Audacity lets you add label tracks to mark particularly important passages. You can create these tracks in Tracks | Add New | Label Track and then add content by clicking in the label and entering text. Alternatively, you can import a longer label in one go via File | Import | Labels. Save the label tracks in a file via File | Export Labels (Figure 7). The syntax could hardly be simpler: There are tabs between the positions specified in samples on the timeline and the text on the label; a newline ends a label.

The notes, which by themselves are useful, impress with qualities that are not immediately obvious: For example, a new label track can combine the underlying soundtracks into a group that can be synchronized separately. The label tracks also prove to be a useful tool when digitizing LPs and music cassettes. If you select File | Export Multiple, Audacity displays a wizard in which the labels can be used to split files and name file. Audacity then automatically numbers the individual tracks.

In the label tracks, Audacity distinguishes between labels for points and regions. A point label can be converted into a region label at any time by dragging it to the label symbol's angular brackets. Tracks | Edit Labels opens all the project's label tracks in an editor designed as a simple table. Alternatively, you can export the labels as simple text files, edit them with your favorite editor, and save the file as a new label track.

Audio editors offer the option of precisely selecting sections of recordings in chronological order and boosting quiet passages or damping louder ones; users apply subtle manipulations using equalizers and filters. But, what if you have a deep humming sound or a nasty crackle between the 14th and 16th seconds in what is an otherwise good recording? You will not want to change the volume for the flaw, but you need a way to eradicate it.

Some tools require very complicated settings and a very sophisticated equalizer to deal with this problem, and undesirable interference can often result. However, the procedure works very well in Audacity: First, switch the display in the drop-down list in the track header on the left from Waveform to Spectrogram. The logarithmic variant Spectrogram log(f) often appears more differentiated and is thus easier to read.

Recognizing the dynamics in the recording may now be more difficult in the spectrogram display, but you do get prominent frequencies displayed as color gradations at each point of the track. The colored stripes represent the intensity of frequencies read from the scale on the left. The selection tool, which only provides vertically continuous stripes in the waveform view, now draws rectangles with settable upper and lower limits. On the right, you will find a small tool that precisely displays the selection frequency in the bottom toolbar.

Paul Licameli, the main developer for this function, is also working on a number of plugins. The names of these three simple, but effective Nyquist modules, reached from the Effect menu, each begin with Spectral edit and provide various filter functions. Spectral edit multi tool [10] fulfills the tasks of a classic filter, which, depending on the selection, executes a high-pass, low-pass, or notch filter.

The filter module determines its settings from your selection in the output material and immediately gets started when accessing items from the effects menu. Selections starting at the top or bottom trigger a high- or low-pass. A selection that starts with 100Hz and ends with 300Hz effectuates the use of a notch filter at 150Hz. The other two modules, Spectral edit parametric EQ [11] and Spectral edit shelves [12] also use this simple and elegant method. These modules also let you adjust the amount of attenuation in a simple interface (Figure 8). These filters are missing in Arch Linux; if necessary, you can download them from the Audacity SVN and copy or link them to /usr/share/audacity/plug-ins/.

Comparable functions in Linux-compatible audio software only appear in the Snd audio editor [13] and Sonic Visualiser [14]. In Snd, such functions can only be accessed via a programming interface, for which you would have to write such operations in Scheme or Ruby yourself. Sonic Visualiser, as the home of the Vamp plugins also used in Audacity, provides additional and more flexible spectrograms; however, operating these features requires more expertise than with Audacity. Additionally, the program proved less stable than Audacity in our lab; Ardour does not yet even provide such functions. You will find more information and a few manuals about dealing with the spectral view in Audacity's online manual [15].