How can i detect amplitude from FFT on Time-Domain signal.

Hi, everyone!

I have a signal of wave song and I have a problem with amplitudes.

%graph 1 time domain
song2 = song(1:size(song));
fs = 44100;
dt = 1/fs;
t = 0:dt:(length(song2)*dt)-dt;
figure(); 
plot(t,abs(song2)); xlabel('Seconds'); ylabel('Amplitude');

Then I put signal in FFT, because I want to get amplitude of detecting peaks(for example; 164Hz).

%graph 2 fft
L = length(song2); 
NFFT = 2^nextpow2(L);
f = fs/2*linspace(0,1,NFFT/2+1);
X = fft(song2,NFFT)/L; 
X = 2*abs(X(1:NFFT/2+1)); 
figure(); 
plot(f,X);

The problem appear when I get the amplitude of the signal (for example; 0.0103) but if I compare with the amplitude of the (time domain) is not the same.

My question is How in the time domain(graph 1) I detect amplitude of the of a frequency(for example; 164 with amplitude 0.0103)?

Any help would be helpful.

[Thomas Casey]

taking an fft IS the process of extracting relative frequency amplitudes from your time domain "signal." that is, each y axis point in your frequency domain spectrum is the relative contribution of the corresponding frequency to your time domain "signal." Is this what you're asking?

[Erik]

Hm, I will rather ask in this way. I detect frequency in frequency domain spectrum as the graph below.

For example Let us take the the first signal (82hz)(amplitude:0.0075) And my question if is posible to detect position of this first signal in time-domain(as the graph below)

[Ella]

Can you please share with us how could you do it ? Im having the same problem I dont know why my amplitude values in time domain and frequency are not same ?