Specially built wood horn known as holztrompete sounds at Met Opera in Wagner’s `Tristan und Isolde’

NEW YORK — About 4 1/2 hours after the first notes of Wagner’s “Tristan und Isolde,” a startling sound emerges from the wings, one many in the audience likely have never heard before.

A nearly 4-foot wooden horn known as a holztrompete, specially constructed to the composer’s somewhat ambiguous specifications, signals the arrival of the ship carrying Isolde and King Marke to Brittany, inspiring a mortally wounded Tristan to hang on to life for a few more moments.