Soundproofing and airflow

[Jakester]

I'm lucky enough to have an outbuilding to go and play in. Until recently it had a very shonky door, but yesterday it's been replaced with a steel security door which has acoustic infill, suggesting a noise reduction of up to 45dB. I suspect this is unlikely. 

The walls are thick stone - about a foot thick, and have recently been tanked with a concrete slurry so mass isn't an issue with the walls. We had to redo the roof a while back due to leaks, so took the opportunity to double up on acoustic insulation in the roof as well.

However, that leaves the final potential weakness, which is the window. 

It's a single pane top-opening window with quite a deep reveal. I'm now considering whether or not just to pack the reveal with acoustic insulation and seal off the window entirely by putting a panel across it, to stop it being a source of noise externally.

However, with the new door in, it's probably now the only source of airflow (the old door had a two inch gap at the bottom!).

The room is/was damp, hence the need for tanking. Even so, we still got mildew and mould in there until I put a dehumidifier in there which is always on. 

If I were to box in the window and seal it off, is it likely to cause problems with the lack of airflow? Would I need to put a plenum chamber in the boxing and an exhaust fan?

[itu]

It is not so complicated to make a silent room. Friend of mine built a very nice music room, but nearly killed himself because there was no functional ventilation.

Build a labyrinth to the vent. There should be lots of ideas in the net:

https://cundallconversations.com/2015/02/20/cold-air-mining/

I would change it so, that the "input" is between the second and third red arrow in the back and the "output" is before the last blue arrow in the front. Then it is easier to build into a wall as a thin but wide construction element. Every inner part is covered with wool which does not contain glass!

[stewblack]

That's fascinating stuff 👆 

[itu]

I want to add, that the vent I proposed earlier is only the other of (a minimum of) two... I wish no one is relying in a single vent. Air has to move, or the room will be like sauna or even dangerous.

[Skybone]

I have heard about building a vent cover with baffles in, in order to break the straight path of the sound waves. 

For example, building a box cover for an air vent, and adding wooden slats into it at about 45 degree angles, on opposing faces, so that the air has to travel further, but you still get ventilation into the room.

[Hellzero]

As explained in the labyrinth vent and by @itu, the lack of airflow will lead to asphyxia for you and wall sweating in humidity.

If you put your labyrinth vent in the ground, say one meter deep, you'll be able to have fresh air in summer and warm air in winter totally naturally. This is why very old houses with thick walls (inertia) and ventilated cellar(s) or so pleasant to live in. It's exactly the same ventilation as with the labyrinth vent explained above without the noise cancelling of the labyrinth.

These old houses are in fact the first organic-o-climatic houses and are, at least, 500 years old in their concept...

But don't forget to have two of these natural air forcing devices, ideally one at the North and the other at the South, this way the air flow will regulate itself permanently.