I played around a bit with an audio signal generator. The falling of bass is noticeable below 100 Hz, and no sound can be heard at 20 Hz. I am very interested how could be the bass boosted with the holes covering or internal felt trick.
I have (had :-( ) another set of ortos, the Technics EAH-820. It has rectangular diaphragm. Unfortunately one side is broken. Could someone give me some advice how can I replace/repair it? I removed the good diphragm, and it could be used as a template for photograhic duplicating. I even purchased some thin aluminium-covered plastic foil, that seems to be similar in thickness. Now I have to transfer the pattern from the good diphragm to the foil with some photoresist method, then etching it. Here is a photograph of the diaphragm: