Help understanding voltage db vs power db.

in

n

ame

ce

ower is

B

a fish

Yes, 20*log(V1/V2), if you must. GH

Reply to
George Herold
Loading thread data ...

s what you do know that ain't so."

t in mind and your troubles will disappear. (These ones, anyway.) ;)

Yeah, but then you'd be talking about 0.301 dB and 0.602 dB and we'd still have people confused about doubling power vs. doubling voltage. What's rea lly different except we are working with 10 times smaller numbers?

When they teach Bells, they should ignore the deci part to get the idea acr oss, then say, "For convenience we commonly use dB in the same way we use d l or cm".

Actually, I don't know of any common usage of the deci prefix other than dB and dl. Even dl is only used in medicine where it is commonly mixed with units like ug/dl for some bizarre reason. The medical community is nuts wh en it comes to units. But they are great at saving our asses!

--
  Rick C. 

  -- Get 1,000 miles of free Supercharging 
 Click to see the full signature
Reply to
Ricky C

And they mess up the calorie unit, speaking of 1000 calories by names like 'food calorie', 'large calorie' or 'kilogram calorie' which are often shortened to just 'calorie'.

I like to tease my doctor friends with something like "Look, we have to deal with hundreds of different products all the time while you doctors have been dealing with a model that's remained unchanged for thousands of years. Yet you still know only a fraction of what there is to know about it."

One doctor came back with "You can turn machines off." To which I replied "Yeah, and you have anesthetics".

Reply to
Pimpom

n dB and dl. Even dl is only used in medicine where it is commonly mixed w ith units like ug/dl for some bizarre reason. The medical community is nut s when it comes to units. But they are great at saving our asses!

Yeah, but who uses the (small) calorie? I've never run into a use of it.

I like the definition of calorie...

/?kal(?)r?/ noun noun: small calorie; plural noun: small calories; noun: large calorie; plur al noun: large calories; noun: Cal

  1. the energy needed to raise the temperature of 1 gram of water throug
  2. the energy needed to raise the temperature of 1 kilogram of water th

sure the energy value of foods.

Two completely different definitions casually placed side by side as if the re is no contradiction at all.

It's a bastard unit and all use of either version should be outlawed on pen alty of eating Twinkies.

The amazing thing is every day I eat enough energy to raise the temperature of a kg of water by 2000 degrees C! What do I do with all that energy???

--
  Rick C. 

  -+ Get 1,000 miles of free Supercharging 
 Click to see the full signature
Reply to
Ricky C

There seems to be a propensity of many branches of science and technology to choose their units such that the most common values end up in the interval 1..100. Never mind that a weird unit is harder to grasp than a very big or very small value.

Cosmologists are real artists in that respect. For example, Hubble's constant is really just the inverse of the age of the universe, but who'd notice?

Jeroen --stamp out convenience units-- Belleman

Reply to
Jeroen Belleman

I do, for one, albeit rarely. We still used cgs units when I was in college.

............

I was idly chatting with two doctors on my porch last year when the talk turned to the discomfort of sitting in a non-airconditioned car in the summer. I did a quick mental calculation and pronounced that, based on the energy value of the food we eat, two adult humans sitting in a car generate enough heat to boil one liter of water every half-hour. They were astounded.

Reply to
Pimpom

I do electromagnetic calculations in Gaussian units (rationalized cgs ESU). It gets rid of all the mu-noughts and epsilon-noughts, which saves blunders. Converting to SI at the end is trivial.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
 Click to see the full signature
Reply to
Phil Hobbs

Yeah, (Well fixing everyone's temperature scale to absolute zero.. is more 'important' than some power ratio. if that was the 1-100 reference?.)

Exponentially speaking, I think my brain starts to shut down around 10^10...

10^20 is almost meaningless. (yeah no real good feel for a mole of atoms.) George H.
Reply to
George Herold

By how much would that half-hour be reduced if the occupants were each given a chicken phall to eat before the experiment commenced? :-D

Reply to
Cursitor Doom

--------------------------

** Most folk connect dBs with sound - ie " dB SPL".

The reference here is a defined ( tiny) change in air pressure, so clearly not power.

But the situation is analogous to using a defined voltage as a reference and letting the actual power take care of itself.

BTW The quantity "Sound Power Level" exists too and takes account the energy in the entire sound field. While not related to the SPL at a point, for a long time the same acronym was used for both causing great confusion.

An air conditioner might carry a "65dB SPL" rating which had nothing to do with the emitted sound level or any noise standard.

.... Phil

Reply to
Phil Allison

Sorry, but decibels are not about any unit. Not power, not voltage. They're only about ratios. They're just a log representation of a ratio, which is a *pure unit-less number*.

RF designers assume they're about power. Audio folk assume they are A-weighted subjective measures of loudness. Etc.... but those are all the assumptions of a specific community, and they're just convenient lies.

Because dB are not *about* any of those units.

Clifford Heath

Reply to
Clifford Heath

LOG(x), without qualification, means the *natural* log, not to base 10.

I assume you wouldn't rather be using those?

Clifford Heath

Reply to
Clifford Heath

The usage I remember from my early days was that "log(x)" usually meant to the base 10, and "ln(x)" was used to represent the natural logarithm to the base e.

From a quick Google-search I find that this does seem to be a common convention.

Writing log10(x) and ln(x) makes it explicit and avoids us being confoosed.

Reply to
Dave Platt

Why not, there is also a ln(x) based ratio known as Neper

formatting link
It was used e.g. in telephony.

lg(x) is often used for base 10 logarithm. Unfortunately, lg(x) is also sometimes used for base 2 logarithm.

Reply to
upsidedown

That's been my experience too. In the days before computers and scientific calculators, log tables at the back of technical books were often presented without specifying the base and were assumed to be to the base 10.

No argument there.

Reply to
Pimpom

Yes, I used Nepers in a presentation about RF range and attentuation but only succeeded in losing most of the audience that had never heard of them.

piglet

Reply to
piglet

-----------------------------------

** 100% WRONG.

The only quantities that can be legitimately expressed in dB are actual power or directly relate to power.

A car can have 3dB more power than another but not 3dB more speed.

** The only convenient LIAR here is YOU.

Bugger off.

Reply to
Phil Allison

You don't get 3dB power gain because the power went from 0.02 to 0.08 in your example and that was a four-fold increase. Hence 6dB.

You doubled (added 3dB to) the voltage but because P=V^2R that quadrupled (added 6dB to) the power.

piglet

Reply to
piglet

Yup, ln(x) is 100% natural log. You could call it the un-common log!

--
  Rick C. 

  +- Get 1,000 miles of free Supercharging 
 Click to see the full signature
Reply to
Ricky C

I never thought I would end up agreeing with you - but this time I do!

--
~ Adrian Tuddenham ~ 
(Remove the ".invalid"s and add ".co.uk" to reply) 
 Click to see the full signature
Reply to
Adrian Tuddenham

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.