How do you add up fan noise levels (db)?

[MrCoyote]

Take for instance you have fans in your case rated as follows:

CPU Fan - 46db
3 Case Fans - 35db each.
---
Total: 151db

Is this how noise levels are correctly added up? Or is it actually lower, and you take some sort of average of the number? The threshold of pain in human hearing is around 115-140db. Of course the case itself will deaden some of the noise of course but not much. I've never been around a system that was too loud that I couldn't handle. But I'm building a new system and am worried that the bigger, faster fans, especially the CPU fan will add considerable noise.

 

[mikef208]

I can't tell you how they are added up, but I am pretty sure you don't just add the total, cuase 151 Db would prolly kill you. LOL

I don't think you take the average either, cause if you have all 30Db fans, i am pretty sure they all produce more then 30Db together. I would like to know for sure as well.

 

[AMDPwred]

Yeah I know you don't just add em. Hehe, I have a 1200 watt system in my car and it gets about 145-155db. I can't ever listen to it like that. I take it to audio competitions and turn it up. That would be loud as hell to have 155dbs of fans in your PC, lol.

 

[jayshah]

I believe you add them logarithmically
IE. 25dB(A) + 35dB(A) is added with the formula

= 10 * log (10^(25/10) + 10^(35/10))
= 10 * log (316 + 3162)
= 10 * log (3478)
= 10 * 3.541
= 35.41 dB(A)

so the 25dB fan would have very little effect on the 35dB one.

Keep in mind that fans inside or at the back of the case will seem quieter becuase of the case itself blocking the sound, while your front mounted fans will seem a bit louder

Also "twice" as loud is approx. 10 dB(A)

BTW, your case would be about 47 dB(A), assuming all fans were outside. Inside the case probably drops the sound to maybe 40-45...That's just a guess.

 

[J3anyus]

Yes, it is done logarithmically. Here's a link to how to do it:

Decibels

It's actually not that hard. I wrote a program for my calculator that adds decibels for me. Here's the main part of the source:

Disp "1?
Input A
Disp "2?
Input B
A/10->Q
10^Q->U
B/10->O
10^O->L
U+L->C
log(C)->D
D*10->E
Disp E

Hopefully you can understand that, it's not terribly complex.

Note: One simple thing with decibels is when you are adding something to itself (i.e. two 35 dB case fans), you just add 3. So 35 dB + 35 dB would equal 38 dB, and 20 dB + 20 dB would equal 23 dB.

Hope that helps!

J3

 

[MrCoyote]

wow, thanks! I'll check out that link here soon.