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AcouVApp Alpha Rating

Calculation of Sound Absorption Ratings



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Freq (Hz) Measured Sound Absorption Coefficient (αs)
Freq (Hz) Pratical Sound Absorption Coefficient (αp)

Freq (Hz) Reference Curve Shifted

ISO 11654, ASTM C423 and Calculation of the Ratings of Sound Absorption

About the ISO 11654 Standard

The ISO 11654 is the internationally recognized standard that specifies methods fo calculating the rating of acoustic absorption of materials. The measurement of sound absorption is based on the ISO 354 and the ISO 11654 is used to calculate the rating and the class.

How the sound absorption is measured?

The sound absorption is measured according to the ISO 354 in a laboratory. The material (a surface around 10m²) is placed in a reverberant room. The reverberation time is measured with and without the materials inside the room and by using the differences between the 2 reverberation times (with and without the materials), the sound absorption of the material is calculated.
This is the calculation of the equivalent absorption area AT (in m2) for each one-third octave band

\[ A_T = 55.3 \times \left(\frac{1}{c_2 \, T_2} \;-\; \frac{1}{c_1 \, T_1}\right) \;-\; 4 \times V \times (m_2 \;-\; m_1) \]

  • V: Volume of the room in m3
  • c: Speed of sound in air (m/s). Typically c = 331 + 0.6 × T, where T is the temperature in °C (15 °C < T < 30 °C)
  • T: Reverberation time in seconds
  • m: Power attenuation coefficient in m–1, calculated according to ISO 9613-1
  • m1: m1 = α / (10 × log(e))

From AT, the absorption coefficient αs (Dimensionless) for each one-third octave band :

\[ \alpha_s = \frac{A_T}{S} \]

  • S: Surface area of the sample in m2

What are the different ratings presented in the ISO 11654?

Based on the measurement of the sound absorption presented above (αs), the ISO 11654 gives the calculations methods to evaluate :

  • The Pratical Sound Absorption in octave(αp): The calculation is done by averaging the 3 third octaves and rounded this value to the closest 0.05. We obtain an octave spectrum from 125Hz to 4000Hz
  • The Weighted Sound Absorption Coefficent (αw) : The calculation is done using a reference curve given in the standard ISO 11654. This curve is in octave from 250HZ and 4000Hz (so without the octave 125Hz). The calculation of the αw is done by shifting the reference curve in steps of 0,05 towards the measured value until the sum of the unfavourable deviations is less than or equal to 0,10. An unfavourable deviation occurs at a particular frequency when the measured value is less than the value of the reference curve. Only deviations in the unfavourable direction shall be counted. The weighted sound absorption µw is defined as the value of the shifted reference curve at 500 Hz.
  • The Shape Indicators (M,L,H): These letters indicate if the absorption coefficient spectra is more important in low, mid and high frequencies. It's calculated by comparing the shifted reference curve with the sound absorption curve, and if the difference is more 0.25 at in the octave, the following indicator is determined (L for 250Hz, M for 500Hz and 1000Hz and H for 2000Hz and 4000Hz).
  • The Class of Sound Absorbers : The classification system is primarily intended to be used for broad-band applications and to compare rapidly sound absorbers.
    The table below presents how the class is defined:

    Sound absorption class αw
    A 0.90; 0.95; 1.00
    B 0.80; 0.85
    C 0.60; 0.65; 0.70; 0.75
    D 0.30; 0.35; 0.40; 0.45; 0.50; 0.55
    E 0.25; 0.20; 0.15
    Not classified 0.10; 0.05; 0.00

    • What is the NRC and SAA and how they are calculated ?

    From the ASTM C423, the sound absorption average, SAA—a single number rating, is the average, rounded off to the nearest 0.01, of the sound absorption coefficients of a material for the twelve one-third octave bands from 200 through 2500 Hz, inclusive.
    The NRC is the round the average of the sound absorption coefficients for 250, 500, 1000, and 2000 Hz octave to the nearest multiple of 0.05.

    How to use the different ratings from the ISO 11654?

    First, these different ratings are used to have a quick representation of the sound absorption performances. It's used also to compare products between them.
    However, for acoustic studies, the measured sound absorption in third octave or octave are used.

    How to use our tool AcouVApp Abso Rating

    The tools is quite easy to use. You just need to copy/paste your data in the table by putting your mouse key in the first row.
    The calculation is automatically done when you change the values in the table. Be carrefull to enter correct values (from 0 to 1 and sometimes little more than 1).
    You have also acces to the shifted curve below the graph.

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