The Algolian musical instrument was a large instrument consisting of many small chimes that were struck with small wooden hammers and played according to certain ceremonial rhythms.
An Algolian musician was aboard the USS Enterprise-D during the biennial Trade Agreements Conference and entertained the visitors of Ten Forward by playing his instrument.
William T. Riker later transmitted the rhythm by subspace as a covert distress signal from the Ferengi Marauder Krayton whose crew had kidnapped him, Deanna and Lwaxana Troi. (TNG: "Ménage à Troi")
The actual instrument shown and heard is bamboo angklung, which originates from Indonesia. (citation needed • edit)