Unit of amplitude on y axis in linear plot.

I have a doubt about the unit of amplitud eon y axis on linear plot of a sound wave in signal analyzer.

In lograthmic scale it is dB but I am confused about what it should be on the linear scale and what is the maths behind the conversion that would be a plus if someone has any idea bout it.

[Renato SL]

I think the linear scale should be the amplitude of the signal. It can be an electrical current or voltage. It's the same as if you do the command:

plot(signal)

The unit of the y-axis is not specified (until you specify it).

Regarding the plot below that, the amplitude is not just the amplitude of the signal in dB. As it is written in the plot itself, it is the power spectrum of the signal, in dB. I think the Wikipedia page (here) is a good place to start.

Anyway, perhaps you can get more insight in the Signal Analyzer App documentation (here).

[AKSHAT NEGI]

Thank you so much for you reply. Yes thats what I had in my plots. which i plotted using line spectrum functions. Here i have plotted the sound signal with respect to frequency. I was using Voltage in my plots...I am just confused that it should be voltage or pascals. Top linear plot is what confuses me, both are power spectrums of signal. What would be the ideal unit to explain the signal for my project.

[Renato SL]

"both are power spectrum of signal"

If so, then the unit for the top plot shouldn't be Volt or Pa. I guess Watt makes sense, but you should check again with the instrument and/or method used to measure the power if it specifies the unit.

[AKSHAT NEGI]

Thank you again for the help... I am not able to find the conversion maths... where i can convert for example the value at 404.2 Hz is 0.004 to -1.26 db in log scale...

I have attached the section of the code below

-----------------------------------------------------

fSep=1/(N*h);

disp(['FFT frequency separation = ' num2str(fSep) ' Hz'])

[Y,f,YdB]=SimpleLineSpectrum2(y,h,0,fny);

figure

subplot(211)

plot(f,Y,'LineWidth',1.5),grid

title(['Line Spectrum of ' name ],'fontweight','bold','fontsize',10)

ylabel('Linear power(V)')

xlabel('Frequency (Hz)')

axis([fL fR 0 max(Y)])

subplot(212)

plot(f,YdB,'LineWidth',1.5),grid

title('Line Spectrum in dB','fontweight','bold')

ylabel('power (dB)')

xlabel('frequency (Hz)')

axis([fL fR dBmin 0])

pause

--------------------------------------------------------

and this is what "Simplelinespectrum " is doing

% normalize by 2*np

np=length(y); % length of the input vector

%===remove the mean

yavg=mean(y);

y=y-yavg;

%===calculate the fast Fourier transform

Y=fft(y(1:np));

%===generate the frequencies in the frequency grid

nint=fix(np/2);

i=1:nint+1;

f(1:nint+1)=(i-1)/(np*h);

%===take the absolute value and normalize

Ya=2*abs(Y(1:nint+1))/(np); % normalization

% Ya=abs(Y(1:nint+1)); % no normalization

%===generate the frequencies and magnitudes to be

% returned consonant with frLeft and frRight

fL=frLeft;

iL=fix(1+np*h*fL);

fR=frRight;

iR=fix(1+np*h*fR);

retYa=Ya(iL:iR);

retf=f(iL:iR);

Ymax=max(retYa);

YdB=20*log10(retYa/Ymax);

-------------------------------------------------

Please let me know if this helps to recognize whats exactly happening..thank you very much for your time and help.

[Renato SL]

For this part

ylabel('Linear power(V)')

please make sure if the signal is power or voltage. As it is, it doesn't make sense to me. Volt is not the unit for power.

Regarding the conversion to dB, it's there in Simplelinespectrum. The comments in the code are there to help you.

It started with removing the mean, then compute the FFT (Y), then the normalization (Ya), and finally in the last line it is converted to dB (YdB).

The formula for the conversion to dB can be read here.

In general, if you have a code, I believe the best way to get an understanding of it is to ask the author.