Incorrect Sound Pressure Level from FFT in Matlab
in [Matlab]
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From: Rune Allnor on 9 Apr 2010 08:34 On 9 apr, 14:26, "Johannes Buechler" <jb_s...(a)gmx.DELnet> wrote: > Rune Allnor <all...(a)tele.ntnu.no> wrote in message <a9ad38ca-84ed-40cd-96ef-b11aa4cb9...(a)11g2000yqr.googlegroups.com>... > > The most likely explanation is that you process the raw, > > uncalibrated data in matlab, whereas the other package > > uses additional calibration information. > > The time signals are exactly identical in PAK and in Matlab, as is to be seen here:http://www.JohannesBuechler.de/TimeSignal_PAKvsMatlab.bmp > (the upload will be done tonight from my home computer) > > As you can see the numerical values are identical up to at least 0.1 mPa... This proofs that the export has already applied all correction/calibration/whatever factors. So please, take this as given. No, it doesn't. It only proves that the same data were exported. > The problem however lies somewhere in Matlab. The problem lies somewhere in your brain. Read what I posted yesterday, and contemplate what I said. Rune
From: Johannes Buechler on 9 Apr 2010 09:27 Rune Allnor <allnor(a)tele.ntnu.no> wrote in message <5a499dbf-8e5f-4969-b0ca-687805178580(a)g11g2000yqe.googlegroups.com>... > On 9 apr, 14:26, "Johannes Buechler" <jb_s...(a)gmx.DELnet> wrote: > > Rune Allnor <all...(a)tele.ntnu.no> wrote in message <a9ad38ca-84ed-40cd-96ef-b11aa4cb9...(a)11g2000yqr.googlegroups.com>... > > > The most likely explanation is that you process the raw, > > > uncalibrated data in matlab, whereas the other package > > > uses additional calibration information. > > > > The time signals are exactly identical in PAK and in Matlab, as is to be seen here: http://www.JohannesBuechler.de/TimeSignal_PAKvsMatlab.bmp > > (the upload will be done tonight from my home computer) > > > > As you can see the numerical values are identical up to at least 0.1 mPa... This proofs that the export has already applied all correction/calibration/whatever factors. So please, take this as given. > > No, it doesn't. It only proves that the same data were exported. True. But a plot in PAK from a regular measurement with a calibration device on the microphone delivers a "1" in Matlab, and 94 dB in PAK, so with this information it is a proof. Because, as I said, if I calculate the overall level in Matlab from the spectra, it also leads to different numerical values than in PAK. > > The problem however lies somewhere in Matlab. > > The problem lies somewhere in your brain. Read what I posted > yesterday, and contemplate what I said. Thanks for pointing this out so nicely. In fact this is what I meant when I said "in Matlab", because the problem is in the Matlab code that I posted and use... There is an agreement that there is no Matlab-bug or anything like that leading to a "wrong" plot. Matlab just calculates what the user tells it to calculate. So I have to find the mistake in my code, which is why I asked in the Matlab group. So let me redefine the problem just for you: I'd like to get the same Levels in the Matlab plot as in the professional 20,000+ EUR software PAK. I also have ArtemiS on my computer which delivers identical results to PAK. They are the two most common acoustical/vibration measurement and analysis tools in the world, together with LMS maybe. Joe
From: Rune Allnor on 9 Apr 2010 09:42 On 9 apr, 15:27, "Johannes Buechler" <jb_s...(a)gmx.DELnet> wrote: > Rune Allnor <all...(a)tele.ntnu.no> wrote in message <5a499dbf-8e5f-4969-b0ca-687805178...(a)g11g2000yqe.googlegroups.com>... > > On 9 apr, 14:26, "Johannes Buechler" <jb_s...(a)gmx.DELnet> wrote: > > > Rune Allnor <all...(a)tele.ntnu.no> wrote in message <a9ad38ca-84ed-40cd-96ef-b11aa4cb9...(a)11g2000yqr.googlegroups.com>... > > > > The most likely explanation is that you process the raw, > > > > uncalibrated data in matlab, whereas the other package > > > > uses additional calibration information. > > > > The time signals are exactly identical in PAK and in Matlab, as is to be seen here:http://www.JohannesBuechler.de/TimeSignal_PAKvsMatlab.bmp > > > (the upload will be done tonight from my home computer) > > > > As you can see the numerical values are identical up to at least 0.1 mPa... This proofs that the export has already applied all correction/calibration/whatever factors. So please, take this as given. > > > No, it doesn't. It only proves that the same data were exported. > > True. But a plot in PAK from a regular measurement with a calibration device on the microphone delivers a "1" in Matlab, and 94 dB in PAK, so with this information it is a proof. Because, as I said, if I calculate the overall level in Matlab from the spectra, it also leads to different numerical values than in PAK. > > > > The problem however lies somewhere in Matlab. > > > The problem lies somewhere in your brain. Read what I posted > > yesterday, and contemplate what I said. > > Thanks for pointing this out so nicely. Y're welcome. > In fact this is what I meant when I said "in Matlab", because the problem is in the Matlab code that I posted and use... There is an agreement that there is no Matlab-bug or anything like that leading to a "wrong" plot. Matlab just calculates what the user tells it to calculate. So I have to find the mistake in my code, which is why I asked in the Matlab group. I have pointed out repeatedly that you don't have all the required information. > So let me redefine the problem just for you: I'd like to get the same Levels in the Matlab plot as in the professional 20,000+ EUR software PAK. I also have ArtemiS on my computer which delivers identical results to PAK. They are the two most common acoustical/vibration measurement and analysis tools in the world, together with LMS maybe. So what? You could have a EUR 17 billion package and it wouldn't matter if you don't get access to the calibration data. Rune
From: Mark Shore on 9 Apr 2010 10:14 snip > True. But a plot in PAK from a regular measurement with a calibration device on the microphone delivers a "1" in Matlab, and 94 dB in PAK, so with this information it is a proof. Because, as I said, if I calculate the overall level in Matlab from the spectra, it also leads to different numerical values than in PAK. > snip > So let me redefine the problem just for you: I'd like to get the same Levels in the Matlab plot as in the professional 20,000+ EUR software PAK. I also have ArtemiS on my computer which delivers identical results to PAK. They are the two most common acoustical/vibration measurement and analysis tools in the world, together with LMS maybe. > > Joe I take a very empirical approach to this sort of thing, so it could be that I'm missing something, but if you have dB values you trust from PAK and want to reproduce in MATLAB (you've managed to create remarkably similar output plots by the way), then why not simply adjust the value of p0 in L_p = 20*log10(abs(B)/p0) until the dB levels match? (And figure out why it works afterward.)
From: Johannes Buechler on 12 Apr 2010 03:42
"Mark Shore" <mshore(a)magmageosciences.ca> wrote in message <hpncnd$3q7$1(a)fred.mathworks.com>... > > So let me redefine the problem just for you: I'd like to get the same Levels in the Matlab plot as in the professional 20,000+ EUR software PAK. I also have ArtemiS on my computer which delivers identical results to PAK. They are the two most common acoustical/vibration measurement and analysis tools in the world, together with LMS maybe. > > > > Joe > > I take a very empirical approach to this sort of thing, so it could be that I'm missing something, but if you have dB values you trust from PAK and want to reproduce in MATLAB (you've managed to create remarkably similar output plots by the way), then why not simply adjust the value of p0 in L_p = 20*log10(abs(B)/p0) until the dB levels match? (And figure out why it works afterward.) Thanks Mark for this comment. It is a quite refreshing remark considering that some people obviously think that they are the most clever in the world and if they don't know something then they haven't been provided with all necessary information. However I have to find the reason for the discrepancy before I can proceed with my work (drawing of the Campbell is not the goal, it is just a way of easily comparing calculation results). The calculation of SPL is defined with p0 so the question is why does the FFT deliver different numerical values already before the logarithmisation. In order to find the reason, I'll have to start with a simple sinusoidal signal. I'll probably try to get some help as soon as I can define the problem in a more detailed way. Thanks a lot so far, Joe |