I’ve pondering this for a while, and I hope someone here might be able to come up with a definitive answer (I think the answer is “yes” by the way.)
Suppose one is inside a quiet building. For say of the example, let’s say there is no noise from machines or people being generated in the building. However, there’s a noisy party going on outdoors. If one were to open all the windows, so the noise from the party could be heard clearly, would the noise level * inside* the building go up? I’m not sure how noise is measured, but I know I’ve read it can be…
Well, since all noise is is vibrations in the air (or other medium, but we’ll just consider air), if the windows are opened, the air inside the building will start vibrating, enabling one to hear the noise. There is no difference to the ear or to a noise measuring thingamabob where the source is.
When referring to the “noise level” of a building, I’m guessing that what is really meant is an average of noise levels gathered from around the building.
Noise level is measured in decibels by an instrument that measures the physical vibrations of the air around you. So, technically “noise” level has nothing to do with the source of the sound, other than the fact that the intensity decreases as you get farther away from the source without any obstacles (say, a building’s window) in the way.
To illustrate, let’s take your example to a further extreme. Say you’re at an airport on a tuft of grass by the runway. If we could isolate that tuft of grass with soundproofing, we’d find it to be relatively quiet. However, that doesn’t mean the noise level is low around the tuft of grass without the soundproofing. There are still planes booming around keeping the level of noise relatively high. (Planes on a runway emit noise at about a 120 decibel level. That’s about 15 decibels below the threshhold of pain).
Noise level has a lot more to do with surrounding conditions (soundproofing, loud parties, etc.) than with what’s emanating from the area studied. Air is what’s important in sound, and it’s everywhere. That’s why in a vacuum the noise level is zero no matter how many physical vibrations you attempt to make.