Buzzing that goes away when you touch the strings

We've probably all run into the buzzing that goes away as soon as you hand is back on the strings, or even if your guitarist touches their strings if you're on the same power strip. What causes this? I thought my bass was shielded. Yes, for this type of interference your instrument is shielded. You have copper foil and / or shielding paint in the cavity of your instrument and essentially have created mini Faraday cages.

But what does a Faraday cage need to work? A path to ground. This is the key, when you touch the strings, they are connected to the foil or paint and your body becomes a ground for the shielding making it now work. So what is happening is that there is not a path to ground through your cable and amp. You can test the cable with a multimeter, it is almost always fine. Unplug the amp and use a multimeter to test continuity from the strings to the ground on the plug. If that's good, test the ground on the outlet using one of the outlet testers you find at Home Depot, etc.

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