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Correct way to calculate decibel level??

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Insomnium

Senior member
I hope someone knows how to do this: what's the correct way of calculating the decibel level in a system. For example, if I have 4 case fans in an Antec SX830 rated at 20dB each, then what is the actual decibel level of the system? What's the equation for figuring this out?
 
There's no real equation since the sound level is relative to position. Therefore I'd test the volume at your head's position while seated at the comp(cause int he real world that's what matters right?).
 
True, that's probably what i'll end up doing. I was just curious because i know there's an equation for figuring out the general dB level, I just can't find it anywhere. Thanks anyway 🙂
 
I'll bite.

Sound levels are usually measured in intensity levels referenced to a small basic level that usually represents the threshold of human hearing.

This is I_0 = 10^-12 Watts/meter^2, which is a power density if you look at the units.

To calculate the sound level in decibels, use the formula

Sound Level = 10 log (I_sound / I_0)

In your case, you just want to add two sound levels that are already in decibels to calculate worst-case sound level in the case, right?

Well, we can calculate the I_sound/I_0 ratio for ONE fan and then add them into the formula above to get the worst case sound level in decibels.

For ONE fan:

20 dB = 10 log(I_sound / I_0)
and dividing both sides by 10 we have
2 dB = log(I_sound / I_0)
and exponentiating both sizes we have
10^2 = I_sound / I_0
or
100 = I_sound / I_0
So ONE case fan has a intensity ratio of 100.

Now we can plug 4 of these babies back into the original formual for sound level:

SL_total = 10 log( (I_fan1 + I_fan2 + I_fan3 + I_fan4)/I_0 )
SL_total = 10 log(100 + 100 + 100 + 100) = 10 log (400)
and plugging this into my handy Casio calculator I get
SL = 26.02 dB

Now, realize that this is just an approximation, and will be worst case as though each fan were a point source of noise adding together at the same point, which obviously isn't the real "case" inside your case, now is it?

Kyle
 
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