Q:

Frequency : Decibel plot using FFT in MATLAB

I am trying to use MATLAB to import a WAV file and create the type of diagram shown below. I am basically trying to pull frequency information and plot it according to decibels. Here is the code I am working with, but it doesn't seem to pull the frequency information correctly:
[x fs]=wavread('filename.wav');
dt=1/fs;%% time interval

X=fft(x);
df=1/(length(x)*dt); %% frequency interval
f=(1:length(X))*df;%% frequency vector

%% frequency domain plot, freq in hertz
figure
plot(f,abs(X))

Please help! Thank you!

A:

In your code "X" contains the waveform information, not the frequency information. To get the frequency information of the soundfile you could use the FFT function. I use this (more elaborate, but still simple) code for what you want to do: 
[FileName,PathName]=uigetfile('*.wav');
[y, Fs, nbits] = wavread(fullfile(PathName,FileName));
length_y=length(y);

NFFT = 2^nextpow2(length_y); % Next power of 2 from length of y
fft_y=fft(y,NFFT)/length_y;
f = Fs/2*linspace(0,1,NFFT/2+1);

semilogy(f,abs(fft_y(1:length(f))));
title('Single-Sided Amplitude Spectrum of y(t)')
xlabel('Frequency (Hz)')
ylabel('|Y(f)|')

I hope this is useful to you. The plot will not be in steps like the one you have shown, but that can also be achieved - using the "stairs" plot function. 

