A water hammer arrestor absorbs the shock wave created when water flow suddenly stops preventing the loud banging noise in pipes. Install arrestors near washing machines dishwashers and quick-closing valves. These devices contain a sealed air chamber or piston that cushions the pressure surge. Most screw directly onto the supply valve between the valve and hose. Water hammer can damage pipe joints and fittings over time making arrestors a worthwhile investment for protecting your plumbing system.