Question
When my irrigation system is running, it makes a loud and unpleasant hammering sound for the entire duration of the zone.  All 9 zones have this problem, and the cadence of the hammering changes based on flow rate.  It is most noticeable, and annoying, during the start of a new zone.  Strangely, when I turn on a faucet inside the house while the irrigation is running, the cadence (and volume) increases.  I've been in the basement and held the copper pipe that feeds the irrigation system, thinking that it must be banging or rattling against something, but no amount of bracing/dampening has relieved the problem.  Since it appears to be flow-rate dependent, could it be an interaction between the pressure breaker and the water meter?

I'm fairly certain that the pressure breaker has a role in this, because when the irrigation is off, and I open up an outside faucet which has its own 3/4" pipe to a spigot, the water meter runs like crazy but no hammering noise occurs.

I'm located in Minnesota, and by code I have a pressure vacuum breaker -- Watts 720A, located outside, at least 12 inches above the highest sprinkler.  My water pressure is high -- about 80PSI -- and I often have the traditional water hammer sound inside the house when the dishwasher or washing machine stop filling.

Also, a few of my neighbors have shared similar stories of loud hammering irrigation systems, and all have the same pressure breaker, however none appear to be as loud as mine.

Any ideas?  

Answer
Water hammer is a condition caused when water in a plumbing system is subject to changes in velocity and or pressure.  Since water is a non-compressible substance, it cannot absorb the shock, and it it demonstated by the sounds and shaking that you see as water hammer.  As far as I know, there are two solutions that are fairly simple.  One is to install a pressure reducing device in your system.  This will help decrease the pressure and velocity of the water in your system, keeping it more even.  The second solution is to install a dampening device in the system that will act like a shock absorber for the hammer.  I would suggest that you consult with a plumber for both of these ideas.