A resonant tube captures and reradiates the near field, defeating acoustic short-circuiting

Evidence

Figure 20.9 Savart’s cup and resonator. Two kinds of resonance are involved in this instructive setup. First, the oscillation in the shape of the cup, in which the originally circular cup takes on an elliptical shape with long axis first in one direction and then, halfway through the oscillation, with long axis perpendicular to that direction, is driven by a cooperative resonance with the bow. Second, the length of the tube can be adjusted so that the air column in the half-open tube is resonant with the cup oscillation, amplifying the sound emanating from the cup resonance. The tube captures the strong near field, phase-shifts it, and reradiates it (NFC mechanism), breaking the deadlock of acoustical short-circuiting. — Eric J. Heller, Why You Hear What You Hear- An…, 1900

Categorization

• DEFINE: Very clear account of how near-field coupling works: capture, phase-shift, reradiate.