Spek is a small application that shows the spectral representation of any audio file in a time-varying graph, usually called spectrogram. The frequency of the signals is represented in the vertical axis on a linear scale, and the intensity of the signals is represented in decibels by a different color.
The program doesn't playback the audio file to obtain its spectrogram; it only takes a few seconds to display it. After this, you can save it as an image file. To open an audio file you can drag and drop it. To save the spectrogram as image, the program uses the same name of the audio file, and uses the PNG extension by default, but you can manually change both the name and the extension file (JPG and BMP image formats are also supported).
One disadvantage I found is that the spectrogram cannot be amplified, you can only maximize the program window for this purpose - the graph is replotted each time that the window size is changed. On the other hand, the program automatically scales the axes to fit the entire content of the audio file in the spectrogram, and includes a function, which was recently added, that allows you to change the lower and the upper limits of the dynamic range in dBFS, with the use of some keyboard shortcuts.