EEVblog #49 - Decibels (dB's) for Engineers - A Tutorial

[jpmcbride]

The 20log and 10log formulas really confuse people. Its easy to explain though ...

First, the dB is defined in terms of power as dB = 10 log (P1/P2)

If you want to solve for voltage, you substitute into the above formula P=V^2/R to get,

dB = 10 log [(V1^2/R) / (V2^2/R)]

Now cancelling the R's above gives us, dB = 10 log (V1/V2)^2,

And the rules of logarithms let us move the 2 down like this,

dB = (2) 10 log (V1/V2) which gives us our dB formula for voltage,

dB = 20 log (V1/V2)

[yeoldeengineer]

Oh, and: Nice tutorial, Dave!

I'm not sure I could explain it that well, dBs have been with me since my teens. The problem below still had me scratching my head though. It's amazing how these supposedly basic things can still trip you up at times.

[yeoldeengineer]

Now if you want something *hairy*, try calculating the voltage and power gain of a unity gain buffer (with an actual source and load). Hint: You can express both in dB, but the value will not be the same. Why?

[pvbinoyable]

@yeoldeengineer the value will be same. for calculating power in decibel you have to use 10 log (P2/P1) and for voltage 20log(V2/V1) . you might have used same formula for both. thank you

[yeoldeengineer]

Since it seems to be a common question: dBs in sound are easy.

dB SPL (sound pressure level) is dB relative to 20 µPa, a value close to the mid-frequency human hearing threshold. (It's AC, so probably an RMS value.)

Now sound pressure is analogous to voltage, and sound particle velocity relates to current in the same way. Hence, it's "voltage dBs".

A microphone will usually have a spec'd sensitivity in dBV / 1 Pa = dBV / 94 dB SPL. This directly links the sonic to the electrical side.

[ElectronicsPubVideos]

thanks a lot for info dave, very nicely explained!

[Jamesrevelations]

Thanks a tone dave,,,,awesome information...i wonder if you can put up a lil more on the use of DB in sound ...loved it all,,,,God bless keep it coming

[piraat6666]

keep up the good work

[5341773]

u re owsum sirrr,u have done

really appreciatable work

[Films4You]

pessimist add 3db and get deaf

[Films4You]

pessimist zero Decibels, leave 'bel' alone LOL

[AxelTiger]

like the 555 shirt lol

[AxelTiger]

I wish you taught me electronics in highschool ha ha! youd make a great tutor

[Luccasio1]

Great vid, very helpful, Thanks!

[sanjosebum85]

I LOVE YOU MAN~!

[DanFrederiksen]

especially with sound intensity and amplification dB is intuitive because a sound has to be 10 times as strong for us to perceive it as twice as loud. same with light intensity. but really, the 'unit' has no merit ESPECIALLY not when they do idiotic shit like have two meanings of db (x10 vs x20). I will make it a point to use factors instead of dB and of course that's better and easier to work with. it's not easy to use log.. please

[istilllikevinyl]

Anything more about conversions, eg from Dbv - DbSPL etc?

[illusionLord]

The glass of water is 6dB down or 3dB down?!:)!! Great video as always! Congratulations!

[indidesidude]

Brilliant ........

[Interdiffusion]

Brilliant presentation, really useful stuff!!!

[miwgub]

Whats a expression :)

[NobleIronCross]

Dear dB, I love you so much.

[ForViewingOnly]

Great video, very clearly explained. I've only watched 3 of these blog vids and I'm hooked! Dave's enthusiasm is infectious!

[heroineworshipper]

15 years of using DB's & could never describe in words what they were or why 1/2 was sometimes 3db & sometimes 6db.

[shodanxx]

I think this is really confusing to sometime use 10 log and some other time 20 log

I didn't really get the why of that

when I learned ham radio I was told that 3 dB is double the power and that was correct and that 30 dB was 1000 time the power I wasn't told about using 20 dB for magnitudes

so when someone whatever was 30 dB I though it was 1000 time more intense but if they were talking about a voltage or magnetic field strenght it was only 316 times more .. no wonder people are confused !!

[salat]

@shodanxx: I'm a ham too and I it's easy to rembember: If you double the voltage in a circuit,the current doubles too and since P=I*U we get: 3 db(V) which leads to a an increase of 3db(W)

e.g. with a 1 Ohm circuit:

from 1V to 2Volts there's a ~6db(V) gain

from 1W (1V * (1V/1Ohm)) to 4Watts (2V * (2V/1 Ohm)) we get 6b(W) gain

that's the reason for 10*log(p1/p2) for power and 20*log(v1/v2) for voltage..

73, DO9SAS

[Zed1967]

An optimist would say the glass is half full

A pessimist would say the glass half empty

An engineer would say the glass is twice as large as it needs to be :)

[m3alnemer]

سوف أقبلك لو إستطعت. لقد أنقذت حياتي. قبلة أخ ليس قوس قزح ؛)

[leocechet]

where do I get a shirt like yours???

[m3alnemer]

@leocechet

zazzle.com.au/eevblog/gifts?cg­=196338080235192904

[m3alnemer]

@leocechet zazzle.com.au/eevblog/gifts?cg­=196338080235192904

add you w's

[VeXorian1337]

Aaaah Dave... 20 mins just for dBs... :P

[zilym]

Nice lecture. Makes sense. Hopefully I'll remember it next time I run into dB's.

[Teboo100]

Very helpful video...

thanks Dave!

[TheCynicalAtheist]

Incredible Video, taught this pom a lot!

Thanks Dave!

[electra]

Great videos. Please continue to do these traditional electrical engineering topics/issues and ignore the people that request simpleton versions or projects and hacks.  There's plenty of that junk on the internet already.

[nblfyb]

i wish my teachers were this energetic when i was in college. haha i was looking for dB in reference to music/sound engineering. i studied EE though but music is better :D

{Benson and Tonic}

[c0dyd0g]

Hi Dave! Love your videos!

If you have any side projects or hacks, can you share those with us?

BTW, I happen to enjoy the low-level theory stuff very much. I understand, though, that it might go over some heads. Thanks!

[JINXZER]

Hi Dave, great videos , i'm learning a lot from you so I would like to thankyou again. All my life I thought you get the money then the power then the respect. As I can see thier is no money in that equation just the knowledge which gives you power and when you share it, you get the repect.

[eydryan]

oh, also, just my two cents, your should make something that appeals to a larger audience next time, that would get you some more viewers.

something that explains a broad but simple to grasp problem/curiosity. no idea what it could be. stay cool Dave!

also, great shirt!

[eydryan]

hmm i distinctly remember commenting on this :))

anyway, what i understand from this is that basically dBs are useful because we're talking a log scale rather than a linear one.

Thank you Dave for making us smarter :) Your videos are always fun to watch

[kchididdy]

Correct me if I'm wrong Dave....

I generally don't like to memorize formulas. I would probably go one step further and state that power is proportional to the square of magnitude. In high school math, we learned that (assume log base 10): log (v^2) = 2 log(v). Since we are dealing with "deci" Bels, we multiply both sides by 10 to yield: 10 log (v^2) = 20 log (v). In other words, dB values are indeed ratios, but it's implied that the ratio is in terms of power/intensity.

[JohnM3D]

You are exactly what my incompetent instructor wished he was: didactic.

Thanks a lot. :)

[rgfcarson]

explained very well :)