# db and dbm

Discussion in 'Wireless & RF Design' started by shahnawaz, Jan 20, 2009.

1. ### shahnawaz Thread Starter Active Member

Dec 30, 2008
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hi dear all
can you plz tell me what is difference between db and dbm.
what is the relationship between both and how difference also.

Apr 20, 2004
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Apr 5, 2008
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4. ### leftyretro Active Member

Nov 25, 2008
394
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DB is a dimensionless value, just a ratio between two values, where as DBM is a ratio of a value referenced to 1 milliwatt. At least that's how I recall the difference.

Lefty

5. ### KL7AJ AAC Fanatic!

Nov 4, 2008
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That is correct.

Eric

6. ### RAH1379 Well-Known Member

Dec 13, 2005
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+3DBs doubles the power -3DBs cuts it in half. +10 DBs its a 100 fold increase, -10DBs is 1/100th the signal strength.

7. ### RAH1379 Well-Known Member

Dec 13, 2005
69
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sorry, i meant a 10DB change is equal to 10 times not 100

8. ### CDRIVE AAC Fanatic!

Jul 1, 2008
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105
Roger that.
The origin of the Decibel is interesting unto itself because it was based on how the human ear perceives changes in audio amplitude. A.G.B determined that our ears are not linear. I've often wondered how many human test subjects A.G.B used for his experiments.

9. ### David Bridgen Senior Member

Feb 10, 2005
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dB on its own means that the figure which precedes it is so much greater than some other, unstated, figure. It is therefore a (logarithmic) multiplier, but without knowing the unstated figure there is no way of knowing just how large either of them is.

e.g. 3dB means a power which is twice as large as another one, or, another way of saying the same thing, a voltage which is 1.41 times as large as another one. In neither case we can't deduce the actual value of the smaller or larger voltage or power.

dBm means that the figure which precedes it is so much greater than 1mW.
e.g. 6dBm means a power which is 4 times greater than 1mW, or 4mW.

10. ### CDRIVE AAC Fanatic!

Jul 1, 2008
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LeftyRetro (Post4) gave the correct and textbook answer.

11. ### David Bridgen Senior Member

Feb 10, 2005
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Yes, I noticed that too - except that textbooks usually get the conventions right. dB, not DB, and dBm not DBM.

And by the way, it's decibel, not Decibel.

Last edited: May 13, 2009
12. ### CDRIVE AAC Fanatic!

Jul 1, 2008
2,219
105
Since we're going to get anal over this I might as well point out that the decibel translates to 1/10th of a Bel but I've never seen a Bel.
Oops, I capitalized again! I guess I just like those caps.