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From: Wayne King on 28 Oct 2009 15:11
"Kaushik " <kaushikkn(a)gmail.com> wrote in message <hc9sev$d1s$1(a)fred.mathworks.com>...
> Thanks Wayne
> I have my pressure measurements in the units of KN/M^2. I also know the reference pressure at 20 mu pascals. I only wanted an info as to the commutativity of the relations weather we take the spectra and find the SPL or SPL and then take the spectra.
>
> KNK
>
> "Wayne King" <wmkingty(a)gmail.com> wrote in message <hc9rve$d12$1(a)fred.mathworks.com>...
> > "Kaushik " <kaushikkn(a)gmail.com> wrote in message <hc9r56$jon$1(a)fred.mathworks.com>...
> > > Hi
> > >
> > > I have a data file which gives the pressure time history of a measurement
> > > i need to find the SPL spectra of this signal. How do i go about using matlab, i know
> > > the use of fft command but confused wheather we need to take the spectra and then find the SPL or find the SPL and take the spectra. Hope i have asked in the right community
> > >
> > > thanks
> > >
> > > KNK
> >
> > Hi KNK, See the following thread:
> >
> > http://www.mathworks.com/matlabcentral/newsreader/view_thread/262003#684001
> >
> > You will need to have some additional information before you can display your spectral estimate in dB SPL.
> >
> > Hope that helps,
> > Wayne
KNK,
Take the Fourier transform first:
dt = 1/20000;
t = 0:1/Fs:1-(1/Fs);
x = 3e-2*cos(2*pi*1000*t); % assume peak amplitude of 0.03 Pascals
% 63.52 dB SPL
y = (2/length(x))*fft(x);
f = 0:(Fs/length(x)):Fs/2;
plot(f,20*log10(abs(y(1:Fs/2+1))/20e-6));
% Component at 1 kHz 63.52 dB SPL

Wayne
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