dB has no units.
dB is a relationship between two values.

Speaking of voltage:   dB=10*log(V1/V2)=10*(logV1-logV2)

for example, if you want to know the gain in dB for an amplifier you take
Vin and Vout and apply the formula:

Vin=0.5v
Vout=10v

10*log(Vout/Vin)=10*log(10/0.5)=13dB


0dB is when the relationship between the two numbers is 1, for example:

10*log(1/1)=0dB

Another way of seeing this relationship is as 'gain' or 'loss'. When you
have a negative value in dB it means that the relationship between the
numbers is less than 1, for example:

10*log(1/5)= -7dB (aprox)

One common reference used when dealing with power is 1 mW, so if you have
1mW of power out of something compared to the reference you have 0dB. But if
you have, for example, an amplifier that gives out 100W of output for each
mW of input, you then have an amplifier with a gain of:

10*log(100/0.001)=50dB


Bye

Calvin


----- Original Message -----
From: "David VanHorn" <dvanhorn@CEDAR.NET>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, May 24, 2001 9:31 PM
Subject: Re: [OT]: How to read dB?


> >
> >If for volume, 20dB, 0dB,-10dB and -50dB which is bigger? Coz I am
> >totally blind in
> >reading value in dB....
>
> Decibels (literally, 10ths of a Bell) are relative power units.
> You can only correlate them to voltage if you already know the impedance.
> A gain or loss of 3dB is half or twice power
> When you express power in dB, you must give it as dB relative to
something.
> 3dB is literally saying "twice as much power" If you want to say an
> absolute amount of power in dB, you relate it like dBm (dB milliwatt) or
> dBW (dB watt)
>
> -50dB is MUCH smaller than +10dB.  In fact, it's 60dB down from 10dB, or
> 1/1000000th the power.
>
> 30dB is a factor of 1000.
>
> My audiologist once told me that at a specific freq, I had a "minor"
> hearing loss of 60dB.
> I asked her how she'd like a "minor" paycut of 60dB. She'd be making about
> $0.10 that year.
>
> I got a new audiologist now.
> :)
>
> --
> Dave's Engineering Page: http://www.dvanhorn.org
>
> I would have a link to FINDU here in my signature line, but due to the
> inability of sysadmins at TELOCITY to differentiate a signature line from
> the text of an email, I am forbidden to have it.
>
> --
> http://www.piclist.com hint: The PICList is archived three different
> ways.  See http://www.piclist.com/#archives for details.
>
>

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