An End Struck Plosive Aerophone, ESPA, or slap tubes.. This smaller set is made from scrap PVC . I’ve installed larger sets made of ABS plastic at Raven Hill Discovery Center, the Charlevoix Public Library Children’s Garden,, Petoskey Central Elementary School, and Grand Ledge’s RiverWalk. A stainless steel version is at Garoon’s Gateway to Science in Lake Zurich, Illinois.