EITHER/OR by BOB GONSALVES
Even before you enter the space proper there are experiments to try: You can stand in the outdoor rotunda and clap your hands, listening for the echoes established by reflections of the sound between the dome and the cement floor. At the other entrance to the building are Doug Hollis' Aeolian harps. Mounted on the roof, the harps, of solid sheet metal, are vibrated by the wind passing over them, and the sound is transmitted by metal cable to resonators arranged in a cluster over the doorway.
Walking inside The Exploratorium one is immediately bombarded by sensation-the air is thick with activity. The museum is always busy-that's its nature. The exhibits: touch, sound and hearing, waves, resonance, electricity, exponentials, motion, vision, color, animal behavior, light, pattern, graphics: are virtually all interactive; the participant manipulates a: button, rod, mirror, string, light, capacitor: to demonstrate the principle of the particular exhibit. Here are a few exhibits that illustrate basic sonic principles.
An analog to the echoes of the outdoor rotunda is the echo tube inside the Exploratorium - a 12' diameter pipe extending some 100 feet to the ceiling. Sounds created at its opening are echoed back many times, seemingly more clearly than the original sound. Sharp impulse sounds, such as a hand clap, also produce a high pitch, similar to the stylized rifle sound used in radio and T.V. Engineer Larry Shaw pointed out that a similar phenomena occurs with electrical impulses in our atmosphere. The sounds, variously called "chirps", whistlers", or "the dawn chorus", can be detected by attaching a coil of wire to an ordinary audio amplifier (or a barbed wire fence, as was done by composer Alvin Lucier).
Next to this tube are the Pan Pipes, a collection of beautiful glass tubes ranging from 5 ft. to 2 ft. long, with a 90 degree curve in the lower end. Like the large tube, they can be shock-excited to yield a pleasant drum sound at the resonant frequency of the tube. They also serve as resonant filters of the complex sound of the Exploratorium environment, which approaches white noise (the simultaneous occurrence of all frequencies), by enhancing a certain frequency and its harmonics. A similar, electronic, device is the analog "bucket brigade" phase shifter, which selectively amplifies frequencies which are integral multiples of the time it takes for the signal to travel down the delay line.
Two exhibits derived from the wonderful world of new music are the Multiplied Glockenspiel, by Jan Pusina, and the Drum, by Steven von Huene. The glockenspiel contains within it, in addition to the metal bars of various lengths/pitches, a ring modulator. The processed output of this electronic device gives us the sum and difference frequencies of the glockenspiel and the modulating frequency, which is variable by turning a single knob. The resulting sound may have a very dense timbre, similar to large bells, or it may be very pure if the modulation pitch is in a simple ratio to the original instrument's. the large Drum, which performs pieces by James Tenney every 1/2 hour, has 32 mallets around its periphery, which are actuated by electromagnets, and appear to be programmed by electronics and a plexiglass disc with black markings indicating strokes.
One of the most visually interesting sound exhibits was, what I call, the Tympanic Membrane. This consisted of a variable speed strobe light, shining on a rubber sheet stretched over a loudspeaker. This membrane is marked with polar coordinates (concentric circles and radii). A pitch, which is variable, drives the loudspeaker. There are three controls to the exhibit, one to control the frequency of the membrane's oscillation, one to control the strobe's flashing rate, and a third marked "freeze". The relationship, in pitch, between the two oscillations (membrane and strobe) creates various effects: at times you can see that it is vibrating in several places, or perhaps breathing slowly. The freeze button causes the strobe to flash at the same rate as the speaker, causing it to appear fixed in place. At some points it appears to undulate like some sort of jello-like organism.
These are only a few of the exhibits in the sound subsector alone, and exhibits from other fields (such as waves and resonance ) make evident acoustic principles. The Exploratorium is open, Wednesday to Sunday, 1 to 5 (Wed. eve, 7 to 9:30) and is located at 3601 Lyon St., in San San Francisco. p.3