Sound pressure levels to Third Octave bands

[najri]

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,
Nathan

 

[GregLocock]

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

https://bksv.com/~/media/literature/Product Data/bp0163.ashx

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

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[SomptingGuy]

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.

Steve