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Sound pressure levels to Third Octave bands

Sound pressure levels to Third Octave bands

Sound pressure levels to Third Octave bands

Dear all,

I'm working with acoustic predictions originating from CFD/computer simulations, and I readily have my acoustic results represented as sound pressure levels (dB). I end up with equally spaced frequencies, from 100 to 20,000 Hz with a step of 100 Hz for instance. I need to represent my results in third octave bands, and I have some doubts on the correct methods of summing/averaging for each different band. I find plenty of resources for addition of noise sources but this is quite different I think, should it be a log average or similar?

Any help would be greatly appreciated, or if you could point me to an appropriate resource?

Thank you in advance,
Kind Regards,

RE: Sound pressure levels to Third Octave bands

RMS=square root of the sum of the squares in each band. Edge cases are complex. A real third octave filter is not a sharp cutoff at each band but has a roll off into the next band. See figures 3-5 in this


100 Hz res means you won't get anything meaningful until about 1000 Hz, you need 3 lines in each 1/3 octave.


Greg Locock

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RE: Sound pressure levels to Third Octave bands

If your CFD simulation is giving you dB values, you'll need to convert them back to pressures and calculate the RMS. People often get confused about this, trying to remember a magic formula and mixing up 10s and 20s in the calculation. Just work from first principles and realise that you are adding squared pressure values.

A simple rule of thumb (to check your calc) is that if you have two levels at X dB each, but at different frequencies, their combined level will be (X+3) dB.


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