A row of trees can filter a voice to sound an octave higher

Evidence

Suppose the high pitch the woman needed to sing was near A4, 440 Hz. Her voice would carry 440, 880,1320,1760,… Hz partials. Assuming trees spaced by 4 m, they would have been standing about 6 degrees off the normal from a line connecting the first two trees in the front row. This would cause the required 1.1 ms repetition delay, removing the 440, 1320, 2200,… components, leaving a resonantly enhanced 880, 1760, 2640,… harmonic series—that is, an octave higher. — Eric J. Heller, Why You Hear What You Hear- An…, 2654