As a Briton, I can't help thinking that once the unit is in place, with its double-wall sandbag enclosure (including 4' service space), I won't have any garden left.
Why bother building the other three walls? Wouldn’t one SB wall between the unit and the exterior wall of the building cut down the noise enough?
Well... supposing I did have a bit of garden left, I might like to enjoy it in peace without hearing a constant noise of machinery.