Higher Order Ambisonics (HOA)

Ambisonics is both a spatialization algorithm and a recording technique. It aims to recreate the acoustic field, by using a specific encoding/decoding technique that confers to this algorithm its unique flexibility.

It is based on decomposing the acoustic field in several dimensions, forming a stream that is directly decoded to the loudspeaker setup, or that can be recorded and later decoded to any other speaker system.

Higher Order Ambisonics simply refers to a more precise version of first order Ambisonics. It allows a more accurate source localization, and works over larger audience areas.

note

To understand the working principle of HOA, check the Spatialization Guide to Ambisonics

Speaker Layout Requirements

HOA was designed to work on 2D and 3D surround speaker systems. Try to use speakers that are as regularly spaced as possible across the entire setup, for a maximum homogeneity. However, HOLOPHONIX is using a decoding method that allows non homogeneous speakers (i.e. the spacing between the loudspeaker dosen't need to be constant).

Speaker placement requirements

This algorithm requires the loudspeaker layout to include frontal and surround speakers. The center of the coordinates system needs to be inside the speaker layout.

Check for error messages in the Logs if an invalid speaker layout is used.

Choosing the HOA Order

Automatic HOA Order

When creating an HOA bus, the Order will be automatically computed depending on the loudspeaker quantity. You can still edit manually the HOA Order.

Always try to match the chosen HOA order with the quantity of loudspeakers. An HOA stream requires at least as many speakers as the number of HOA components for the given order. You can refer to the table below.

tip

It is not always necessary to choose the maximal order, as in practice, a 3rd or 4th order HOA bus generally offers satisfying results.

HOA Order	2D	3D
Order 1	3	4
Order 2	5	9
Order 3	7	16
Order 4	9	25
Order 5	11	36
Order 6	13	49
Order 7	15	64

When a stream is decoded to a smaller quantity of loudspeakers, its precision will correspond to a lower order.

HOA Parameters

All of the parameters listed here are available in the Algorithm Tab of HOA buses.

In its basic form, Ambisonics is designed to offer a good sound field reproduction at the center of the speaker system. HOLOPHONIX integrates advanced optimizations to adapt HOA to large audience areas and irregular loudspeaker setups.

warning

• The various HOA decoding Method and Type settings feature strong level differences. Activate the Compensation parameter to limit those level differences when changing those setting.
• Level compensation is not available on dual-band decoders. Change the decoder settings with care.

Decoder Type

Ambisonics, by design, uses all the loudspeakers of the setup to spatialize sounds. Its standard decoder is suitable for listeners at the center of the speaker setup, but not for listeners are close to the loudspeakers. These listeners will perceive the sound localization as coming from the closest loudspeakers.

HOLOPHONIX offers multiple optimizations coming from the scientific research, to overcome this challenge. Choose the Decoder Type corresponding to your use case based on audience size.

Choose the decoder type according to the proximity of the listeners to the loudspeakers, and the audio content fed to the system.

tip

For venues where the listeners are close to the loudspeakers, In Phase, Basic/In Phase or In Phase/Max R_e will often be the best settings, depending on the audio content.

Single-Band Decoders

Basic

The Basic decoder ensures a proper reconstruction of the wavefront at the center of the listening area for low frequencies. With this decoder, all the loudspeakers contribute; they feature level and phase differences.

recommended use

It is recommended to use Basic decoding only when the listeners are close to the center of the venue.

Max R_e

Max R_e was designed based on psychoacoustics criteria to offer better localization than Basic decoding, for frequencies above 700 Hz, at the center of the setup. It concentrates the energy in the direction of the virtual sources. With this decoder, most of the loudspeakers contribute, and feature phase and level differences. However, the level differences are stronger than Basic decoding.

recommended use

It is recommended to use Max R_e only when the listeners are close to the center of the venue.

In-Phase

In-Phase was created for venues where some listeners are close to the loudspeakers. When computing the loudspeaker feed signals, the in-phase decoder type tries to cancel out of the phase differences, as well as the side and rear lobes that would appear with Basic or Max R_e decoding. As a result, only the speakers close to the source are fed with the decoded stream, and they all have a coherent phase.

recommended use

It is recommended to use In-Phase decoding when the audience features off-centered listeners.

Dual-Band Decoders

Combined decoders

It is possible to combine decoding types, each one working on a specific frequency band of the spectrum, with adjustable Cross Over Frequency. The first decoder in the combined name refers to the low frequencies decoder type, and the second to the high-frequencies decoder type.

Cross Over Frequency

Only available for dual-band (combined) decoder types, to adjust the crossover frequency between the two decoding bands.

Blur

This parameter reduces the resolution by lowering the HOA Order (progressively eliminating the high-order components), thus giving a blurring effect to the audio scene.

Phantom Speaker

Only available for 3D HOA, the phantom speakers allows you to create a virtual speaker placed above (Top) or under (Bottom) the speaker setup. When speakers are not evenly distributed, this can solve homogeneity issues.

Level Compensation *

Applies a power compensation to balance the level differences between the decoding types and methods. It is not currently available when using dual-band decoders.

Decoder Method *

The decoding method offers an optimization depending on the loudspeakers layout. By default, HOLOPHONIX uses the Energy-Preserving decoding method, which is suitable for homogeneous and non-homogeneous loudspeaker setups.

Energy-Preserving

Energy Preserving was designed to keep a constant sound energy when a source is moved across an irregular loudspeaker setup.

All-Rad+

All-Rad stands for 'All-Round Ambisonics Decoder'. It superimposes two panning methods by first simulating a perfect HOA setup on virtual speakers with Direct Sampling method, and then renders those virtual speakers on the real setup using VBAP. All-Rad+ is a more recent version of All-Rad that was designed to offer level homogeneity characteristics competitive with Energy-Preserving, while still offering the localization precision of All-Rad.

All-Rad 2

All-Rad 2 is an alternative version of All-Rad, specially crafted for 5.1.4 ITU loudspeaker setups. It offers constant loudness when moving a virtual source across the setup, where other decoders might not achieve this.

Orientation *

The orientation of the B-Format stream can be adjusted with Yaw, Pitch, Roll, to rotate the entire audio scene. In HOA2D, the only available setting is Yaw.

See rotation angles.