I tested out the hollow metal of this installation at Durham Botanical Gardens after accidentally knocking it and getting an impressive sound.
Knocking it in different places gave different notes. The highest note from the narrowest section.
This was a bit deeper.
This was a very low note. Since they are all part of the same space, it can't be the air that is vibrating. Maybe stationary waves are forming across the metal. Wider metal would be bigger wavelength and thus deeper note.
