The acoustic complication was born from a practical need: to read the time in the dark, without looking at the dial. Then it became the most expensive and difficult technical demonstration in mechanical watchmaking. The principle is simple to describe, impossible to execute well: one or more hammers strike gongs or bells fixed inside the case, producing chimes that encode hours, quarters and minutes.
The fundamental distinction is between à la demande striking and au passage striking. The minute repeater — minute repeater — acts on demand: you activate a slider or lateral pusher, the mechanism reads the position of the time wheel train and translates that information into sequences of chimes. The hours strike on a low tone, the quarters on a double sequence (low-high), the minutes on a high tone. Sonnerie au passage, instead, sounds automatically at every hour and quarter, without the wearer's intervention. Grande sonnerie combines both logics: it strikes the quarters at passage and also allows repetition on demand; in twenty-four hours it generates approximately 912 hammer blows distributed across 96 distinct sequences. It is the most demanding variant from an energetic and mechanical standpoint.
The carillon adds harmonic complexity: instead of two hammers, it employs three or more, each tuned to a different note. The result is a structured melody — not a simple rhythmic code — that marks the hours, quarters and minutes on precise musical intervals. The quality of the sound depends on the tuning of the gongs, the thickness of the case, the material and the tension of the springs that move the hammers. A case in gold sounds different from one in titanium or steel; sapphire crystal transmits vibrations differently than mineral glass. Some manufactures have worked on the inner edge of the crystal as a resonant surface.
There is also the decimal repeater, less common: it divides the hour into ten-minute segments instead of fifteen, making acoustic reading more direct but changing the logic of the intermediate chimes.
From my workbench I have learned that the quality of an acoustic complication is evaluated in three seconds of listening: the sound must be clean, sustained and without parasitic resonances. A poorly fixed gong or a hammer out of regulation are immediately noticeable. The centripetal regulators of the striking mechanism — such as the silent centripetal flying strike regulator — serve precisely to eliminate the mechanical noises that degrade the useful sound. I don't know everything that happens inside certain proprietary movements, but I know how to recognize when a repetition sounds good.