The gong is the vibrating element that produces sound in acoustic complications: minute repeaters, quarter repeaters, sonneries, and mechanical alarms. It is not a bell — it is a tempered steel wire, curved around the movement and fixed to the plate or case, struck by a hammer at a precise point. The distinction matters, because gongs and bells have radically different acoustic behaviors: a bell concentrates vibration within itself, while a gong propagates it through its anchor point toward the case, which acts as a resonator.
The number of gongs determines the sonic grammar of the watch. With two gongs you get two tones: a low note for the hours, a high note for the minutes, and a combination of both for the quarters. With three gongs — a Carillon configuration — you produce a third intermediate tone, and the quarters are marked with three notes in sequence rather than two. With four gongs and four hammers, you can reproduce melodic sequences like the Westminster Chime, the same melody as Big Ben.
Tuning is the most delicate work. Each gong is hand-filed, millimeter by millimeter, until its vibration frequency reaches the desired pitch. Curved gongs — those that wrap almost twice around the movement, sometimes called cathedral gongs — require more complex tuning than round gongs, because the curvature alters the distribution of tensions in the metal. I have disassembled enough vintage repeaters to know that a poorly tuned gong sounds fine in the workshop and stops doing so when the case is closed: the case changes everything.
The material and geometry of the case affect the final result. Grade 5 titanium amplifies and clarifies the chimes better than steel, which in turn sounds different from gold. Audemars Piguet developed with the Swiss Federal Polytechnic of Lausanne the Supersonnerie system, which anchors the gong not to the plate but to a separate device designed as a resonance chamber — the same principle as nineteenth-century pocket chronometers. Chopard patented a different solution: gongs carved directly from monoblock sapphire crystal, so that vibration propagates outward without passing through the movement. These are opposite approaches to the same problem, which is always the same: sound dissipates, and recovering it costs years of work.
Gongs visible through the caseback or sapphire crystal dial — those polished mirror-bright — are an aesthetic detail, but not a secondary one: a perfect mirror reduces the micro-irregularities on the surface that dampen vibration. I don't know if the difference is measurable with instruments; I know that the best tuners search for it anyway.