
With Spectrogram settings... from the Spectrogram menu, you can determine how the spectrogram is computed and how it is displayed. These settings will be remembered across Praat sessions. All these settings have standard values ("factory settings"), which appear when you click Standards.
To see how the window length influences the bandwidth, first create a 1000Hz sine wave with Create Sound from formula... by typing 1/2 * sin (2*pi*1000*x)
as the formula, then click View & Edit. The spectrogram will show a horizontal black line. You can now vary the window length in the spectrogram settings and see how the thickness of the lines varies. The line gets thinner if you raise the window length. Apparently, if the analysis window comprises more periods of the wave, the spectrogram can tell us the frequency of the wave with greater precision.
To see this more precisely, create a sum of two sine waves, with frequencies of 1000 and 1200 Hz. the formula is 1/4 * sin (2*pi*1000*x) + 1/4 * sin (2*pi*1200*x)
. In the editor, you will see a single thick band if the analysis window is short (5 ms), and two separate bands if the analysis window is long (30 ms). Apparently, the frequency resolution gets better with longer analysis windows.
So why don't we always use long analysis windows? The answer is that their time resolution is poor. To see this, create a sound that consists of two sine waves and two short clicks. The formula is 0.02*(sin(2*pi*1000*x)+sin(2*pi*1200*x)) + (col=10000)+(col=10200)
. If you view this sound, you can see that the two clicks will overlap in time if the analysis window is long, and that the sine waves overlap in frequency if the analysis window is short. Apparently, there is a tradeoff between time resolution and frequency resolution. One cannot know both the time and the frequency with great precision.
The Spectrum menu also contains Advanced spectrogram settings....
© ppgb, January 28, 2011