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What this technique does is eke out as many as possible of the sympathetic resonances, or harmonics, latent in the reverberating strings, thus imitating the natural harmonics found in plucked strings and struck metal bars.

Natural harmonics are ‘hidden’ notes, usually quite high in pitch, which are found within any given sound, like the additional colours of the spectrum that are contained within white light.

In “April 2007,” the distance traveled is the length of an entire string, but the pitches seem wild and uncertain, yet still in unison: The players are using natural harmonics, listening to each other to locate the same glassy whistles and ghost tones — pitches that seem to suddenly dive down deep or leap up high.

Mr. Riley and Ms. Wu began their set with an improvisation that touched on the techniques and accents of their instruments, often in direct comparison: natural harmonics on the guitar were answered with bright plucked tones on the guzheng; assertive guitar strumming called forth equally vigorous strumming on the guzheng; and Ms. Wu’s pentatonic melodies found a response in sometimes modal, sometimes diatonic runs by Mr. Riley.

Mr. Young was then developing his drone style with the Theater of Eternal Music, letting the natural harmonics of long-held tones ring out loudly, sometimes for hours.