Immersive audioSurround soundAmbisonics

Immersive audio basics: formats, arrays, and spatial cues

Channel count alone does not create immersion. Recording must preserve direction, distance, and room reflections, and playback must reproduce those cues coherently.

By · DADAO Acoustics Lab · DADAO AUDIO8 min read

Diagram comparing stereo, horizontal surround, and three-dimensional loudspeaker layouts with a height layer.

Contents

What this article covers

6 chapters · About 8 minutes

Chapter01

Definition

Channels around the listener do not guarantee spatial impression

Industry writing often uses “immersion” and “envelopment” interchangeably. Here, immersion means the listener’s engagement with the complete sound field, while envelopment means spatial cues from the sides, rear, height, and room reflections. Sending the same mono signal to many loudspeakers adds source locations but does not necessarily form a stable spatial impression.[1] For loudspeaker playback, the height layer is the main distinction between three-dimensional layouts such as 7.1.4 or 9.1.4 and conventional 5.1 horizontal surround; binaural rendering can also present height cues over headphones.[2][5]

Figure 1 · Dimensional differences between stereo, horizontal surround, and layouts with a height layer.

Chapter02

Formats

Read the layout notation first

5.1 contains five main channels and one LFE channel. In Dolby’s 7.1.4 notation, the first number counts ear-level channels, the second counts LFE channels, and the final number counts height channels.[3] ITU-R BS.2051 defines 22.2 as 22 main channels plus 2 LFE channels, with the main channels distributed across upper, middle, and lower layers.[2] “.1” describes an LFE signal channel; it does not mean the venue can use only one subwoofer. A format sets delivery and monitoring boundaries, while the recording array must still suit the sources, room, and spatial cues you want to preserve.

Figure 2 · Main-channel, LFE, and height information in four loudspeaker layouts.

Chapter03

Ambisonics

Ambisonics is a sound-field representation, not a spherical microphone model

Ambisonics represents a sound field with spherical harmonics and is not tied to one hardware design. A common first-order microphone uses four nearly coincident capsules in a tetrahedral arrangement to capture A-format, then converts those signals into the W, X, Y, and Z channels of B-format. That is only one way to produce first-order signals.[4] Three-dimensional Nth-order Ambisonics requires (N+1)² spherical-harmonic signals. Higher orders usually increase spatial resolution and enlarge the useful playback area, but results still depend on frequency, decoder, and loudspeaker layout.[4][5] Delivery to 5.1, 7.1.4, or headphones always requires rendering for the target system.

Figure 3 · Relationship between Ambisonics order, signal count, and rendering target.

Chapter04

Spaced Array

The 2L spaced array is a case study, not a universal template

Spaced arrays use arrival-time and correlation differences between microphones to capture spatial impression. DPA has described Morten Lindberg’s 7+4 omnidirectional array and stated that its microphones were at least about 1 m apart; in another interview, Lindberg explains that he changes the array for each ensemble and venue, with dimensions ranging from roughly 40 cm to 150 cm.[1][6] The sources describe different configurations, which is exactly why “7+4” should not be copied as a fixed coordinate set. Study the direct routing and relationship between lower and upper layers, then size the array for your own programme material.

Figure 4 · Conceptual layers of the 2L 7+4 spaced array; the dimensions change from project to project.

Chapter05

Decorrelation

Decorrelation can diffuse the field, but it leaves processing traces

Decorrelation is not simply copying one track to more channels. Common methods use all-pass filters, time-varying filters, or short delays to reduce inter-channel correlation while trying to preserve spectrum and transients. It can synthesize diffuse sound and, in some reinforcement systems, reduce low-frequency variation caused by multiple sources.[7] Excessive processing can smear transients, alter timbre, or broaden localization. After processing, check correlation, mono compatibility, downmixes, and several listening positions instead of judging width only at the sweet spot.

Figure 5 · Uses of decorrelation and the side effects that need to be checked afterward.

Chapter06

Approach

Set priorities before choosing the array

When precise localization and post-production rotation matter, begin with a coincident or near-coincident approach. When hall envelopment matters more, consider a spaced array. Pressure omnidirectional microphones usually extend well at low frequencies, but they also capture more room sound and low-frequency noise.[1] The final plan must account for array size, reverberation, monitoring layout, and delivery format together. Routing each microphone directly to one loudspeaker channel is a valid method for some productions, not a rule for every immersive recording.