Here are some animations and figures from my presentation at the COMSOL Conference in Cambridge last year, showing how sound fields can be manipulated using topology optimization. You can cite  if you want to recreate any of these.
Mode Converter I - 01-to-02 mode:
Mode Converter II - Evanescent_01-to-00 mode:
Mode Converter III - 3D complex mode to 00 mode (Before and after):
Tweeter Phase Plug:
You can of course read much more about the details in the paper and presentation.
 R. Christensen, "Acoustic Topology Optimization – Implementation and Examples", COMSOL Conference 2019 Cambridge, https://www.comsol.dk/paper/acoustic-topology-optimization-implementation-and-examples-81961