special unpitched stops also appear in some organs. among these are the zimbelstern (a wheel of rotating bells), the nightingale (a pipe submerged in a small pool of water, creating the sound of a bird warbling when wind is admitted), and the "effet d'orage" ("thunder effect", a device that sounds the lowest bass pipes simultaneously). standard orchestral percussion instruments such as the drum, chimes, celesta, and harp have also been imitated in organ building.