This assumption will lead to a pronounced bass boost for speaker rigs of small diameter, which increases with Ambisonic order.
If speakers have to be moved very close, care must be taken to ensure they still cover the entire listening area with reasonably flat frequency response.
For horizontal-only content, horizontal systems provide more stable localisation at high frequencies than full-sphere ones, as shown by a simulation of the energy vector
Since multiple speakers will inevitably radiate very highly correlated content, a moving listener may experience a phasing effect that affects the perceived timbre and can upset localisation.
Phasing problems usually become evident in walk-around environments, and are of less concern for a seated audience, unless the interference pattern is so dense that it is perceived by small head movements.
Generally, the higher the order and the more physically accurate the reproduction, the more robust it is, up to the point where occlusion produces realistic effects that are consistent with the affected listener's visual perception.
For low order systems however, reconstruction can easily fail entirely when line-of-sight to speakers is blocked, which has led to odd seating arrangements in listening tests.
Horizontal-only playback rigs are the most commonly deployed and most extensively researched Ambisonic speaker systems, because they constitute an economic next step after conventional stereo.
Actual speakers are point sources and will inevitably leak energy along the vertical axis, which has consequences for near-field compensation and the tuning of dual-band decoders.
Consequently, for predominantly frontal sound stages, Benjamin, Lee, and Heller (2008) have observed a preference for rectangular layouts over squares.
[8] Second and third-order material can be played satisfactorily over the ITU 5.1 layout, but due to the problems with first-order reproduction, it should not be considered for Ambisonics except as a compromise when 5.1 content predominates.
When used with one speaker in front, the hexagon can be abused for native 5.1 playback at the expense of a significantly wider and more blurry stereo stage (120° as opposed to 60° between L and R as per ITU-R BS775).
Alternatively, reasonably sharp virtual speakers at the canonical ITU locations can be created with second-order panners - this is an interesting option if a phantom center is tolerable, and it will also work with a two-in-front orientation, which leaves more room for a TV or projection screen.
For first order, phasing artefacts might become obvious under non-reverberant listening conditions due to the use of significantly more speakers than required, and Solvang's results (2008) suggest slightly increased timbral defects outside the sweet spot.
[13] Since stacked rings are somewhat wasteful at higher elevations and necessarily have a hole at the zenith, they have been largely surpassed by hemispherical layouts since mature methods for decoder generation have become available.
As they are difficult to rig and require overhead points, hemispheres are usually found either in permanent installations or experimental studios, where expensive and visually intrusive trussing is not an issue.
Before the development and adoption of modern mathematical tools for the optimisation of irregular layouts and the generation of T-designs and Lebedev grids with higher numbers of speakers, the regular polyhedra were the only tractable options.
Despite Gerzon's somewhat over-enthusiastic description (which pre-dates the introduction of Ambisonics and the proper formulation of its psychoacoustic criteria), the tetrahedron exhibits the same stability problems in 3D that plague the triangle for horizontal-only reproduction.
Goodwin (2009) has suggested a slanted octahedron with separate front center (which he calls 3D7.1)[17] as an alternative way of using 7.1 systems to achieve with-height Ambisonic reproduction in games, and to allow reasonably accurate native 5.1 playback.
Cuboids are easily fit into standard rooms and provide precise localisation in first order for a single listener plus enjoyable envelopment for one or two more, and they can be built using off-the shelf 7.1 components.
If all speakers are placed in room corners, their acoustic loading and resulting bass boost will be uniform, which means they can all be equalised in the same way.
For the sake of consistency, we consider the vertices of the regular polyhedra as speaker positions, which makes the twelve-vertex icosahedron the next in the list.