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Surround sound is a technique for enriching the fidelity and depth of sound by using multiple audio channels from speakers that surround the listener and provides a more realistic audio environment for a home or movie theater compared to the classic stereo configuration.

Mono:

The first audio reproduction used one microphone and one speaker. Subsequently, more microphones were used all mixed into the same monophonic channel.

Stereo, 2.0:

In the late 1950s, stereo players and phonograph records emerged using left and right channels for a more pleasing sound experience.

The prevailing standard of audio reproduction remains the CD with 2-channel stereo sound. Technical innovations notwithstanding, there is still only one good place to hear the recording (at the peak of the isosceles triangle based on the speakers). This "sweet spot" gives fascinating stereophony both in width and depth. You have a box seat in the comfort of your own home, and can listen to events in the concert hall as if directly through a horizontal aperture.

Surround quadraphonic 4.0:

In the 1970s, four-channel sound appeared but was never popular

Surround 5.1:

Today, the most common surround system is the 5.1 layout, which uses six channels in a 2D horizontal plane around the listener at ear level. It comprises left, right and center front channels along with left and right channels toward the rear, plus subwoofer.

While surround reproduction broadens the horizontal perspective, the sound retains the two-dimensional image of stereophony.

However, the human ear has keen directional sensors that perceive sound not only from side to side, but above and below. The reflection of even the subtlest sounds from ceiling and floor will reliably tell us how large a room we are in. And at a concert, we naturally hear the orchestral brass sounding from a higher platform, with the choir behind them and the organ in a gallery further above ...Three-dimensional portrayal of sound is thus an important step forward and in fact an absolute prerequisite in the quest for natural music reproduction at home.

Compared to the 2D horizontal plane of speakers at ear height, immersive Sound adds speakers on the wall or in the ceiling to create a true 3D effect. In 2010, using existing workflows and delivery formats, Auro-3D introduced the first end-to-end immersive sound solution

Auro-3D:

Auro-3D is an immersive 3D audio format developed by the Belgium-based company Auro Technologies.

Dolby atmos

Number and positions of speakers

When defining a speaker configuration for Immersive Sound, it is important to take all these elements into account. This is exactly what happened when defining the Auro-3D compatible systems, starting from Auro 8.0 Various tests and studies have shown that at least a cube, using quadraphonic Height layer on top of a quadraphonic ear-level layer, is needed to reproduce a convincing 3D space (cube) . The Top or Height layer needs to be positioned with approximately 30º elevation to achieve a consistent sound field and avoid that the different layers become too distinct, reducing the naturalness of the experience (see 2.2.2) As a result, all Auro-3D speaker setups include this important Height layer, capable of reproducing the vertical stereo field all around the listener. As part of the foundation of the format they are as such also defined as channels in all Auro-3D formats, allowing for easy to-produce and predictable, consistent results during the reproduction.

In cinema, Dolby Atmos provides only 2 such channels, which are then positioned directly above the listening area. This has two effects:

a) overhead channels cannot contain the allimportant reflections that define a truly, natural 3D sound field

b) the distance with the Surround layer becomes too big (ca 45º-60º elevation), eliminating the vertical stereo-effect.

As a consequence, these channels become ‘detached’, greatly reducing their ability to reproduce natural sound, especially with ambiance-rich content such as sports events or music. In the Home version of Atmos, the Top channels are even limited to be used for rendering of objects (or object-clusters) only, reducing the possibilities for natural, life-like sound even further since a real Immersive Sound field can not be achieved with a 2D Surround bed combined with distinctly placed sounds in 3D.

All Auro-3D speaker setups, from Auro 8.0 up to Auro 13.1 and AuroMax 26.1, contain at least the 4 Height speakers that are crucial to reproduce a natural-sounding immersive and true 3D sound field, delivering a consistent and compatible experience with all Auro-3D content. The additional speakers, including the third layer above the listener (the Top), are useful to maintain the optimum experience as the room grows.

Dolby Atmos, however, starts with 5.1.2, using only 2 Top speakers placed above the listener, with limited added experience as it is not able to fully reproduce a real 3D space. The experience when playing back the same content over the various setups, from 5.1.2 to to 5.1.4, 7.1.2, 7.1.4, etc… can therefore vary dramatically and leaves content makers as such with a more ‘unpredictable’ result.

Channel-based configurations

Type	Dimensions	Channels
mono		1
stereo	1D	2
surround	2D	6 (5.1)
surround	2D	6 (2-2-2)
surround	2D	6 (6.0)
surround	2D	8 (7.1)
immersive	3D	10 (5.1 + 4 height)
immersive	3D	12 (7.1 + 4 height)
immersive	3D	12 (5.1 + 5 height + 1 top)
immersive	3D	14 (7.1 + 6 height)
immersive	3D	14 (7.1 + 5 height + 1 top)