Posted 24 October 2004, 18:23
Over a bed of sruti-box-like drones, a slow chord sequence plays through twice, first in closed voicing, then in open voicing. Best listened to with headphones.
Duration: 8 minutes, 5 seconds.
This piece was inspired by a section of David B. Doty’s excellent book The Just Intonation Primer. Chapter 2, “Acoustic and Psychoacoustic Background”, pages 17-19, discusses the phenomena of difference tones, summation tones (collectively referred to as “combination tones”) and the periodicity pitch. I won’t get into a detailed description of these terms, but essentially they describe pitches that are synthesized by our ears and/or by our higher-order neural processing in response to hearing a set of two or more simultaneous tones. These tones are not always perceivable by the listener, but are theoretically always present, or at least potential.
When I read this part of the book, I was fascinated by Figures 2.10 (page 17) and 2.11 (page 18) which show in musical notation the chords that emerge from certain simple-ratio intervals when these combination tones are perceived. I decided to make a Csound instrument that, given an interval, would produce a chord consisting of the original dyad plus its derived combination tones. My initial motivation was to simply make these chords audible with properly-tuned intervals (not possible on my equal-tempered keyboard), but when I heard the results, I decided to write something using these materials.
The instrument I eventually came up with (instr 2 in the Csound score), takes as input a starting pitch and a ratio (which together describe the base dyad) and computes an eight-note chord consisting of the dyad plus three difference tones (first-, second-, and third-order), two summation tones (first- and second-order), and the periodicity pitch. Of course, for a given dyad, the resulting combinations tones are not always unique, so there are not always eight distinct pitches. A chord based on an interval smaller than an octave will generally cover a wide range (i.e., it’s in an open voicing), but I wanted to be able to hear what a close voicing version would sound like, so I added the capability of “reducing” the chord such that all the tones could be transposed as necessary to be restricted to a given interval, such as an octave. The chord sequence in this piece is played in close voicing the first time, and in open (or natural) voicing the second time. I also added a little flanging to “fatten” the sound.
The chord sequence in the piece is based on a series of simple-ratio intervals, all except one (9/8) taken directly from the Doty figures. The complete set of ratios used (in the order in which they appear) are 8/5, 7/5, 6/5, 7/6, 9/8, and 5/4.
The drone in the background is based on an instrument I found in the Amsterdam Catalog of Csound Computer Instruments v1.2, which implements Risset’s design for a harmonic arpeggio. The drone consists of four instances of this instrument on the pitches 1/1, 3/2, 2/1, 2/1 (tonic, fifth, tonic+8ve).
The entire piece is based on a root frequency of 60Hz. If I were to make a “European” version, I would base it on 50Hz (you can do this yourself by commented out line 256 in the score, and uncommenting line 259).
I wrote the piece with Csound version 4.23f12 (the “canonical” version), using the 64-bit Windows executable. The piece renders fine in real time, at least on my machine (try this yourself by uncommenting line 19 and commenting out line 20 in the score).
Copyright © 2004, Dave Seidel. Some rights reserved. This work is licensed under a Creative Commons Attribution License.