Dead spot in the fish tank in Maryport

In the aquarium at Maryport there is a fish tank with a spherical viewing cavity that you can crawl into. The picture above shows from the outside through the fish and then the cavity. I crawled in. There was the odd sensation of being able to hear in one ear and not the other. I moved around. It became apparent that there was an acoustic dead spot. My first thought was that it would be a node on a stationary wave set up inside. Thinking about it later, it occurred to me that the dead spot was almost where you'd expect the focal region of a spherical mirror to occur. But that should be loud not quiet. I'll try to take a tape measure next time I go to get some better data. Below is the view from the inside looking out.