Iain,
> If I place 4" 705 on a wall with a 4" air space behind it how low do you think that would absorb? <
It will absorb down to below 20 Hz. However, it won't absorb very much that low! Absorption of porous material like rigid fiberglass falls off as you go lower in frequency, but it's still absorbing something.
The better question to ask is "Where do I put four inch thick 705 to get the most benefit at low frequencies?" The answer is in the corners first, and then maybe put some more on the front and/or rear walls.
--Ethan
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