Undecoded UHJ
In a world in which the majority of listeners do not have Ambisonic decoders, most listeners to 2-channel UHJ-encoded material will be listening undecoded.
Some engineers have preferred the perceived additional spaciousness, width and stability of undecoded 2-channel UHJ to conventional panpotted stereo mix techniques. The late John Timperley, for example, used the UHJ Transcoder (a simple method of Ambisonic mixing) extensively and produced several series of UHJ recordings even though he was fully aware that few people would ever hear them decoded, and mixed material deliberately for the best undecoded result. Unfortunately these recordings have not yet been fully documented.
Similarly, on the Tina Turner album, Break Every Rule, the backing vocals are in fact UHJ encoded - and in this case there is no point in the listener decoding the tracks, because the rest of the mix is in regular panpotted stereo and contains no surround information - the vocals were encoded purely for the spacious effect without decoding.
When listening to undecoded 2-channel UHJ material, the rear information is folded down over the front stage at 3dB down, so that the soundstage retains some degree of 'distance' between front and rear. For example, a sound panned around the listener in surround will, when auditioned undecoded, appear more distant as it passes across the 'rear'; in addition, the interchannel phase relationships associated with rear localisation make the sound attempt to appear behind the listener as a result of a form of 'aural decoding'. However, while this is generally effective in the case of Ambisonically-mixed content, in the case of material recorded with a soundfield microphone (SFM) system the ambience, for example in a concert-hall, may be sufficiently high in level when undecoded to make the recording appear over-reverberant, where this is not the case when the UHJ is decoded into surround. This has been cited in the past as a criticism of some SFM recordings by some record reviewers: the problem can be ameliorated by careful mic array positioning (e.g. a position closer to the musicians).
Read more about this topic: Ambisonic UHJ Format