引导声(外声) External Sound BNS 9 此燕声需具其特性,需要有强力的引导能力,方能有效把燕屋外部的燕群,引进燕屋内部。 External sound. Effective to attract and lead swiftlets into house. 共有多款成功外声: Some of the successful external sound : 1. BNS 12 2. NTSG id 2 (2021 new sound)