Liminal Kicks is my latest album, released on the new label Superpang based in Rome, Italy, with cover art designed by Joe Gilmore. It’s a collection of tracks made with drum synths and saturation. I used Mathematica to generate MIDI files with up to one million notes, where the timing between notes was programmed to change exponentially, with the exponent set to the Golden ratio (1.618 or 0.618). ‘Liminal’ derives from the ancient Greek limen (λιμήν), meaning ‘threshold’ or ‘doorway’.
In physiology, psychology, or psychophysics, a limen or a liminal point is a sensory threshold of a physiological or psychological response. It is the boundary of perception. (Wikipedia)
Each of these tracks crosses this perceptual threshold. As the drums increase or decrease in tempo and cross the threshold, our perception of it changes between rhythm and pitch. At lower speeds, we hear separate events as increasing/decreasing rhythm, but beyond the threshold we perceive a note or frequency, increasing/decreasing in pitch.
Rather than playing around that threshold, these tracks go well beyond it, exploring what happens when the tempo is very high. As the sounds speed up or slow down, there is often a cyclic or fractal pattern in the frequency shaping of the sounds. This is probably due to a kind of Moiré or wagon wheel effect, caused by the constantly changing difference between the length of the drum sounds and the tempo. You can see an example of the fractal kind of pattern in the spectrogram of track 4, ‘kick2tom1048576′:
The sound is made more complex by a saturation effect, which adds some non-linearity to the equation. Sometimes it also produces an effect like a Shepard or Risset tone, an audio illusion of constantly rising/falling pitch, where it generates tones that are overlapping (in time) and parallel (in frequency). You can hear this most clearly in the ‘snare16384’, and see it in the yellow lines in the image below:
In ‘kick5’, a symmetrical track, you can hear the cyclic patterns in the upper frequencies. In the spectrogram they form similar shapes at different scales. Counter-intuitively, as the duration between notes deceases, the size of the patterns increases, and vice versa. This is probably due to the exponential rate of change, which starts fast and slows down, or vice versa.