The Octave Band method estimates the sound level at the ear, taking into account the frequency content of the noise. This method is more complicated than the NRR method and HML method, but is the most accurate of the three and better for noise with high tonal or low frequency content.
In order to use the Octave Band method you need the following:
The formula to calculate the level at the ear is:
L'_{A} is the effective A-weighted sound level at the ear
f is the octave band centre frequency in Hz
L_{f} is the measured octave band sound level in band f
A_{f} is the frequency weighting for octave band f
APV_{f} is the assumed protection value of the hearing protector for octave band f
Some regulations, such as those used in the UK, recommend adding 4dB to the final result to account for real-world factors, such as the poor fitting of protectors.
As there are only eight bands of interest for these calculations, it is easy enough to expand this to a usable form:
Frequency (Hz) | f | 63 | 125 | 250 | 500 | 1000 | 2000 | 4000 | 8000 | |
Sound Level (dB) | L_{f} | 69.6 | 74.0 | 75.9 | 77.3 | 100.1 | 105.8 | 77.9 | 57.1 | ⇐ Example data |
A Weighting | A_{f} | -26.2 | -16.1 | -8.6 | -3.2 | 0.0 | 1.2 | 1.0 | -1.1 | |
Protector APV (dB) | APV_{f} | 8.6 | 12.2 | 13.9 | 15.8 | 16.4 | 22.9 | 24.6 | 22.7 | ⇐ Example data |
L_{f} + A_{f} - APV_{f} | 34.8 | 45.7 | 53.4 | 58.3 | 83.7 | 84.1 | 54.3 | 33.3 |
Now take each of the numbers on the bottom row, divide by 10 and anti-log:
L_{SUM} = 10^{3.48} + 10^{4.57} + 10^{5.34} + 10^{5.83} + 10^{8.37} + 10^{8.41} + 10^{5.43} + 10^{3.33}
And finally anti-log the result and add 4dB for the real-world factor:
L'_{A} = 10 log (L_{SUM})
L'_{A} = 86.9 dB
So in this example, after all that work, these particular hearing protectors are inadequate.
NoiseMeters provides a range of online applications for calculating noise exposures along with other noise related calculators. These applications are free to use.
You can use our Octave Band hearing protection calculator to carry out the above calculations and produce a simple report. All you need are the octave band sound level measurements and the octave band APV figures from the hearing protector specifications.