Those who study science usually receive explanation and understanding at the same time as their eyes are dazzled by mathematical equations, but those who study the humanities (in our case, art, design and architecture) usually use verbal explanation and must understand the rest on their own.
Therefore, in a nutshell, the difference between the concepts NRC and wæ without the use of mathematics.
NRC – a mathematical number that determines the sound absorption property of the ceiling layer (coefficient).
The number ranks the random incidence of noise attenuation in a reverberation chamber when a sound source is activated from several locations on a decimal scale from 0 to 1. (For those who are more in-depth, morphology and physics are also found in the American standard ASTM 423), the measurement in the laboratory expresses the arithmetic mean of the sound absorbed in three-quarters of an octave at frequencies of 250, 500, 1000, 2000 Hz.
The result obtained is rounded to the nearest number of 0.05. That is, if the mathematical average obtained is 0.62, it follows that NRC=0.6 and if the mathematical average obtained is 0.64, it follows that NRC=0.65.
It goes without saying that in the laboratory, many repeated tests are performed and each test usually yields different results depending on temperature changes, endurance tests and frequencies of the computer systems and sound systems, therefore the reliability of the tester is cardinal to the published result.
The designer must understand very well that the weight of the characterization of the product name and the manufacturer’s name is much higher and more valuable than the weight of the numerical characterization of various acoustic coefficients.
This empirical method has been in the world for many years, but its popularity has been declining since an alternative called weightedæ was introduced. (For more information, see the EN ISO 11654 standard, which describes how the disadvantage of the lack of indication of the spread of energy against frequencies was overcome and the evaluation methods developed to overcome this)
weightedæ – a mathematical number (coefficient) that ranks the weighted sound absorption in a reverberation room when a sound source is activated from a number Points on a decimal scale from 0 to 1.
The measurement expresses the arithmetic mean of the sound absorbed in a third of an octave (according to the evaluation method specified in the EN20354 and/or ISO 354 standard).
In arithmetic interpolation, the results obtained are converted into octaves at frequencies of 250, 500, 1000, 2000 and 4000 Hz (this operation is called alpha P, i.e. practical noise absorption due to the addition of more frequencies). This is then depicted in a line graph and compared with another standard graph used for comparison.
Then the variation of the measured graph with the standard graph is checked until the results of the moving average, (pæ ) under the standard graph is less than or equal to 0.1. Then, a vertical line is drawn on the 500 Hz axis and all the intersection points of the measured graph are taken into account as pæ.
Using the operations described above, the absorption can be expressed only in increments of 0.05 without rounding the measurement to the nearest number. (This method prevents the weighting of the variation obtained from the measurement frequencies according to the American method, thus supposedly preventing the correction that can be made to a low result by rounding up, i.e. a measurement result of 0.68 cannot be obtained at all, let alone be rounded up to 0.7.
Furthermore, if during the measurement in the reverberation chamber a result is obtained that is 0.25 or more higher at one of the frequencies, then the frequency at which the high result was obtained is also indicated, for example: at 250 Hz, the letter (L) is indicated, at 500 or 1000 Hz, the letter M is indicated) (And at 2000 to 4000 Hz the letter H is indicated) ) This allows us to know with greater approximation the behavior of the ceiling layer in the laboratory.
In conclusion, according to the American standard NRC – an average frequency index of 250 /500 / 1000 / 2000 Hz and rounded the figure by 0.05 points upwards
It was replaced by a revision with the SAA index standard – which is an average index in 12 thirds of the octave from 200 to 2500 Hz and rounded the figure by 0.01 points upwards
According to the European standard – the figure wæ is a comparative figure.
wæ measures in a third octave and uses a comparative absorption curve (calculates the average absorption like NRC in steps of 0.05 and checks that there is no negative deviation in the result above 0.01) If the sum of the negative deviation exceeds 0.01, the absorption is measured again at 500 Hz.
At each frequency where absorption is measured to be 0.25 higher than the comparative graph, this is indicated by the letters
L – for 250 Hz
M – for 500 and 1000 Hz
H – for 2000 and 4000 Hz
Shmuel Zoref Hataf
Innovation and Sustainability Manager
