Thanks for the answer!

I think I'm beginning to understand why headphones typically have wonky treble frequency responses - presumably it's all centimeter-length resonances in the shell of the driver and the cavity of the cup. On that note, I recall that some string instruments like the Oud, which have ribbed backs, always have an odd number of ribs. Supposedly it's to minimize unpleasant resonances. If you build a 5 or 7 sided polygonal cup - would it kill all the cup resonances and have a really flat treble response? The only reason we make cylindrical cups is for mass production purposes; but with 3D printing this is no longer an issue if you just want to make one for yourself.