Research on Noise Reduction Coefficient (NRC)

What is NRC?

Let’s understand what do we mean by Noise Reduction Coefficient, also NRC.
NRC basically, is a single number which ranges from 0.0 to 1, which describes the sound absorption capacity of an acoustic material. The higher the NRC, the better the sound absorption capacity of that material. If the NRC value is 0.0, it basically means the material does not attenuate mid frequency sound and is mostly likely to reflect the sound back. In short, an NRC of 0 value indicates perfect reflection while NRC of unity indicates perfect absorption.

NRC=0
At 0 NRC, there is no sound absorption. The sound bounces off of a smooth surface and back into the room, as shown by the red waves.

NRC=0.5
At 0.5 NRC, only 50% of the sound is being absorbed by the acoustic product, while the other 50% is being reflected back into the room.

NRC=1.0
At 1 NRC, 100% of the sound is being absorbed by the acoustic product, and no sound is being reflected back into the room.

What objects have the most NRC?
Let’s say in general, objects which are soft, pliable or porous like (cloths) serves as good acoustic insulators i.e absorbing most sound where as, dense, hard, impenetrable materials like metals reflect most.

How do we calculate NRC?
Now every product we choose has an octave band frequency, we can calculate the NRC by seeing the absorption coefficient i.e the absorption capacity at 4 different frequencies in Octave Band Frequency.
The frequencies are as follows;
1- 250 Hz
2- 500 Hz
3- 1000 Hz
4- 2000 Hz
Once we get an average of these frequencies on octave band, we get the value of NRC.
Note: These are not a concrete information but the best informations provided on internet and it gives an approximate number on how much of a product to be used in a particular room.

Timber Acoustic deals with production of panels of pinewood, sheesham and roxul rockwool.
Let’s have a look at NRC values of various products.
Abbreviations: OBFV – Octave Band Frequency Value

 

Products At 250 Hz At 500 Hz At 1000 Hz At 2000 Hz NRC Value
Normal Wood 0.11 0.10 0.07 0.07  8.75% 
Plywood [10mm] 0.34 0.25 0.19 0.15 23%
Concrete [150mm] 0.3 0.4 0.6 0.09 35%
Curtains 0.12 0.25 0.35 0.40 28%
Rockwool thickness = 50mm , 80 kg/m^3  0.6 0.92 0.90 0.88 82%
Rockwool thickness = 50mm, 40 kg/m^3 0.59 0.86 0.86 0.86 79%
Fibre absorber, mineral fibre, 20 mm thick, 3.4 kg/m^2, 50 mm cavity 0.56 0.82 0.87 0.70 73%
Fibre absorber, mats of porous flexible fibrous web fabric, self- extinguishing 0.07 0.2 0.41 0.75 35%
Fiberglass Wall Panel [2inch] 0.50 0.80 0.90 0.80 75%
1” Thick (6 PCF Fiberglass) 0.32 0.82 1.04 1.02 80%
2” Thick (6 PCF Fiberglass) 0.82 1.10 1.04 1.01 100%
3” Thick (6 PCF Fiberglass) 1.05 1.11 .1.11 1.08 110%

Unless you are a sound engineer, you probably won’t have the required equipment to rate your acoustic equipment. Worry not, those tests are carried out under controlled conditions and are readily available on the packaging.
Here’s how to anyway:
– A microphone and a speaker are placed on opposite sides of the room.
– To get a proper control, we play the sounds at different frequencies to get a baseline.
The frequencies are 250Hz (Bass), 500Hz, 1000Hz, and 2000Hz as these typically cover a human vocal range. They are also adequate for sounds created by music, mechanical equipment, and other low-frequency sounds. More specifically, dedicated testing can cover a range of up to 4000Hz. (Treble)
– Next, the chosen soundproofing method is installed and the test is repeated.
– The results are measured against the existing baseline, determining the NRC rating.

Materials that absorbs sound wave the best:

Porous Absorbers:
Mineral wool
Curtains
Clothing
Cork
Textiles
Carpets
Certain types of foam plastic
Let’s talk about Acoustic Fabrics:

Under a research I have noticed a recent uptick in the selection of fabrics comprised of polyethylene. Basically, 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 acoustic wall panels 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).

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.

Recently, Henderson Engineers tested a polyethylene fabric in our test lab to assess its acoustical transparency.
Two important items can be drawn from this test:
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!).
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.
It’s important to note that the effectiveness of acoustic products depends largely on the size of the space and how the product is installed. All of our products receive high NRC ratings, making them the perfect choice for bringing peace to your space.
Ready to hear the difference? Check out Timber Acoustics acoustic panels , made up of New Zealand Pinewood frame , Roxul Rockwool insulation material (NRC 1) and acoustically transparent Jute based fabric.

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