db formula

[Fullmetal Chocobo]

Does anyone know of a formula to calculate db(a) with several devices, such as if you have 8 25db fans going at once, what is the ambient db of the area? I don't see how they could be additive, so I'm pretty sure it would merely be a curve on a graph, and not a steady line.
Tas.

 

[Brian23]

ambient db of the area is always 0 dba.

 

[Fullmetal Chocobo]

I wish. And nice quote by the way. I also like "If my video card has more memory (256mb) than your system, it's time to upgrade". But seriously, is db something like
fan1x(1db) + fan2x(.5db) + fan3x(.25)...
Tas.

 

[f95toli]

Of course it depends on how the fans are arranged. However, I guess you just want an approximate value.
Look here.

 

[The Boston Dangler]

Decibel is a relative measurement, while dBmV, dBa, et cetera are absolute.

A fan measured to be 40 dBa is exactly 10 times louder than a fan at 30 dBa. The difference is 10 dB.
A fan at 35 dBa is exactly twice as loud as a fan at 32 dBa. The difference is 3 dB.

3 dB= 2x
6 dB= 4x
9 dB= 8x
10 dB= 10x
20 dB= 100x

That being said, there are so many variables involved, only a good SPL meter can give you an accurate final number.

 

[TuxDave]

You want a nice formula, ok....


10*log(base 10) of [10^(db1/10) + 10^(db2/10) + .....]

 

[Fullmetal Chocobo]

Okay, fair enough. I'll just have to play with it. So for now, I'll submit my formula:
1 + 1 = 3 Try to figure that one out. Thanks for everyone help!!!
Tas.

 

[Extrarius]

Originally posted by: TuxDave
You want a nice formula, ok....


10*log(base 10) of [10^(db1/10) + 10^(db2/10) + .....]

This is the correct way to 'add' decibel values - you convert from the logarithmic scale back to linear, add, then convert back to the logarithmic scale

Decibels are a logarithmic system: db = 10 * log10(value)
Thus, to undo it, Value = 10 ^ (db / 10)

Then you add up the values to the the sum. To convert that back into decibels, db_sum = 10 * log10(value_sum)

Or, for the question of 8 * 25db fans:
value = 10 ^ (25/10) = ~316.23
value_sum = 8 * value = 2529.84
db_sum = 10 * log10(value_sum) = 10 * log10(2529.84) = 10 * 3.4 = 34 db

All this only works if all the db values are relative to the same base. Also, this is only in an ideal environment where they all add perfectly and no dampening or amplification occurs. In reality, the total noise should be somewhat lower or higher.

 

[f95toli]

You need to use Tuxdaves formula if you want to calculate the number directly, another option is to use the diagram on the pag I linked to.

 

[TuxDave]

Originally posted by: tasburrfoot78362
Okay, fair enough. I'll just have to play with it. So for now, I'll submit my formula:
1 + 1 = 3 Try to figure that one out. Thanks for everyone help!!!
Tas.

But... my formula works..

 

[KoolAidKid]

Originally posted by: Extrarius

Originally posted by: TuxDave
You want a nice formula, ok....


10*log(base 10) of [10^(db1/10) + 10^(db2/10) + .....]

This is the correct way to 'add' decibel values - you convert from the logarithmic scale back to linear, add, then convert back to the logarithmic scale

Decibels are a logarithmic system: db = 10 * log10(value)
Thus, to undo it, Value = 10 ^ (db / 10)

Then you add up the values to the the sum. To convert that back into decibels, db_sum = 10 * log10(value_sum)

Or, for the question of 8 * 25db fans:
value = 10 ^ (25/10) = ~316.23
value_sum = 8 * value = 2529.84
db_sum = 10 * log10(value_sum) = 10 * log10(2529.84) = 10 * 3.4 = 34 db

All this only works if all the db values are relative to the same base. Also, this is only in an ideal environment where they all add perfectly and no dampening or amplification occurs. In reality, the total noise should be somewhat lower or higher.

This is equivalent to adding 3 dB every time the number of fans is doubled. 8 fans = 3 doublings -> a 9 dB increase. So if one fan is 25 dB, 8 will be 25 + 9 = 34 dB.

 

[Fullmetal Chocobo]

Awesome... Thanks everyone! All this, only to find out that it will require a lot of soldering to replacy my 8 80mm fans. So I'm just going to work on the dual 120mm fans.
Tas.

 

[Calin]

Now take into consideration that some of the sound generated is generated by air speeding over fan's blades, over heat sinks and so on. You can't reduce all the noise (but using two 120mm fans will still give you less sound, and better (easier to hear) sound