newbie: is cutting amplitude in half -3 or -6dB?

[Swiss Frank]

Ultra-newbie question:

Is cutting the amplitude of a sound wave in half the equivalent half -3 or -6dB?

[ferez21]

-6db will be half

[djeroen]

-6.020599913279624 dB to be more or less exact

[ferez21]

-6.020599913279624 dB to be more or less exact

Did you type it from memory?

[camsr]

-6db is half voltage, combines to equal 0db. -3db is half power, which is half the wattage.

[BertKoor]

Ultra-newbie question:

Is cutting the amplitude of a sound wave in half the equivalent half -3 or -6dB?

An answer using deductional logic:

Audio on Compact Disk uses 16 bits.
Each bit doubles the amplitude.
So CDs have a dynamical range of either 3x16 = 48 or 6x16 = 96 dB.
Cassette tape has a dynamical range of 55dB at most (without Dolby)
CDs do much MUCH better than cassette tape.

So the correct answer has to be -6 or -6.0205999132796239042747778944899 dB.

[hugoderwolf]

An answer using deductional logic:

Audio on Compact Disk uses 16 bits.
Each bit doubles the amplitude.
So CDs have a dynamical range of either 3x16 = 48 or 6x16 = 96 dB.
Cassette tape has a dynamical range of 55dB at most (without Dolby)
CDs do much MUCH better than cassette tape.

So the correct answer has to be -6 or -6.0205999132796239042747778944899 dB.

Please don't do this ever again, it might induce cancer.

[ion_one]

i would say that half the signal equals -3 db as the formula is 10*log P2/P1 for powers
and the formula is 20*log V1/V2 for tensions so -6 db

careful it's logarithm in base 10 not other base...

[tomtoo]

Maybe interesting ?

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

Cutting the amplitude to the half is cutting the voltage to the half what is -6.xxx db.

Am i wrong ?

[camsr]

Not wrong, just different
As far as music recordings go, a dynamic range of 30dB tends to sound like 3 different volume levels, so this effect is very strong.

[mystran]

i would say that half the signal equals -3 db as the formula is 10*log P2/P1 for powers
and the formula is 20*log V1/V2 for tensions so -6 db

careful it's logarithm in base 10 not other base...

If you're working with regular PCM signals, then half the signal is (roughly) -6dB. If you're working with squared quantities (eg power, RMS) then half is -3dB (essentially all we're doing is dividing the result by 2 to undo the exponent once in log-domain, which saves having to take a square root first). This way you get the same dB values independent of whether you squared your signal first. Since this probably refers to the problem of adjusting and/or measuring regular signals (as they are sent to an audio interface) the 20log10 or ~6dB rule is what you want.

To the OP: to convert decibels into multipliers (eg to adjust volume by dB) use the formula pow(10, dbGain/20), and to convert measurement values to dB use the formula 20*log(value)/log(10). The ~6dB is a convenient approximation; it's useful when talking about things in terms of powers of two (rather than ten) but it's not exact.

[AnalogGuy1]

-3dB cuts the voltage by .7071, and the power by half.
-6dB cuts the voltage by half, and the power by a quarter.

The equation is voltage in dB = 20 log10(voltage/ref voltage) or
power in dB = 10 log10(power/ref power).

[Swiss Frank]

Thanks all.

I knew CDs were about -96dB s/n and I should have figured it out from there.

[davidguda]

Now this kind of confusions is why I prefer to see signal levels as multiples rather than decibels!

[xoxos]

perhaps adding fuel to the fire, what i have managed to glean is that you're pretty safe using the pow(10,dB/20) mystran gave..

x = pow(b, y);
b = pow(x, 1 / y);
y = log(x) / log(b);

perhaps the -3dB in RMS is where the confusion comes from, because i, and apparently a lot of other people, have used apps where -3 is used to reduce the signal by half (was this cooledit? been years, i wonder where it came from) before i decided to secure a definition. i believe there are certain groups who make up their own definition of decibels, perhaps changing them routinely to confuse outsiders. old sow got caught in the fence last spring.