# Plot spectrum.

What is the minimum time interval of the track for plotting the spectrum?

It depends on the “Size” setting and sample rate.

The minimum time interval is the same as the “Size” setting in samples. So for the minimum size setting of 128, and a sample rate of 44100 Hz, the minimum duration is 128 / 44100 = (about) 3 ms.

It’s determined by the [u]size[/u] which is the number of samples grouped together for analysis. (i.e. At a sample rate of 44.1Khz, 1/100th of a seconds is 441 samples.)

The math gets complicated but obviously you can’t get the frequency from one or two samples and the more samples you use the more accuracy/resolution you get. But since most real-world sounds are constantly changing, it turns-out to be a mathematical compromise.

Size
This controls how many frequency divisions are used for the spectrum, or how many samples are used for autocorrelation. In Spectrum, a larger size gives more accurate frequency resolution (narrow bands), but averages the result over a longer period of time (because more samples are needed for the calculation). In Autocorrelation, a larger size looks for repeating patterns over a larger range of time offsets, and so will detect lower frequency patterns. With either algorithm, Audacity will show a warning if the selected region is too short for the chosen size.

Thank you! And is it possible to build a spectrum of not just a whole track but for particular selected audio?

By analogy with the export selected audio. You can, of course, select, then delete all, that is out of the allocated, and plot spectrum. But can it be fulfilled directly?

When there is selected audio, Plot Spectrum analyses the selected audio only, not the whole track.
If you modify the selection after opening Plot Spectrum, click the “Replot” button to update the spectrum display.
If multiple tracks, or parts of multiple tracks are selected, Plot Spectrum will analyse a mix of the selected parts of the selected tracks.

Yes, just select the audio first and then open “Plot Spectrum”, or if “Plot Spectrum” is already open, click the “Replot” button.

Thank you!