Noise x is occurring at any given time, and noise y is occurring simultaneously.

Noise x dba > noise y dba

Is the total noise equal to noise x dba or x dba plus y dba

Mind blown

op rewords as "why don't my eardrums explode when i drop a box of pins"?

If a tree falls and no one is around
And a box of pins falls at the same time

Does the additional noise created by the box of pins have any bearing on the overall noise?

Blown

yes, if you stop measuring in dba

e: i am actually not sure which device will give you the results you are after (an increase in "total noise" equal to the sum of all "noise makers"), but i am sure that a dosimiter is not the utensil for the job

This post was edited by dude_927 on Dec 6 2014 11:44am

It's not as simple as just adding decibel. Noise (or sound in general) is a wave propagating through air. If you add additional waves on top, they interact in rather complex ways. For example, it's actually possible for two "noises" to cancel each other and create silence. That's how noise-cancelling headphones work. They create a noise of their own to cancel the ambient noise they "hear" through destructive interference of waves.
The noises can also be at difference frequencies and not actually change the volume (decibel) at all. You'll just hear more things, but they will not be any more or less loud. So no, noises volume doesn't simply add, although noise 1 CAN increase the volume of noise 2 under certain circumstances.

compare 5 kids in a lunchroom vs. 300 kids in a lunchroom

Look up standing waves in a college physics textbook for the full explanation