I was delighted to read of the idea of a direct audio link between England and France (NCE 30 November). However, I would take issue with the reported hemispherical shape of the reflectors described in the article.
A hemisphere does not have a focal point and would lead to a poor signal to noise ratio.
Surely the designers intend to use a parabolic dish which does have the essential focal point.
On the high cost of making moulds, the designers might learn from the world of optical astronomy. Large parabolic mirrors have been made 'simply' by gradually cooling molten glass in an opentopped constantly rotating container.
Any liquid naturally forms a parabolic shape when spun this way. The faster the spin, the shorter the focal distance from the centre of the dish. I am sure this technique could be applied to wet concrete and achieve the desired tolerance.
Peter J Felton, Wilinson Associates, 119 Richmond Road, Kingston Upon Thames, Surrey KT2 5BX.