All art, instructions, material selections and information in this post and following posts (collectively the "SuperBAF HD800 BTA Absorber mod") are covered by theCreative Commons Attribution-Sharealike 4.0 international license. Any derivative work may be sold as long as instructions and art are distributed freely and correct attribution (mention of the name of the mod, SuperBAF and a link to this thread) is maintained. See this: http://www.superbestaudiofriends.org/index.php?threads/superbaf-hd800-mod-formerly-btamod.254/ And this: http://www.superbestaudiofriends.or...800-mod-formerly-btamod.254/page-5#post-25585 Z28 MOD v0.30 MEASUREMENTS: http://www.superbestaudiofriends.or...ks-and-experiments-a-la-hd800s.352/#post-8028 Z28 MOD v0.30 DRAWING: http://www.superbestaudiofriends.or...-experiments-a-la-hd800s.352/page-3#post-8328 LISTENING TEST: http://www.superbestaudiofriends.or...-experiments-a-la-hd800s.352/page-4#post-8947 @Serious ' INDEPENDENT IMPLEMENTATION AND MEASUREMENTS: http://www.superbestaudiofriends.or...-experiments-a-la-hd800s.352/page-3#post-8490 -- BEGIN ORIGINAL POST First of all, we'd like to thank Jude for posting the photos of the drivers and inside of the HD800S cups. This is a branch of the HD800S thread, specifically concerning the Helmholtz resonator concept in the driver. Note that this approach is very different from the prior type mods which seek to reduce the diffraction and resonant effects of the cups. Could be a semi-closed. The calculations don't seem to imply the need for multiple cavities. Too many small cavities might also present issues with airflow. Anyway, a very cheesy quick mockup below. I used a drinking straw encased around with foam. Cheap tape was used to wrap around the foam to prevent air leakage (otherwise the resonator won't work.) Figure 1: HD800 with DIY Resonator v0.1 Figure 2: Measurements of stock HD800 and HD800 with DIY Resonator PURPLE = cheesy resonator v0.1 GREEN = stock These are the V1 sponge method measurements. I marked placement of the cup precisely and took multiple measurements to make sure I had it right. I also placed the dust covers back on (so more roll-off seen in the last octave). A bit smoother and more extension in the treble. I look forward to Bill-P's tweaks.