As Part 1 introduces, we get decibel using the formula

I'd blame my poor English ability that I didn't realise what the name *decibel* really meant for quite a few years. It's called deci-bel so apparently there's a *Bel* first.

The bel (B) and the smaller decibel (dB) are units of measurement of sound pressure level (SPL) invented by Bell Labs and named after him

from: Wikipedia

That's him! As stated, using *deci-*, people can make *bel* smaller, and meanwhile we have to add the
$10 \times$
in the formula, to make *bel* become *decibel*.

You might notice that when talking about voltage and current, the formula becomes

So why 20 here? I was very confused. I asked around and got this answer: For measuring something like a **magnitude**, we use
$20 \times$
, otherwise for the **power** stuffs, use
$10 \times$
... Okay then it's time to test my English again: what kind of thing is a **magnitude**? And what is not? I reckon it's a correct but not so good answer.

Finally I met a guy, he took out a piece of paper and wrote some high school maths on it:

Yeah that's Ohm's Law, I get it. Then if $P_r$ and $U_r$ are the references and $U$ is the voltage we are measuring, from the original $10 \times$ formula we have:

That's how $20 \times$ comes up. Same process for the current (Use $P=I^2 \times R$ ). Maths can be scary, but useful.

from OZLab

## Top comments (0)