WHISTLE FREE - Multi-pattern corrugation disrupts the vibrations that cause whistling. INDOOR / OUTDOOR - Great for gas fire pits, gas log sets, fireplaces, and other fire features. SPECIFICATIONS- 3/4-inch MIP to 3/4-inch FIP fittings and 3/4-Inch ID BLACK STAINLESS STEEL - Stainless steel construction painted black for durability and longevity. NG/LP - Works with natural gas and propane appliances. WHISTLE FREE - Multi-pattern corrugation disrupts the vibrations that cause whistling.