poctave() return value for acoustics analysis
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Mark Thompson
on 23 Nov 2018
Commented: Mark Thompson
on 12 Apr 2024 at 1:13
Hi all,
I need some assistance understanding the "units" of the data that is returned when calling the MATLAB function [p,cf] = poctave();
I am analysing an audio signal that has been recorded with a microphone. The data is imported into the MATLAB workspace and converted to a calibrated pressure value with units of Pascals (Pa). So the units of the data that I pass to poctave() are Pascals (Pa). The following code snippet shows how I'm using poctave().
flims = [20 Fs/2]; % set the frequency limits of my analysis
bpo = 3; % Third-Octave analysis required
opts = {'FrequencyLimits',flims,'BandsPerOctave',bpo}; % prepare these options for passing to poctave()
% apply the 3rd-Octave filter-bank to my data
[p, cf] = poctave(pressureData,Fs,opts{:});
(Where "pressureData" is my audio data to be filtered, and "Fs" is the sampling frequency of the data.)
My first question is:
1) What are the units of the octave spectrum data returned unto variable 'p'?
I need to know this for my conversion of the data into a dB value using 10*log10(p/pref).
I understand that if I use poctave() as follows:
poctave(pressureData,Fs,opts{:})
then by default it assigns "pref" a value of 1 and the results are as in the following image:
Without units though, the result seems meaningless...
My second question is:
2) what is pref in the previous equation to ensure correct dB values that correlate with a sound pressure level? do I use:
pref = 0.00002; % 20 micro Pascals reference sound pressure level
or:
pref = 0.00002^2; % 20 micro Pascals (squared); i.e. is p returned from poctave() a pressure squared value or a power value??
I want to create a resulting 3rd-Octave spectrum whos dB values match those as though it was being displayed on a Sound Level Meter. I will then correlate these values with those on our Sound Level Meter to validate the data to be processed within MATLAB.
Thanks so much for any assistance you can offer!!
2 Comments
Mickey Yoseph
on 25 Mar 2020
Edited: Mickey Yoseph
on 25 Mar 2020
Hello mate - did you get an answer to this in the end? In the exact same boat as you for both questions.
Require dB SPL but clearly poctave returns negative dB values which aren't helpful so need 'pref' to be 0.0002.
Any help?
zein
on 25 Jun 2020
i think that the output in the graph is 10log(p) where p is the spectrum in 1/3 octave and to calculate the SPL-1/3 OCTAVE
SPL-1/3octave=10log(p/pref.^2)
Accepted Answer
More Answers (2)
andytodd.msp
on 27 Dec 2023
Hi All, I'm just curious as to the solution to this - as we are in a similar position (using calibrated input data and trying to have results displayed in typical dB SPL values).
I've tryed using the methods above, but I'm not sure if I'm missing something - as we do not seem to get reasonable answers using this method.
For example, the output of poctave at 1kHz is -14.7dB (Average Power). We know the SPL at 1kHz is actually 79.4dB (SPL). I'm struggling to find the link between these two values so that we can convert between the two.
Any help is much appreciated!
Andy
3 Comments
andytodd.msp
on 8 Jan 2024
Hi Mark,
No - the data we are importing is already in Pascal units. Its coming from a calibrated measurement system.
What I'm trying to do, is have poctave display the octave values as dB re 20 micropascals - rather than the default 'dB Average Power'.
Does that make sense?
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