How to calculate dba?

[ndee]

OK, I have 2 fans, which are 12dba loud each, how many dba's are they if I want to calculate the total dba they're generating?

 

[dszd0g]

The general rule is CADET: Can't Add, Don't Even Try.

Let's say you have 38dB and 42dB. It does not equal 80dB.

Divide each by 10: 3.8 and 4.2
Raise 10 to the power of each: 6310 and 15849
Add those two number: 22159
Take the log base 10 of that number: 4.3
Multiply by 10: 43dB

So 38dB + 42dB = 43dB.

 

[dszd0g]

I would like to add that distance plays in, so this is the ideal case where they are occupying the same space. If the two sources are close together it shouldn't be too far off.

 

[MikeMike]

Originally posted by: dszd0g
The general rule is CADET: Can't Add, Don't Even Try.

Let's say you have 38dB and 42dB. It does not equal 80dB.

Divide each by 10: 3.8 and 4.2
Raise 10 to the power of each: 6310 and 15849
Add those two number: 22159
Take the log base 10 of that number: 4.3
Multiply by 10: 43dB

So 38dB + 42dB = 43dB.

now thats confusing! how about 6 fans? i dont know there db level though 6 would be crazy!

 

[dszd0g]

Let's say you had 6 38dB fans, because you would rather have a lot of quiet fans than one loud one. Is this a good choice?

Divide by 10: 3.8
Raise to the power of 10: 6310
Multiply by 6 (number of fans): 37857
Take log10: 4.58
Multiply by 10: 45.8dB

Those 6 38dB fans probably offer more cfm than one 46dB fan.

 

[ndee]

Originally posted by: dszd0g
The general rule is CADET: Can't Add, Don't Even Try.

Let's say you have 38dB and 42dB. It does not equal 80dB.

Divide each by 10: 3.8 and 4.2
Raise 10 to the power of each: 6310 and 15849
Add those two number: 22159
Take the log base 10 of that number: 4.3
Multiply by 10: 43dB

So 38dB + 42dB = 43dB.

Thx a lot. Uh, that's kinda confusing.

 

[dszd0g]

Divide each by 10: 3.8 and 4.2
Raise 10 to the power of each: 6310 and 15849
Add those two number: 22159
Take the log base 10 of that number: 4.3
Multiply by 10: 43dB

Let me try and explain it with more steps.

1) Divide each of the dB numbers that you are adding by 10.
If adding 38dB and 42dB divide each by 10.
38/10 = 3.8
42/10 = 4.2
2) Take 10 to the power of each number:
So for 38dB, take 10^3.8
For 42dB, take 10^4.2

10^3.8 = 6310
10^4.2 = 15849

3) Add the results from the previous step:
6310 + 15849 = 22159

4) Take log_10 of that number:
log_10(22159) = 4.3

5) Multiply previous result by 10
4.3 * 10 = 43dB

If you want it as one equation:

10*log_10( 10^([First dB]/10) + 10^([Second dB]/10) + ...)

 

[Goi]

Here's an easier one...for every doubling of the number of input fans, just add 3dB to the final dB value.

For example, 2 fans which are 12dB each will be 15dB combined. 4 of those fans would be 18dB. the dB scale is a logarithmic scale, so you can't add them, and they aren't proportionate, or even linear.

 

[arcas]

Last winter I wrote a program to do exactly what you're asking.

Here