Knock on Wood is a collaboration between composer Tom Johnson and artist Martin Riches. Control system and software are by Dipl.Ing. Manfred Fox. Shown for the first time at Circuit, centre d'art contemporain, Lausanne, for 6 weeks from 9th Feb. 2019

The installation uses ready-made wooden percussion instruments playing complex rhythms automatically that would be difficult - but not impossible - to play accurately by hand. Each group of instruments is made from a different kind of wood, each with its own defining timbre.

The installation consists of five different groups of instruments, each playing in a cycle ranging from approximately 20 minutes to one hour. Each group plays its own melody independently of the others followed by a silence:-

This results in drifting layers of sound with one, two or more groups playing at the same time punctuated by occasional periods of silence.

Solution 571 This is one of the 572 sets of nine-beat rhythms in measures of 18 beats, calculated by the mathematician Franck Jedrzejewski. It is played on a woodblock with a high-pitched incisive sound.

Call and Response A trapezoid woodblock plays the Calls. The Responses are produced by a frog-like instrument played by a scraper mechanism.

Three Tempos In the recording the three instruments were played standing side by side but in an installation they can be spaced out 20 meters apart.

Quartet Played very fast on a log drum: four different pitches from one piece of wood. Quite difficult for a live player; the four notes are not lined up like they are on a marimba.

Deep Rhythms It combines a Latin salsa instrument, the cajón, and five Chinese temple blocks. The cajón keeps the beat and the temple blocks produce the deep rhythms according to rules laid down in The Geometry of Musical Rhythm by mathematician Godfried Toussaint.

Deep Rhythms is on a low platform - 5. The other instruments are mounted on open-backed pedestals made of brown MDF that contain the control systems.

The murals, the ovals in the exhibition layout above, are enlargements of the diagrams drawn by Tom Johnson to explain his compositions. They were projected onto the walls with a beamer and skillfully traced with an acrylic felt-tip pen.

1983   The Reversibles / Flute Playing Machine / DAAD Galerie, Berlin
1990   The Chord Catalogue / Sound Machine / Ballhaus Naunynstrasse, Berlin
1993   Theme music: Atelier de création radiophonique / Sound Machine / Paris
1994   Linear Percussion / 24 Piece Percussion Installation / Ak.der Künste, Berlin
2000   8 Pieces / 24 Piece Percussion Installation / Parochialkirche, Berlin
           Do it yourself / Installation / Parochialkirche (1st of TJ's tilework series)