Helium gas inside enclosure. Overall +2dB gain as speaker back-pressure is reduced.
Anechoic chamber response — blue/purple is speaker-forward facing; red is speaker-rear facing (into absorptive lining).
Ported vs Sealed. Ported gives overall more bass output (+5dB), all other factors approximately equal. Green is ported; blue/purple is sealed.
Ported vs Sealed and Insulated. Overall you DO NOT want to have sealed box nor have any “stuffing” inside the box. Green is ported. Lavendar is sealed and stuffed / insulated. The EQ is dramatically worse and power-wasteful. This goes directly against professional advice from many years ago.
Lab room acoustics / reflections — tremendous unwanted comb filtering.
Response of speaker in tunnel. The room acoustics are notably reduced. Troughs are thought to be material (tunnel wood) resonances not adding to / reflecting sound back into tunnel but rather coupling it out into the larger room atmosphere/environ.
Testing efficacy of bath towels draped on tunnel walls. Some reduction of mid- and high-end energy sucking.
Effect of Pyramid Foam (expensive). Green is raw tunnel. Lavendar is Pyramid Foam.
Standard fiberglass acoustic ceiling panels (quite good) except for being fiberglass.
Egg cartons (cheap). Light. Not recommended compared to other treatments.
Egg Cartons vs Standard Fiberglass Acoustic Ceiling Panels
Toilet Paper — remarkable high-frequency equalization. Possibly the geometry of the rolls reduced reflective comb interferences.
TP vs Ceiling Panels. TP comby in mid/low but excellent high EQ.
TP vs raw tunnel.
Rockwool pads (wool, not fiberglass).
Rockwool pads vs Fiberglass Ceiling Panels.
Pads plus Pyramids. Actually worse.