# How to plot (1) amplitude vs time and (2) amplitude vs frequency plots from .wav file

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Linny Nguyen on 26 Aug 2020
Commented: Star Strider on 16 Dec 2020
Hi, I am having some trouble trying to figure out how to plot an Amplitude vs Frequency plot from a .wav file. I think I managed to figure out how to plot the Amplitude vs Time plot though.
Here is my code:
[y,Fs]=audioread('HornC5.wav'); %Reads .wav file into 2 matrices for amplitude (y) and sampling frequency (Fs)
% sound(y,Fs); % see what it sounds like
y=y(:,1); % Converts to a single channel
dt=1/Fs;
t=0:dt:(length(y)*dt)-dt; %Matrix for time points
% I don't really now how to code the Fourier Transform part :(
figure(1)
subplot(2,1,1)
plot(t,y); title('Horn C5'); xlabel('Time (s)'); ylabel('Amplitude'); % Amplitude vs Time plot
subplot(2,1,2)
plot

Star Strider on 26 Aug 2020
The amplitude - Frequency plot would go something like this:
[y,Fs]=audioread('HornC5.wav'); %Reads .wav file into 2 matrices for amplitude (y) and sampling frequency
L = size(y,1);
Fn = Fs/2;
FTy = fft(y)/L;
Fv = linspace(0, 1, fix(L/2)+1)*Fn;
Iv = 1:numel(Fv);
figure
plot(Fv, abs(FTy(Iv,:))*2)
grid
xlabel('Frequency')
ylabel('Amplitude')
There could be two channels in the .wav file. This will plot both of them in the same plot. Note that ‘Fn’ is the Nyquist frequency, ‘Fv’ is the frequency vector, and ‘Iv’ is the index vector for the plot or other analyses.
I do not have your .wav file so I could not test this with it. It should work.
##### 2 CommentsShowHide 1 older comment
Star Strider on 16 Dec 2020
The frequency units (here) are Hz, amplitude is whatever the transducer produced (e.g. mV) or was transformed to (e.g. Pascals if calibrated with respect to Sound pressure level).
It’s complicated!

Sara Boznik on 26 Aug 2020
figure(1)
subplot(2,1,1)
plot(t,y); title('Horn C5'); xlabel('Time (s)'); ylabel('Amplitude'); % Amplitude vs Time plot
subplot(2,1,2)
f=1/t
plot(f,y)
I will do it like that.