The Quiet-Duct Ultra™ / Low and the Quiet-Duct Ultra™ / Green silencers are designed for low-frequency noise control and green building applications. The Quiet-Duct Ultra™ / Low Series is available in three models: ULS, ULM, and ULL in three configurations. The Quiet-Duct Ultra™ / Green Series is constructed of 100% environmentally friendly, using recycled acoustic fill material.