FFT - How to transform my amplitudes into dB

Hello,

I have done a code to obtain the FFT of an audio recorded with Matlab. I'm getting my result in amplitude but I'd like to get it in dB in the frequency domain... Could someone please help me?

Here is my code and my curves:

Thank you for your help.

Answer 1

[Frantz Bouchereau]

Here is a popular MATLAB doc page that explains the relationship between FFT and true power spectra: Power Spectral Density Estimates Using FFT. Use this to scale the FFT to obtain true power values.

Answer 2

[Diya Tulshan]

Hii Jacques,

I understand you want to plot the data in db.

Kindly look into the code given below, it should solve your problem:-

Fs = 16000;          % Sampling rate (Hz)
Channels = 1;        % Number of audio channels
bits = 16;           % Number of bits per sample

r = audiorecorder(Fs, bits, Channels);
duration = 10;

disp('Recording started');
recordblocking(r, duration);
disp('Recording stopped');

X = getaudiodata(r);
N = length(X);
f = (0:N/2) * (Fs/N);
t = (0:N-1) / Fs;

subplot(2,1,1);
plot(t, X);
xlabel('Time (s)');
ylabel('Amplitude');
title('Time Domain Plot');

Y = fft(X, N);
Y = abs(Y(1:N/2+1));
fftResult_dB = 20 * log10(Y); % covert into dB

subplot(2,1,2);
plot(f, fftResult_dB);
xlabel('Frequency (Hz)');
ylabel('Amplitude (dB)');
title('Frequency Domain Plot');
xlim([0, Fs/2]);
grid on;

Hope this helps!

[Jacques]

Thank you Diya for your help!

I've tested your code and this is what I get:

Getting negative values for my dBs disturbs me...I don't know how to interpret them

[Paul]

Converting any number less than 1 and greater than 0 to dB will yield a negative result. db(0) is -Inf.

20*log10([2 1 .5 .1 0])

ans = 1×5

6.0206 0 -6.0206 -20.0000 -Inf

So the negative values in dB tells you that the corresponding absolute amplitude is in the range 0 < amplitude < 1.