# Decibel Calculations

#### Eboy87

##### Well-Known Member
Somehow, my notes on this have disappeard, and searches online just give me the calculator, but don't show the formulas. I'm trying to figure out decibels, relative to a reference, i.e, what exactly 3 dBu is. I can't for the life of me remember the formula. I have the formula for finding dB in a system, dB = 10 log P1/P2, but I don't have the formula I need.

I do remember something along the lines of putting for dBu, .775 = something log something, but I'm stumped. If anyone can help, I'd be eternally grateful.

#### Eboy87

##### Well-Known Member
yes i did, and it didn't help; just gave me the formula I know already.

#### Schniapereli

##### Active Member
Well, that was the extent of my knowledge...

#### mbenonis

##### Wireless Guy
/Reaches over and grabs copy of The Sound Reinforcement Handbook

Zero dBu is defined as 0.775V. The 0.775V comes from the formula P=V^2/Z where P is one milliwatt and Z is 600 Ohms.

So 3 dBu would be double that, or 1.55V.

#### Eboy87

##### Well-Known Member
Thanks mbenonis. I lent my copy of the SR handbook to someone in my class last semester and havn't seen it since.