So I was thinking... If a newly born infant was sealed in a room since birth, and over speakers in this room scientists would play a certain tone (lets say a B flat), and this tone was steady and consistent. Would that infant become accustomed to this constant sound and thus define it as their 'silence'? Since silence is the lack of sound to the perceiver, what happens when a sound is no longer recognized due to its persistence? And if in fact this is what would happen, what sort of sounds have we all tuned out due to it being a constant throughout our lives?

As I was thinking about your post, I suddenly became aware of my computer's cooling fan, and that I had not been hearing it for a long time, even though I have been here for a while and it's a constant drone.

I also remember years ago, somebody ran around a city with a sound level meter, looking for a quiet place. One of the ironies is that he or she went to the city morgue (as in "quiet as a morgue") and measured about 50 decibels (as I recall) because of the refrigeration equipment.

So, maybe there's part of your answer. We now monitor the environment with electronic impersonal equipment, so it is hard to see how any widespread global background sound would escape detection.

But maybe I have misunderstood your question.

Unless it was pointed out to the child and the tone stopped, it would just be background noise I think...the kind of stuff we filter out everyday; the freeway we live by, the hum of the fridge motor, that kind of stuff.

I guess we could detect sounds with electric instruments, but I wonder what we forget we're hearing sometimes. As fluffy bunny mentioned, what if that sound the child was accustomed to turned off? How strange would it be to be totally used and accustomed to a constant background sound, and then one day not having it be there? Would you even know it was there, or would it have taken notice that its gone to bring it to your attention?

I'm really, really good at tuning out my kids' incessant chatter. When they stop chattering, I notice right away (although that may be because I am conditioned to understand that silence from my kids leads quickly to "It was an accident!" shortly thereafter.)

But that's a good point, Purplos.

It reminds me of a phenomenon people notice when there is a solar eclipse in the warm months, that the birds fall silent. Few people, I think, really attend to continual birdsong, even though it is not a constant signal, but readily notice its absence when it stops.

So, I think this is yet another "subconscious" function - part of you is listening, but the greater-you are happy to attend to other things, relying on the subconscious listener to give you a heads up if anything out-of-the-ordinary occurs.

I would also like to toss one other idea into the mix. When a human being enters an anechoic chamber, what they often immediately notice is that they can hear their own heartbeat and perhaps even some of their own aortal sound. That sound is always there, and has always been there, like the OP's hypothetical. It is loud enough to move our hearing receptors, so why do we not hear it except under unusual circumstances?

That stuff about the birds is pretty sweet, I'll have to keep an ear open next time theres an eclipse.
So the Earth its constantly making sound, imagine how loud all that magma has to be below our feet, molten rock churning and tectonic plates melting and all that jazz. I also heard about some sort of resonance originating from the core, I wonder if those sounds have been 'automatically' tuned out since birth... (that is to say if the sounds permeate up here in the first place)