Formula to work out total db's in case.

[blockers]

Hi.
Did a quick google search and couldnt find any formula that made some sense.

Is there on to work out the total db's in your case? or do you just add them up?

Will

[joshwa]

no, it's something or other which means you can't just add it up...

found this though:

Adding dB falls under the CADET rule.
(Can't Add, Don't Even Try)
This is because the scale is exponential.
Still, calculating the final dB level is
quite easy.

I won't go into detail, but if we for example wanted to
add two fans that each produced 18 dB we'd go about
it like this.

18/10 = 1.8
10^1.8 = 63 (aproximately)
63 + 63 = 126
log 126 = 2.1 (aproximately)
10 X 2.1 = 21 dB

18 dB + 18 dB = 21 dB (an increase of 3 dB!)

Now the increase of 3 holds true for every addition of
two equal dB values. If they're not equal you'll have to
do the backwards conversion before you add them
together and then convert them back to dB in the
end.

[Axion]

because Db scale is exponential of base 3 whereby a doubling of sound is expressed by 3Db

basically 3Db is 2x as loud as 1Db
9Db is 4x as loud as 1Db

so one can 'add' them just not using conventional mathematics

[fondie]

it is conventional mathematics, just not primary school level

it's a ratio of 2 numbers, though there are a number of reference points giving rise to the various log scales, it's a logarithmic scale of base 10.

the dB scale that you're probably thinking of regarding sound levels is dB SPL, which is caculated using this formula :-

dB = 20 × log (SPL2/SPL1)

where SPL2 and SPL1 are the sound pressure level you are comparing, with a reference of 0.0002 dyne/cm2 or 20 uPa

to combine sound pressure levels yo can use this forumala :-

SPLC = 10 x log ( ( 10 ^ ( SPL1 / 10 ) ) + ( 10 ^ ( SPL2 / 10 ) ) )

where SPL2 and SPL1 are the sound pressure level you are combining and SPLC is the calculated combined SPL.

this scale is used when producing the tables you may see of the various dB levels associated with various sources and most probably what manufacturers use for their claimed dB figures. of course SPL depends hugely on how far away the source is and a number of other factors. so adding up various different manufacturers claimed dB levels beomes failry meaningless. if you want you can measure SPL yourself using a SPL meter (£25), but due to ambient noise and the limitations of the meter they become rather inaffective at very low SPL levels, so professional equipment and help maybe required to get accurate readings.

our ears however are more complicated, and percieved loudness depends on other factors, such as the frequency of the sound, so there are various weighted dB scale's to account for this.

i think the moral of the story is, don't bother, just use your ears!

[joshwa]

i think there was a website somewhere that would add them up for you if you put in the figures, but i don't know where it is

[fondie]

http://www.doctorproaudio.com/doctor...ladores_en.htm

there's an online calulator, under SPL addition, if you want to add more than 4, just sum 4 then sum more on top. i was just saying the results you get won't be very accurate if you're just pluggin in the manufacturers specs, and the actual loudness you percieve won't follow this scale at all.

[blockers]

Cheers for the info guys.
I think i will have to put all the fans on some sort of manual variable resistor that i can tune to my liking. Sleeping time=low noise, games = lows of noise. Therefore keeping comp cool