Are Decibels Additive?

[Lufkin]

More specifically, If I had two fans producing noise at 10dB each, would I hear a combined 20dB of noise from the two of them? Or would I hear 10dB of noise from both of them? Or would I hear some level of noise 10db<noise<20dB?

I ask because I have a coolermaster with 2 loud case fans, and 1 loud AMD cpu fan. I would like to replace them all with 80mm panaflos (buy an SK7 or SLK800 heatsink to replace my current coolermaster fan/heatsink combo). The tower sits on my desktop next to my monitor about 2 feet away from my desk chair, so I'd like to reduce the nosie level as much as possible without sacrificing too much air flow.

 

[JamesM3M5]

The DeciBel scale is logarithmic. I'm too lazy to look up the exact math of it, but doubling the noise is a 3dB change. If a case fan runs at 27dB, two of them will run at 30dB combined. I'm pretty sure of that, but I could be wrong. Check google for more links regarding dB scale and relative noise.

 

[Fallen Kell]

Well, you could probably take the natural log of the values, then add them together and then an inverse natural log... somtehing like that...

 

[Lufkin]

Thanks for the replies...I googled up this information --

It is very important to remember that decibels and similar acoustical quantities have properties different from more conventional units. Sound pressure levels, for example, cannot be added together as can kilograms. The combination of two noises with average levels of 60 dB does not give a sound pressure level of 120 dB, but 63 dB.

The addition of a noise with a level of 70 dB to a room with a level of 80 dB will result in no measureable difference in the overall level. This does not mean, however, that a large number of secondary sources can be introduced into an environment without increasing the overall level. If ten 'negligible' 70 dB noise sources are combined with one 80 dB noise source, the resulting level will be 83 dB. Fortunately in building acoustics, there is seldom a need to combine noise levels in this way or to do many complicated calculations with decibels or other logarithmic units. These examples merely emphasize the peculiarities of the decibel scale.


Information provided by: http://www.nrc.ca

 

[labgeek]

You also need to remember that it's from 2 different sound sources. If there are differences between frequency and phase, the combined db won't be 100% - so 27+27 might not be 30 in the example above. Place 2 speakers next to each other with the same source signal (mono not stereo). Then reverse the wiring on 1 so that it's 180 degrees out of phase with the other. Watch what happens.

 

[pelikan]

I just replaced all my fans with 80mm Panaflo L1A's. Three for my case, one for my video card heatsink (SVC GC-68), and one for my slk-800. It's the best move I've made in awhile.