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just by typing 'what is db' into google, i got a we definition of a db.
aswell as having loads of links to realted stuff, this would have told you it stood for 'decibel'. typing decibel into google will probably answer most of you questions.
dB is not really a unit of anything, although a lot of people believe it to be. It is a way of comparing values. Look it up on Google as the other person said.
We usually use dB to describe a current value of a variable X wrt a benchmark variable X0.
ie X(dB)=10log(X/X0)
If you understand dimensional consistency, you'll discover that the RHS is unit-less! Ie dB is NOT a unit, just a different representation of a value.
Now, you often hear about 3-dB bandwidth. It's when a certain value drops to half its original value.
So let X/X0 = 1/2. You'll find that X(dB) = -3dB! So every -3dB in decibels is equivalent to halving.
the thing is the true measurement is Bel and it is the log of the ratio of power w.r.t. a normalised value.
but a Bell is quite big for everyday use (a bit like gramme) so they make is a DeciBel. Thus equ for the measurement it
dB = 10*log*(P1/P2)
Now since the power dissipated in a resistor is V^2/R This can be arranged as
dB = 10*log*( V1^2/R /V2^2/R)
This reduces to
dB = 10*log*(V1^2/V2^2)
dB = 10*log*((V1/V2)^2)
dB = 20*log*(V1/V2)
The -3dB point is the point where the power is dropped by a 1/2 (go back through the 1st equ to see).
This equates to 1/SQRT(2) of the voltage (or 70.7% of the starting voltage)
It's simply a ratio, the actual value it's expressed in is the 'Bel', but this is usually too big, so the 'Decibel' is used - this is what the '10' in checkmates equation is for, to convert from Bel to Decibel.
It's commonly used for audio or RF signal applications, mainly because of the 'log' in the equation, this gives it a logarithmic response which makes things simpler.
Because it's just a ratio (and not an absolute value of anything) you need to specify what it's compared to - 0dB if often set as 0.775V, basically line level. So a negative figure is lower than that, and a positive figure is higher.
You should also be aware that dB's can be used for both POWER or VOLTAGE, but they are not the same values!.
It's often said that people who talk about dB's don't know what they are on about 8)
And the simple answer is:
-20 dB means 0.01x.
-10 dB means 0.1x.
0 dB means the quantity is unchanged.
10 dB means 10x.
20 dB means 100x.
30 dB means 1000x.
As noted, it's just a multiplier, although in our context it most often is used for voltage gain. I can correctly say that I had two pet rabbits five years ago, and now I have 200, so I had a 20 dB gain in rabbits.
Further to what has already been said, the dB is a measure of a POWER ratio. It can only relate to a VOLTAGE ratio when the voltages being compared have the same impedance. Some amplifier manufacturers wrongly quote the gain of their device in dB although input and output iomedances are not quoted.
The qoted dB value can indicate an absolute value when it has a recognised suffix appended. See this link for more information:
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