Thanks Henry

Now, you will have to forgive my ignorance over the next few threads as I am not very technically minded, and the next three pictures are of pictures of pictures so are not the best!

Dave, please correct any inaccuracies, don't sue me!

The acoustic lenses as we know them were a bi-product of an experiment to get more sound out of a tweeter.
What Dave and his team did, was to fire a tweeter into two kind of dishes made of hardened clay, in the hope that this would make a louder, directional sound. These concave indentations were, for obvious reasons, nicknamed 'The inverted Dolly Parton', and where the focal point of both met, generated a 'phantom' tweeter.
I believe they are in the top, black rectangle on this picture.