This is a very informative thread.... Thanks to the Op....
For sure 3-D "out of the head" soundfield, is related to the headphone acoustic tuning and design, and to the recording acoustic trade-off of each music album.... Most headphones, at least the 8 i ever owned, dont have acoustical design making them able to externalize the soundfield and improve on the acoustic original recording translation.....
But with my new AKG K340 i listen and perceive an externalization out of the head with many of my albums, often in some recordings, a filling of my room with the sound in a3-D impression .... Not in my head at all in many cases.... I never like headphones because of this "inside the head entrapment" forever......
Few years ago , it takes me a few hundred acoustic experiments to recreate a 3-d soundfield filling the room around me with my speakers... I did not liked when the sound come only from the plane of the speakers.... Then I used one hundred Helmholtz resonators distributed around the speakers and around the room at some critical location...... Mechanically tuned.... I used also a foldable screen... And reflective or absorbing surfaces and diffusive one in a balanced distribution ...
I succeed in recreating a surround sound relative to each recordings ....Then i begun to love my speakers/room at last....
But i lost my house now because of retirement then i was in the obligation to go back to headphone...Without any enthusiasm at all...
5 years ago i begun to modify a bit my AKG K340, the best by a great margin of my 8 others headphone, including dynamic, electroacoustic and even a planar one...
Now by this fate and turn of event in my life, i decided to optimize seriously my K340 this time with, new pads, a new dedicated clean amplifier, vibrations shells control, a new clean dac, etc and WOW! the result was 3-D externalization of sound out of the head accordingly to each albums acoustic atmosphere and peculiar to each original sound recording trade-off and on a level prescribed by each different recordings albums...
For example listening
Organ work of Bach by Marie claire Alain, with the k340 i can hear the organ in my room
out of my head completely and feel the deep bass in my feet touching the floor , a real extraordinary illusion of the sound presence by transmission of the bass through my body vibration resonance...
The K340 is by far the best designed headphone i know of....
It is underrated because it must be optimized to work well and most people dont want to touch an headphone and modify it a bit....And it need more power swing capability than most headphone amplifier can give too... But this hybrid with his 5 passive resonators/diffusers in the shell creating a dual chamber is an acoustic marvel for me... It replace not a Smyth Realizer for sure but with it i dont feel the need for one at all...
It rival my dedicated room, and my other headphone are
unnatural sounding timbre compared to it and they recreate only a claustrophobic headstage inside the head... I hate that with a passion...
The waves timing and a balance between direct and indirect one was crux of the matter in my room, and this 5 passive resonators inside the K340 are pure genious for me and do a similar job .... They help to recreate a virtual speaker in each ear with the hybrid drivers which work like 2 small, one real+ one virtual speakers.... When we hear 2 speakers in a room, in reality we hear 4 speakers, because each speakers present itself differently in time and sound level for each ear... The 5 passive resonators in each cup create this direct/reflected waves balance necessary to recreate localization in a real space like my resonators diffusers critically located were doing in my room...
Calling this 3-D impression an "illusion" or a useless artefact is forgetting that all sound experience is made by informative real "illusion"...These real illusions are the artefacts created by the brain to inform us about a reality....
I now prefer headphone listening for the first time in my life.... But not without the K340 though.... i dont even know what other specific headphone could replace it without these 2 original technology inside the shell.......
The K340 was also better in the bass range and in the high frequencies than my speakers so good they were, they were limited for sure anyway by their price range ...
But what was lacking with my room before my acoustic experiments and with all my headphones, was this surround 3-D out of the head experience with an headphone, and this surround 3-D out of the speakers plane , which only the K340 ever gave me after my dedicated acoustic room happy experience ... All other headphones sleep in a drawer now.....And i will no longer need an acoustic dedicated room either.... Thanks to AKG....
i dont undetrstand why they quitted this design forever and never tried to improve it.... How many hybrid headphone so well done there is with passive acoustic resonators/diffusers inside the shell to this day ?