First of all, grounding prevents damage from lightening to electrical equipment and connected loads.  It acts as an intercept where extremely high voltage lightening occurs in the atmosphere.  The voltage tends to sink immediately, especially where there is a low impedance path to ground present. 

Secondly, how low of an impedance is needed to accomplish the second purpose of grounding which is to provide good RFI shielding, especially for sensitive high impedance loads such as microphones or for that matter any low level signal that is processed in high gain amplifiers.  Naturally, it goes without saying that the lower the impedance to ground one can achieve, the better the shields will be.  The National Electrical Code requires a maximum grounding impedance of 25 Ohms.  However, the benchmark standard for RFI shields that is the accepted maximum impedance for having effective shields is 5 Ohms. 

There are numerous ways that 5 Ohms impedance can be attained in most areas of the country.  Euffer grounds (rebar encased in a cement slab foundation) are probably the easiest to construct on new construction projects.  A burried electrode of some size and length, such as a 1" copper pipe or larger run for at least 20 feet below the frost line in moist soil is usually another effective means of meeting the standard.  Several ground rods (3 or more) spaced at least 5' apart works in more favorable areas. 

However there is one system that in our opinion isn't used nearly often enough that excels in efficiency and in how it often results in 1 Ohm impedance to ground or less.  That system would be the use of GEM25 (Ground Enhancement Material) in a "made electrode" system.  The GEM25 material is also called BENTONITE.  Bentonite is a concrete like clay material that is electrically conductive and is sold commercially in 25 lb. bags.  It is applied just like any bag of dry cement.  We recommend digging a 40 ft. long trench about 3' deep (in most locales) with a 2"-3" layer of the bentonite layed in the bottom running the full length of the trench.  Then, run a bare 4/0 copper wire on top of the bentonite and apply another 3" of ventonite on top of the wire.  Then water the mixture down according to the manufacturer's directions and let the mass harden.  What you will have when you are done is a 4/0 copper wire electrode that will never corrode encased in electrically conductive cement.  You will likely have only 1 Ohm of impedance to ground for your entire bulding and sensitive electronics and have very effective RFI shielding throughout.