Converting dB/Hz --> G^2/Hz for PSD

So I have a list of 500000 data points taken from an accelerometer with a sampling frequency of 100000 Hz for 5 seconds and I wish to pass it through an FFT to see it's frequency response and later calculate it's PSD. I've done some reading around and managed to write my own code and this is what I came up with. This is intended to be a vibration analysis so that we can find the natural frequency of the metal pipe we are testing.

Fs = 100000;%sampling frequency

DATA = xlsread('FFTData 1.xlsx');%reading data in from excel file

t = DATA(:,1);%time values from DATA file in Second's

x = DATA(:,4);%Acceleration values read from DATA file in G's

L=length(x);%the number of values read from the data file (500000)

NFFT = 2^nextpow2(L);%better fft makes zero padding.

dt = t(2) - t(1) ; %time sampling interval

Y = fft((x - mean(x)),NFFT);%fourier transform of the signal subracting DC Bias Voltage

%Also need to specify NFFT, if we don't, fft() defaults to 512 points

f = Fs/2*linspace(0,1,NFFT/2+1);%span of frequency we want to run through, all the way up to NFFT
%value

figure(1);

plot(t,x);%plotting time vs. acceleration

title('Drop Shock Test 1');

xlabel('Time (s)');

ylabel('Acceleration (G''s)');

figure(2);

plot(f,abs((Y(1:NFFT/2+1)))*dt);%plotting frequency vs. acceleration. FFT is in Volts*Seconds.

title('Drop Shock Test 1 FFT');

xlabel('Frequency (Hz)');

ylabel('Acceleration (G''s)');

h = spectrum.welch;    % Create a Welch spectral estimator. 

Hpsd = psd(h,x,'Fs',Fs);             % Calculate the PSD 

figure(3);

plot(Hpsd)

I am wondering if in the PSD I am able to convert db/Hz to G^2/Hz and what the steps would be to accomplish this. Any reference, or guidance, is greatly appreciated!

Here is a link to the plots I have come up with so far. The third plot is the one I am asking about.

https://picasaweb.google.com/109805715000182735269/DropShockData?authuser=0&feat=directlink

[Daniel]

Hi,

Did you ever figure it out?