Thanks. That was the info I was out for. I’ve always loved these tunnels for their acoustic. As a kid, I’d never cross one without trying different shouts or even playing harmonica.
I had one of these tunnels right across the street from where I grew up; across the Tervurenlaan in Brussels. One day in the 70’s, an older guy came toward me inside that tunnel, asked me to pass him the harmonica and played like a god. I like to believe that could have been Toots Thielemans 
