Alex Hay wanted to pick up body sounds: brain waves, muscle activity and eye movements. This is a difficult problem. Our bodies are very quiet. You need to be able to detect and amplify signals on the order of 50 microvolts and have a sufficiently high signal-to-noise ratio on your pre-amplifiers. Of course, such equipment already existed in the medical field, but filled a room and cost tens of thousands of dollars.

Pete Kaminski, Fred Waldhauer and Cecil Coker built a battery-driven differential amplifier which had a peak gain of 80 db from 1/2 Hz to 10 Hz. The whole unit, batteries and all fit into a 1 by 3 by 5 inch box. To do this in 1966 was no mean feat. The signal from the differential amplifier was fed into a voltage-controlled oscillator, then to an FM transmitter, that sent the sound to the speakers.

Electrodes were placed on Hay's head and body to pick up brain waves, muscle activity, and eye movement. A large plastic plate was fastened on Hay's back on which per placed the specially designed differential amplifiers with a peak gain at low frequencies of 80 db and voltage-controlled oscillators that picked up the sounds from the electrodes on Hay's head and body; and FM transmitters sent these sounds to FM receivers in the Control Room and then to loud speakers around the Armory.

Hay entered the space of the Armory in a flesh-colored suit and began to carefully and slowly lay out squares pieces of duck cloth of the same color in a regular grid. The squares were stenciled with the numbers 1 through 64.  When he finished, he sat facing the audience in front of a television camera. Behind his back was a television projector, and the image of his face was projected on the screen behind him. He sat motionless but opened and closed his eyes and moved them from side to side. Steve Paxton and Robert Rauschenberg enter wearing suits of the same color and carrying long poles, used the poles to pick up the cloths in numerical order and put them in two piles on each side of Hay. The piece ended when Rauschenberg and Paxton had finished picking up all the cloth squares.

The sound for Grass Field was produced by various body sounds.  In the first part, when Hay was laying out cloths, there were sounds of the body at work; in the second half the sounds were of a stationary body under intense concentration.  Hay has described the sounds:  "I could recognize the sounds from eye movements and in the last half of the concert, I started picking up very good brainwaves.  I had a regular microphone attached to my throat.  It was in the audio range and could be transmitted very well.  I had an electrode placed right below my chest and it was recording some sort of lung activity.  I was getting a cardiac sound and a sort of siren sound."