showed that an acoustical suspended ceiling is generally very helpful in bringing sufficient sound absorption into the room, but the α_w (or NRC) rating thereof is not the only consideration to keep in mind.

According to the study a highly absorbing acoustical product can give the impression that the requirements in terms of the reverberation time are achieved, when this is not the case.

The calculation of the absorption capacity of an acoustical product assumes a diffuse sound field in the room, which is often not present with a single acoustical treatment. The higher absorption potential of products with higher α_w and/or NRC ratings is therefore not utilised when the real sound field in the room is not sufficiently diffuse, and they will not achieve the required results.

Furthermore, it should be noted that a very high level of sound absorption in the ceiling can lead to “excessive damping” of the room when many people occupy it.

Conclusion

The study concludes that it is advised that an acoustic consultant should be involved from the planning phase to achieve acoustic comfort in an educational setting.