Yup. A bass trap with springs in it, that do ... something!

It is called: "A low-frequency bass trap with Pistonic/Helmholtz action"
Here's more info on that... https://www.msr-inc.com/downloads/pdf_f ... ntary3.pdf
Once again, I'd love to see some real test results from a real independent lab! It would be fascinating to find out what that thing actually does...
- Stuart -