The design of speaker systems for Ambisonic playback is governed by several constraints: the desired spatial operating range (horizontal-only, hemispherical, full-sphere), the predominant resolution (= Ambisonic order) of the expected program material, the desired localisation performance and size of listening area versus the available number of speakers and amplification channels, and the theoretically optimal distribution of speakers versus the actually available placement and/or rigging options. This page attempts to discuss the interaction of these constraints and their various trade-offs in theory and practice, as well as perceptional advantages or drawbacks of specific speaker layouts which have been observed in actual deployments.
General considerations
Near-field effect
In its original formulation, Ambisonics assumed plane-wave sources for reproduction, which implies speakers that are infinitely far away. This assumption will lead to a pronounced bass boost for speaker rigs of small diameter, which increases with Ambisonic order. The cause is the very same proximity effect that occurs with directional microphones. Therefore, appropriate near-field compensation (bass equalisation) is beneficial.
Speaker distance vs. angles
This same plane-wave assumption makes it possible to vary the distance of speakers within reasonable limits without upsetting the correct function of the decoder, provided that the difference is compensated with delay, the power is adjusted for uniform loudness at the center, and that per-speaker near-field compensation is used. Distance does not affect the decoder matrix.
Variable speaker distance is therefore the most important degree of freedom when deploying idealized layouts in actual rooms. It is constrained by the reverberation of the room which leads to uneven direct-to-reverb ratios between speakers at different distances, and the power handling capability of the most distant speaker. 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.
Speaker angles on the other hand should be adhered to as precisely as possible, unless an optimised irregular decoder can be generated in-the-field.
Horizontal vs. full-sphere accuracy
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 . Therefore, if occasional horizontal-only reproduction at the highest precision is desired, full-sphere layouts with a dense horizontal ring are preferable.
Phasing
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 artefacts are most prominent in dry rooms on very precisely calibrated systems. They can be reduced by adding height speakers, which tend to smoothen the effect, or tuned to a subjective minimum by introducing staggered delays to the speakers, with the understanding that this may adversely affect low-frequency localisation if overdone.
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.
Loudspeaker occlusion
For multi-listener environments and auditoria, the occlusion of speakers by other listeners must not be under-estimated. 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.
With-height systems usually provide more unhindered lines-of-sight per direction for a given audience, which might increase their robustness.
Number of loudspeakers vs. source material resolution
Solvang and others have shown that using much more than the minimally required number of speakers can be detrimental. The reason is simple: more speakers with constant angular resolution means higher crosstalk and thus higher correlation between speakers. If not managed, this leads to stronger comb-filtering effect and phasing artefacts when the listener moves.