Help with DB Calculations

[DaCarlson]

Decibels (dB)

It has been known for a very long time that human ears cannot resolve very small differences in sound pressure. Originally, it was determined that the smallest variation that is audible is 1dB - 1 decibel, or 1/10 of 1 Bel. It seems fairly commonly accepted that the actual limit is about 0.5dB, but it is not uncommon to hear that some people can (or genuinely believe they can) resolve much smaller variations. I shall not be distracted by this!

dB = 20 * log (V1 / V2)

dB = 20 * log (I1 / I2)

dB = 10 * log (P1 / P2)

As can be seen, dB calculations for voltage and current use 20 times the log (base 10) of the larger unit divided by the smaller unit. With power, a multiplication of 10 is used. Either way, a drop of 3dB represents half the power and vice versa.




So, if I understand this correctly. Lets say we have an amp with pushing a current of 2amps. The formula would be...

dB = 20*log[(2)1/(2)2]

I came with up with an answer of 40 db. Does this sound correct? Or am I missing something?


I tried using the formula for Amperage but it doesn't work for me either.

dB = 20 * log (I1 / I2)

[yggdrasil]

In your example I1 would be 2. But you havent stated what I2 should be.

If you mean 2A vs 0A, you'll have a problem dividing by zero... dB = 20 log(2/0).

If you expect to calculate the sound pressure created by an amp pushing 2A into a loudspeaker, you will have to know other factors as well.

[DaCarlson]

Such as which?

[Robh3606]

Take a look at this, It should help

Rob

http://www.jblpro.com/pub/manuals/pssdm_1.pdf

[toddalin]

Decibels (dB)


It has been known for a very long time that human ears cannot resolve very small differences in sound pressure. Originally, it was determined that the smallest variation that is audible is 1dB - 1 decibel, or 1/10 of 1 Bel. It seems fairly commonly accepted that the actual limit is about 0.5dB, but it is not uncommon to hear that some people can (or genuinely believe they can) resolve much smaller variations. I shall not be distracted by this!

dB = 20 * log (V1 / V2)

dB = 20 * log (I1 / I2)

dB = 10 * log (P1 / P2)

As can be seen, dB calculations for voltage and current use 20 times the log (base 10) of the larger unit divided by the smaller unit. With power, a multiplication of 10 is used. Either way, a drop of 3dB represents half the power and vice versa.




So, if I understand this correctly. Lets say we have an amp with pushing a current of 2amps. The formula would be...

Db = 20*log[(2)1/(2)2]

I came with up with an answer of 40 db. Does this sound correct? Or am I missing something?


I tried using the formula for Amperage but it doesn't work for me either.

dB = 20 * log (I1 / I2)

Also looks like you need a new calculator.

dB is a comparison. If I go from 1 amp to 2 amps, I calculate a 6 dB increase.

20 log (2/1) = 6.02 dB

Same thing if I go from 2 amps to 4 amps.

So, if dB is a comparison, what does it mean when a speaker is rated at 100 dB at 1 meter?

Zero dB is defined as a pressure of 20 micropascals. So all sound level projections are referenced to this 20 micropascal value. Therefore, sound can actually have a negative dB volume. A pressure of 10 micropascals would represent a level of - 3 dB.

[just4kinks]

Decibels are used throughout electrical engineering to express ratios logarithmically. The most familiar use of the term is as a measure of sound pressure, but "Decibel" can express any ratio of two numbers.

Don't think of the notation "dB" as a unit, it is dimensionless. Think of it as a type of measurement... kind of like a "%" sign. Note that the unit of sound pressure is actually written "dB(SPL)", although it is often abbreviated.

Your voltage and current equations are for calculating signal amplitude, they have nothing to do with sound pressure (not directly anyways).

http://en.wikipedia.org/wiki/Decibels


Tell us specifically what calculation you are working on, and maybe we can help.

[ratitifb]

dB expression takes sense only if you define a reference !
(reference can be voltage, current, pressure, intensity, power, sensitivity, efficiency, acceleration, speed, deplacement, ... any system unities you want to describe according a log way)

10log or 20log only depend on what system you are thinking (power (intensity) or voltage (pressure), respectively).

Regarding pressure, don't forget mathematical (large scale representation) and psychoacoustical (hearing) aspects of the log
(Weber-Fechner law ...)

[rs237]

online dB Calculator :

http://www.sengpielaudio.com/calculator-db.htm

regards

juergen

[ratitifb]

The human relationship between stimulus and perception is logarithmic : why not with money ?

For a long time i suggested to express my salary in dB (referenced at the minimum salary (according to french laws).
So i could claim to my boss only 3dB for increasing my salary by year.
10log or 20log in that case ? Doesn't matter ...

3 (dB) is always small compared to 50 (%) that's too big for my boss