Does 21db + 21db = 42db?

[Id studios]

Members of AT,

I hope this is the correct section of the forum to discuss this,

I am planning to purchase extra Cooling devices for my case, let it be Cpu cooling, vga cooling or case cooling. The problem is (i guess this applies for everyone) that does it mean i will literraly get x amount of noise; (x being the number of db per unit combined)

As i can remeber from my physics classes, sound waves can either ADD up or CANCEL each of their wavelengths, Now if i were to purchase 2 fans with the similar db readings or different, does that i mean i have to add the db to get a fair indication of the amount of noise?

Thanks.

 

[sborchardt]

You do not have to add the two sound sources together to figure out what the resulting sound level will be. The decibel (dB) is a logarithmic value. Therefore if you double the sound energy the increase will be 3 dB. Therefore 2 - 21dB sources will produce 23dB. A 3dB increase in sound pressure levels is barely noticeable, but you will be able to distinguish between 1 and 2.

 

[govtcheez75]

I'm 100% positive that two quiet fans, don't equal one loud one. I am also about 80% sure that 2 quiet fans is a tiny bit louder than 1 quiet one. I can't say for sure what the formula would be, but I would have to guess it would be something like this: 21db+(21db*.15)=24.15db. It wouldn't add much though, since they are very similar, and will be about the same distance way from your ear while working. I'm guessing that you'll "hear" the one that's closest to you the loudest, which will drown out much of the other fan noise. On a similar note, if you get one loud fan (say 60db), and a bunch of quiet ones, the total noise output will be very close to the loud one, since it will itself drown out much of the noise coming from the quieter ones.

 

[govtcheez75]

Originally posted by: sborchardt
You do not have to add the two sound sources together to figure out what the resulting sound level will be. The decibel (dB) is a logarithmic value. Therefore if you double the sound energy the increase will be 3 dB. Therefore 2 - 21dB sources will produce 23dB. A 3dB increase in sound pressure levels is barely noticeable, but you will be able to distinguish between 1 and 2.

how did you come across that 3db value? I'm just curious, since I was typing up my response before I saw yours...and my 15% is pretty close (was a total guess on my part).

 

[akira34]

The +3db increase for two sources of the same rating is posted on at least one site that puts up what the different db levels are like in the real world. One site is here with links to other sites with more information...

An excerpt from that site dealing with two sources at the same level...

"The result of this logarithmic basis for the scale is that increasing a sound intensity by a factor of 10 raises its level by 10 dB; increasing it by a factor of 100 raises its level by 20 dB; by 1,000, 30 dB and so on. When two sound sources of equal intensity or power are measured together, their combined intensity level is 3 dB higher than the level of either separately. Thus, two 70 dB cars together measure 73 dB under ideal conditions. However, note that when the AMPLITUDE of a single sound is doubled, its level rises 6 dB."

Another place to look is here for information...

It's really 'amazing' what a quick, simple, and even 'dirty' google search will get you... :shocked:

 

[Blain]

Originally posted by: Id studios
Does 21db + 21db = 42db?

With that formula... Can you imagine how loud the world would be?
Suicide would be common place :shocked:

 

[arcas]

No. Sound is logarithmic; each doubling of intensity results in 3db increase.

Some years ago I wrote a short application that takes as input various sound sources and computes the overall db level. You can take a look here.

 

[L00PY]

There are also online calculators that do the math for you like this one.

 

[acivick]

Ah, Physics III coming back to haunt me (or us). I believe you were confusing when two sound producing sources are in or out of phase and the resulting wavelength changes with some sort of increase of intensity. I could be wrong, since I haven't taken that class in 3 years.

 

[Id studios]

Thanks guys,

Though when thinking about it, if the formula i posted was true, we would go insane as BLAIN suggested previously.

Thanks Team, Now i can purchase a cpu cooler without any worries.


"Ill have one of those ThermalRight Xp-120 please "

 

[Jeffyboy]

Depends how far apart the sources are. 21db + 21db = more 21db coverage ;-)