I have seen the picture before, but it wasn't until now that I figured out the Helmholtz double entendre.

Helmholtz resonation is a way to enhance certain frequencies by blowing air into a cavity shaped to resonate with the desirable frequencies, named after German scientist Hermann von Helmholtz.

Helm means helmet in German. Holtz means wood in German (nice name: wooden helmet?) but in most other languages it resembles the word "hold" making this a helmet held Helmholtz resonator. Say it fast.