Introduction
In this blog, we will talk about what is NRC, and what benefits (or lack of benefits) this measure has to offer.
Noise Reduction Coefficient (NRC) is defined as the average of the absorption coefficients at the 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz octave bands. 125 Hz and below, and 4000 Hz and above octave bands are ignored in this calculation.
How Sound works
Let’s address what is absorption coefficient, and what are octave bands.
To do that, let’s understand how sound works. We know that sound travels in a medium (learnt that in our schools yeah?). Sound waves travel by moving the air particles. So, let’s say one particle moves, or vibrates actually, causing subsequent air particles to vibrate and so on. There is always some energy loss as this vibration moves forward.
When these particles meet another surface, which is a different medium, there will be some interaction. For the sake of simplicity, let’s assume that a perfectly rigid medium comes in the pathway of sound waves. The sound waves, or the vibrating air particles, will hit the rigid surface and bounce back. This is how sound is reflected. Since surfaces are not perfectly rigid, the actual interaction looks a bit different.
The incident energy gets transmitted, absorbed, and reflected. That means the moving air particle will influence the surface particles. The hitting particles will push some particles off the surface. Those particles will then similarly influence adjacent particles, some will move only inside the surface and lose their energy while some will influence particles beyond the surface and this energy will get transmitted. Some particles will definitely bounce back but now with reduced energy. The ratio of the difference between the incident energy and the reduced reflected energy by the incident energy is the ABSORPTION COEFFICIENT of sound. If the difference between the incident energy and the reflected energy is zero, the sound is completely reflected and the absorption coefficient is zero. If the difference between the incident energy and the reflected energy equals the incident energy, the sound was entirely absorbed and the absorption coefficient is 1.
An easier way to understand this (maybe) is through tennis courts. Let’s talk about tennis for a while. The great Rafael Nadal is considered the king of clay courts. The great Pete Sampras has never won the French Open (I think… not an expert there). They are both great players but their style of play is very different. Clay courts are the slowest courts, the ball loses its speed after bouncing. While on grass courts, the ball does not lose its speed as much, but it also does not spin as much. And obviously, hard courts are the fastest. Similarly, air reflects differently on different surfaces.
An even better example is there in the game of cricket. Almost all of us have played cricket with rubber, cork, and leather balls. Have you tried bowling a leather ball on a grass pitch? It will just not bounce at all. To bounce properly, it needs a really hard surface. But the rubber balls will bounce even on a pitch of grass. Their “coefficients of bouncing” so to speak, are very different.
Imagine that you had to play a professional cricket match but the curators said we will have a ground and pitch all covered with the same grass. That game cannot be played with a leather ball. You need a different surface for the pitch so that the leather ball can bounce. Similarly, you cannot have a material that only absorbs the high frequencies and does nothing to the low frequencies in the context of auditorium acoustics. To understand why there has to be a balance in the absorption, do read the blog about "How to improve speech clarity in an auditorium."
I do not know the motivation for introducing NRC as a measure. I did not care to find who introduced it. Another issue I have with it is the name, which is the most misleading of the things. Whereas it is a measure related to the absorption of sound in a space, it does not guarantee that noise will be reduced. This unnecessarily misleads people into designing incorrect acoustics for their auditoriums or spaces.
Now that you know what NRC is, you can put it to better use to deal with the material suppliers and ask for a frequency distribution of the absorption coefficients. As an architect or a designer, you are no longer at the mercy of the material suppliers and now can make an informed decision about which materials to use while designing auditorium acoustics.
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