The quietest flushing toilets are one-piece gravity toilets such as KOHLER's San Raphael, Rialto, and Rosario. These toilets are very quiet because flush noise is caused both by the velocity of water flowing into the bowl, and by water refilling the tank. Quiet toilets rely on gravity for water to flow into the bowl. The quietest toilets are the one-piece design toilets because these toilets flush without a large "head" pressure (water falling a longer distance). Typically the loudest toilets are those that use pressurized flush valves or tanks.