A water hammer arrestor is a plumbing device designed to absorb the shock wave and pressure spike that occurs when the flow of water is abruptly stopped by a quick-closing valve or appliance. This phenomenon, known as "water hammer," causes loud banging noises and can damage pipes and appliances over time. 
How They Work
Water hammer arrestors work by providing a cushion that compresses to absorb the kinetic energy of the moving water. Most modern arrestors use an air-filled chamber with a sealed piston or diaphragm that moves back and forth as pressure changes, effectively silencing the noise and protecting the system. Unlike traditional air chambers (which can become waterlogged and require periodic draining), mechanical arrestors are generally maintenance-free. 
 
Key Benefits
Noise Reduction: They eliminate the irritating banging, thumping sounds in your walls and floors.
System Protection: By preventing extreme pressure surges, they extend the lifespan of your pipes, joints, and connected appliances like washing machines and dishwashers.
Leak Prevention: Reducing stress on the plumbing system helps lower the risk of leaks and costly repairs. 
Installation
For maximum effectiveness, arrestors should be installed as close as possible to the fixture or valve causing the water hammer. 
Appliances: For washing machines or dishwashers, you can use specialized arrestors that screw directly onto the appliance's water supply valves.
General Plumbing: For general household systems, they can be installed in-line within the pipework, often in place of an elbow fitting or using a tee connection near the source of the problem.
Professional Help: While installation can be a DIY job for simple applications, complex whole-house issues may benefit from a professional plumber's assessment to ensure proper sizing and placement.