# Decibel Calculations

Discussion in 'Sound, Music, and Intercom' started by Eboy87, Mar 12, 2007.

1. ### Eboy87Well-Known Member

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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.

2. ### SchniapereliActive Member

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HAve you checked Wikipedia?

3. ### Eboy87Well-Known Member

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

4. ### SchniapereliActive Member

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Well, that was the extent of my knowledge...

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/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.

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