Many of your questions seem to be answered (to an extent) by the published material, although perhaps not to a satisfactory standard of evidence. Good studies are lacking, probably in part because of decades-old assumptions—based on limited data—about the audibility of bass spatiality in small rooms. As far as I'm aware, the audibility of the effect is not disputed in large spaces such as concert halls or in free space.
In a good concert hall, sufficiently widely spaced microphones yield substantially uncorrelated signals in the subwoofer range. If the recorded signals are then reproduced by independent drivers, it is theoretically possible to produce interaural amplitude and phase fluctuations similar to that which would occur in the hall. If the bass is summed to mono, this possibility is lost.
The "reverberation time" (really a misnomer in small rooms) is related to how well damped the room modes are. As the damping increases, the direct sound becomes more dominant. As the direct sound becomes more dominant, interaural amplitude and phase become less dependent on the modal behavior of the room and thus more dependent on the interchannel amplitude and phase in the stimulus.
One could do binaural measurements in the playback space (using in-ear microphones, a head and torso simulator, etc.), though accurately relating these measurements to perception isn't so trivial. The interaural cross-correlation coefficient is quite high at low frequencies due to head dimensions, but the human hearing apparatus is quite good at detecting interaural time differences. Griesinger proposed a measure that he dubbed the diffuse field transfer function, which is
described in some detail in this paper.
Thanks for putting together a cogent answer to the specific questions raised.
However it is in your above answer to the reverberation time question, repeated below, that I think the issues get a bit difficult: –
…How, step by step, is the reverberation time at home in (sub)bass connected to the sensation of 'patterns' originating in a concert hall's reverberation?
…The "reverberation time" (really a misnomer in small rooms) is related to how well damped the room modes are. As the damping increases, the direct sound becomes more dominant. As the direct sound becomes more dominant, interaural amplitude and phase become less dependent on the modal behavior of the room and thus more dependent on the interchannel amplitude and phase in the stimulus.
Firstly, at long wavelengths the behaviour of the room and the speakers is not comprised of direct sound plus room modes. Even if you remove the room modes, the listener experience is still the sum of speakers plus room, it is not just direct sound from the speakers. The sound waves are still hitting the listener from all directions.
Secondly, the principal means of removing the room modes is to equalise at the listening area. The most effective way to achieve this is to use multiple mono subwoofers placed in optimised locations with optimised amplitude and delays. Nobody is going to achieve sub-bass damping of the room modes in a domestic living space by literally adding damping to the walls of the room. All of these examples being provided and reports of hearing the benefits of decorrelated bass, are occurring using stereo speakers without equalisation, so the room modes are in full play during these demonstrations. Your suggestion of damping the room modes is in reality best done by reverting to multiple mono subwoofers, and your suggestion that this step will enhance ability to perceive the decorrelated bass of two separate bass channels is a conundrum and somewhat ironic.
So, when we add the following factors together ....
- the observations of audible enhancements from casual home experiments are from
sighted listening reports;
- the SPL at the listener's ear in these home tests is probably different, and would need to be matched to control for the risk of that variable being the reason for an observed difference instead of AE;
- the frequency response at the listener's ear in these home tests is probably different, and would need to be matched to control for the risk of that variable being the reason for an observed difference instead of AE;
- with the mono sub-bass implementation, there is plenty of decorrelated bass being delivered into the room (if not so certainly to the listener's ear) above 80 Hz, and indeed to some extent below 80 to perhaps 60 Hz, and all the AE effect that it can deliver, but it is coming from the main speakers;
- a multiple subwoofer mono sub-bass solution with MSO delivers much smoother bass frequencies than decorrelated so-called 'stereo' bass and this smoothness has been proven in multiple independent controlled trials to be of high significance for the listener's pleasure evaluation, in stark contrast to the absence of controlled trials in domestic playback conditions demonstrating not just detection, but also preference and high significance, of the presence of AE specifically below 80, or maybe 70 or even 60, Hz;
.... the most rational conclusion from the current discussion seems to be that the certain losses from not having summed and equalised sub-bass are much more significant than the uncertain gains of having decorrelated sub-bass below 70-ish Hz, in domestic replay systems.
cheers
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