Does the width of the air gap in a double glazed unit make any difference?

Only a little, until you reach very wide air gaps; usually a minimum of 50mm. This is because it is only at these wider air spaces that there is enough air within the gap

To sufficiently dampen the effect of the noise. The use of different gases in the air space, i.e. argon, has no effect on the sound insulation properties of the double glazed unit.