Digital Sound & Music: Concepts, Applications, & Science, Chapter 2, last updated 6/25/2013
45
figure;
freqs = [0: (sr/2)-1];
plot(freqs, fftmag(1:sr/2));
When you do this, you'll see that all the frequency components are way over on the left side of
the graph. Since we know our frequency components
should be 110 Hz, 220 Hz, and 440 Hz, we might as well
look at only the first, say, 600 frequency components so
that we can see the results better. One way to zoom in on
the frequency response graph is to use the zoom tool in the
graph window, or you can reset the axis properties in the
command window, as follows.
axis([0 600 0 8000]);
This yields the frequency response graph for our composite
wave, which shows the three frequency components.
Figure 2.47 Frequency response graph for a 3-component wave
To get the phase response graph, we need to extract the phase information from the
fftdata. This is done with the angle function. We leave that as an exercise.
Let's try the Fourier transform on a more complex sound wave a sound file that we read
in.
y = wavread('HornsE04Mono.wav');
As before, you can get the Fourier transform with the fft function.
fftdata = fft(y);
You can then get the magnitudes of the frequency components and generate a frequency
response graph from this.
fftmag = abs(fftdata);
Aside: If we would zoom in
more closely at each of these
spikes at frequencies 110, 220,
and 440 Hz, we would see that
they are not perfectly horizontal
lines. The "imperfect" results of
the FFT will be discussed later in
the sections on FFT windows and
windowing functions.
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