Schallloch (or Schall-Loch; before 1996: Schalloch) (n)
(with) definite article: dasfingerhole in a stopped semidrone, allowing the drone to sound when open; when closed the drone is mute
MGG: Die Musik in Geschichte und Gegenwart (Kassel, Bärenreiter, 1949–1968, supplemented 1973-1986) / 2nd ed. 1994-2007, supplemented 2008 (➺ Schalloch).
Duden: Die deutsche Rechtschreibung (Duden Band 1). Mannheim, &c., 1991 (20, revised)