When I first saw this Henry Moore sculpture, simply entitled Composition, I was fascinated by its delicate curves and the feeling of fluidity as you trace your eyes across its surface; even without touching it (a definite no-no in the Tate Modern in London where I came across it), you can feel your hand gliding over its smooth, polished contours.
With this in my mind, the first thing I wrote was a very simple melody using the standard Western 12-tone scale, with prominent rising and falling leaps, seven phrases in length. This is the melody that you can hear played by the gliding, siren-like synthesiser. I harmonised this with gentle synth chords, using major thirds and augmented chords to create a sense of wonder—I love the mood created by this kind of harmony in pieces such as Alban Berg's Piano Sonata.
A big thing I wanted to explore in Tree Rings was the relationship between pitch and rhythm. This is something I'm really fascinated by. We think of notes and rhythm as separate components of music, but really they are the same thing: a note is just a sound vibrating very fast (most musical notes are vibrations of 20 to 5,000 Hz, where Hz means "times per second"). To put it another way, if you transpose a piano, guitar, voice or any instrument down a few octaves, you'll hear a regular beating sound like a drum beat.
The way I put this into practice in Tree Rings was by assigning each note of the melody a low frequency, ranging from less than 1 Hz to around 10 Hz (i.e. frequencies we hear as beats rather than notes); lower frequencies for low notes, higher frequencies for high ones. For each note of the melody, I doubled it at a slightly higher pitch to generate a beating sound at this frequency; exactly the same technique as the pulsating, meditative drones created by binaural beats. The tempo also fluctuates as the melody moves from note to note, the number of beats per minute always being 15 times this beat frequency. To explain this a bit more clearly, here's an example: for one of the melody notes, I assigned a beat frequency of 4.75 Hz, so I doubled the melody with a note 4.75 Hz higher to create a sound that pulsates 4.75 times a second, and the tempo of the music at this point is 71.25 bpm (i.e. 15x4.75). My scribbled notes about these beat frequencies actually ended up on the artwork of the CD version of the album!
The buzzing bass interjections are also doubled with higher notes based on these beat frequencies, but because the bass is so low, these changing frequencies can actually be heard as separate notes that crawl up and down above the bass line. Like in Afro-Cuban music, I decided to use claves to mark the tempo as it continually accelerates and slows down. This is actually the only acoustic sound in the piece; everything else was creating using synthesisers.
To provide the final layers of atmosphere, I generated some additional bleeps, bells and plucked sounds using very simple electronic sounds (sine waves, pulses, square waves). To maintain the total integrity of pitch and tempo, for each note of the melody, I converted the corresponding beat frequency I'd chosen into audible pitches (6, 7 and 8 octaves higher). For the note I mentioned above, for example, I'd chosen a beat frequency of 4.75 Hz, so I used pitches of 304 Hz, 608 Hz, and 1216 Hz (a very flat E-flat). I wanted these frequencies to ring out like wind chimes beneath each note of the melody and give it a feeling of buoyancy.
This piece just scratches the surface of the infinite possibilities that exist when we explore the natural relationships between pitch, rhythm and tempo.
You can listen to (and download) Tree Rings and the rest of Sanctuary (Overtones and Deviations) here.