Spectrograms of Alvin Lucier’s new album, made with Sonic Visualiser, using the following settings:
Amplitude scale: dBV^2, colour palette: green
Sample window (x axis): Hann; 4096 samples; 93.75% overlap
Frequency bins (y axis): all, log scale
Using a larger window size allows better representation of lower frequencies but decreases the temporal resolution (because of the time-frequency uncertainty principle). Applying more overlap makes up for some of this loss. However, there are lots of visual artefacts: smears, shadows, reflections, interference. Nevertheless, some of the music’s main features are represented, like the point at which the beating frequencies of the two guitars are at their slowest in the mid-point of Criss Cross. You can distinguish the piano from the other instruments in Hanover, and see the structure of ascending/descending/static notes.
Changing the bin setting from ‘All’ to ‘Frequencies’ looks quite different:
Sonic Visualiser documentation says:
If set to Frequencies, each peak’s bin will be drawn with a single line at a position that corresponds to an estimate of the actual frequency present within the bin, rather than the theoretical frequency range of the bin. This instantaneous frequency estimate is arrived at by comparing the theoretical phase difference between consecutive time frames at the nominal bin frequency with the measured phase difference.
This setting reduces the visual artefacts, increasing the contrast between sound and silence, but loses much of the structure that was previously visible, although perhaps it better represents the music’s minimalism. Squeezing the horizontal scale and using a larger window (16,384 samples) shows the structure of Criss Cross more clearly. Now the two separate frequencies are visible at the start and end, and they cross in the middle, as one guitar descends a semitone from C to B and the other does the opposite: