I had a conversation recently that I’ve had many times. I figured it’d be useful to share the gist of it and fill you in on some (just a little) of the techie background.
Hum and Guitar String Ground
So here's the problem:
You’ve got your guitar plugged in. You’re not playing it and there’s a noise. A hum. It’s not terribly pleasant.
Thing is, you touch the strings and it’s gone.
The response: It's ok. It's not a problem. That’s all as it should be. Guitars pick up interference and that comes out the amp as a hum. When you touch the strings, it's supposed to get quieter.
Who’s grounding who?
If you’ve ever noticed this or discussed it with other players, someone might have mentioned that the noise reduction is because you’re grounding the guitar.
Actually, it’s the other way around.
If your guitar is working properly, there’s almost certainly a string ground: A wire from the bridge/strings to a ground-point inside the guitar (see this post from a little while ago and this one from longer ago). From that ground-point, there’s a wire connected to the ground or shield of the output jack. That ground continues through your guitar lead and into the amp. Through the amp and… well, we don’t really care what happens after the amp right now.
The thing is, there should be a low-resistance path to ground through your guitar and amp circuitry. In comparison, a human is a really crappy, higher-resistance way for a signal to get to ground. Especially a musician as they're probably wearing scruffy sneakers with rubber soles. ;-)
So, when you touch your guitar strings, it’s you that’s being grounded through the guitar.
The reason the noise quietens is that when you’re grounded, you no longer act as a fleshy meat antenna (careful, now). Ordinarily, you’re standing around soaking up various electrical interference from the environment. Touching the strings means a lot of that interference goes off to ground where it dies and is mourned by nobody.
There’s an argument I’ve heard that a player also acts as a ‘shield’ of sorts when grounded like this. While there might be something to this, I’m guessing the (relatively) higher resistances of the body probably mean this isn’t a major effect.
So it's all working fine?
Yep. Guitar picks up up some environmental noises, you touch the strings, noises are reduced. That's all good.
But what about when I don’t touch the strings?
When’s that? If you’re playing the damn thing, you’re touching the strings. If you want to put down the guitar, or you want to put your hands in the air like you just don’t care, turn down the volume.
There’s more to grounding that most people think. I think I’ll revisit this soon. #nerd