This command analyzes the frequency spectrum of audio, via a Fast Fourier Transform (FFT). The result is displayed as a plot in which the horizontal axis is frequency in Hertz, and the vertical axis is amplitude in decibels. The FFT output can also be exported to a tab-delimited text file, using the Export button. For real-time analysis, use the spectrum bar instead.
If a selection exists, only the selection is analyzed, otherwise the entire document is analyzed. The analysis has various parameters, described below. After changing one or more parameters, press the Refresh button (or Alt-R) to re-compute the FFT and update the plot. It's not necessary to refresh after changing Frequency Axis or View, because they don't require re-computing the FFT.
| Window Function | This selects which window function to apply to the audio prior to the FFT, to reduce spectral leakage. All are bell-shaped or triangular, except for the rectangular window. |
| Window Size | This is the size of the FFT, in sample frames. Larger values give increased spectral resolution (more frequency bands), which is helpful when analyzing low frequencies. |
| Frequency Axis | This determines whether the plot's frequency axis is has a linear or logarithmic scale. A logarithmic scale allocates more space (visually) to low frequencies. |
| Channel Mode | This determines how multi-channel audio is handled. Select "Combine" to sum all channels prior to analysis, or "Separate" to analyze each channel individually. |
| View | If channel are analyzed separately (see above), this option lets you select a single channel to show in the plot, or select "ALL" to view all channels at once. |
| Frequency Range | To limit the plot to a particular frequency range, uncheck "Auto" and enter the start and end frequencies in Hertz. To restore the default range, recheck "Auto". Note that the frequency range controls may be hidden if the dialog is too narrow. In this case, widen the dialog until they're visible. |
The plot has a tooltip, which displays the frequency and amplitude of the nearest data point. The tooltip appears when the mouse cursor is positioned on or near a data point (or the line between two data points), and held there for half a second. Once the tooltip is shown, if the cursor is moved along the plotted line, the tooltip tracks the cursor and updates itself immediately. To hide the tooltip, left-click; it stays hidden until it's reset by moving the cursor outside the plot window, and then back.