Morning SoundonSound,

I am currently studying sound engineering and I've got a question if you could help me solve.
It's a topic under Decibels. The question is:

How many Decibels are in between 4mV and 800mV

I can't seem to figure out the formula for it but I have the answer. The answer is +46dB

Can anyone help me?

You must have been given the formula!

Decibels is a ratio of two quantities, in this case signal voltages.

Decibels = 20 x log(v1/v2)

So 800/4 is 200

Log 200 is 2.301

20x2.301 is 46

Answer is 46dB

I claim my test marks!

Hugh

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Technical Editor, Sound On Sound

Hi Hugh,

First of all, thank you so much. Does this work for V, pascals and the others as well?

Sort of:
The dB is actually a ratio of power, not voltage, and is defined as dB = 10 * log (p1/p2), but power is proportional to the square of voltage, so dB = 10 * log ((v1*v1)/(v2*v2)) = 20 log (v1/v2).

Thus for other things you need to decide what relation they bear to power, (linear or square law) in order to decide what the multiplier needs to be used.

Regards, Dan.

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Audiophiles use phono leads because they are unbalanced people!

Quote Fruitfly:

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Does this work for V, pascals and the others as well?

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Pascals? Basically convert the ratio in question to a logarithmic value : 10x log(desired value/reference)

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

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C.LYDE
http://soundcloud.com/c-lyde

Fruit Pascals?

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www.jackruston.com

Quote Fruitfly:

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Does this work for V, pascals and the others as well?

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For Pascals you have to use the same formula as for V. So, dB=20log(Pa1/Pa2).

As for "others", I don't know what you mean

Quote Jack Ruston:

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Fruit Pascals?

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http://www.loverslanestudios.co.uk

Thanks! you guys have been great

But I've actually tried both the formulas to calculate the dBs of Voltage and others Which is:

10 * log ((v1*v1)/(v2*v2)) = 20 log (v1/v2)


I cant seem to get the right answer for it - is all the answer suppose to be negative?
I'm doing an online quiz and it seems to be telling that it's not the right answer.

the questions are:

1. How many dBs between 50 MicroPascals and 200 MicroPascals
I got 16 for this.

2. How many dBs between 2V and 200V
I got -10.

3. How many dBs between 5mW and 500mW
Answered 18

They all seemed to be wrong. The Bels are killing me

Quote Fruitfly:

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They all seemed to be wrong. The Bels are killing me

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get your bracketing right!!! Also - what Log are you using on your calculator?

As in all exams, showing your working is everything. Just giving an answer means nothing. So, show your working for the first one and it'll be easy to show where you're going wrong.

Quote narcoman:

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Quote Fruitfly:

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They all seemed to be wrong. The Bels are killing me

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get your bracketing right!!! Also - what Log are you using on your calculator?

As in all exams, showing your working is everything. Just giving an answer means nothing. So, show your working for the first one and it'll be easy to show where you're going wrong.

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Indeed!^
And who uses microPascals? I make 50 of 'em ~ equal to 8dBSPL? But then I am not well you know!

Dave.

Underwater acoustics (My day job) uses 1uPa as the default reference level.....

Regards, Dan.

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Audiophiles use phono leads because they are unbalanced people!

eyes going.


though that said underwear acoustics

Quote Fruitfly:

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I cant seem to get the right answer for it - is all the answer suppose to be negative?

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You'll get the same answer with a + or - symbol depending on which value to put as v1 and v2.

If the question asks for the difference then only the value is relevant, not the sign.

Quote:

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1. How many dBs between 50 MicroPascals and 200 MicroPascals
I got 16 for this.

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I get 12dB. Pascals is a pressure, so 20xlog ratio

Quote:

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2. How many dBs between 2V and 200V
I got -10.

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I get 40dB

Quote:

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3. How many dBs between 5mW and 500mW
Answered 18

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I get 20dB (this one is a power, so 10xlog ratio.)

It really is very simple: for powers it is 10x the log of the ratio of the two values. For other quantities like voltage, sound pressures etc, it is 20 x the log of the two values.

Hugh

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Technical Editor, Sound On Sound

Quote narcoman:

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eyes going.


though that said underwear acoustics

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And with the 'subtly enhanced and padded' versions available nowadays for those who want to simulate butt extensions, such acoustics could actually be measurable!


Martin

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YewTreeMagic