Anytime you listen to the terms immersive audio, 3D audio or spatial audio, it's about sounds coming from everywhere - not only from your front, like stereo or even around you, but coming from the sides, the front, above and behind you! 

Understanding these concepts is the key to integrating 3D audio into your workflow. In this article, Nuno Fonseca from Sound Particles aims to give you an overview of all of these new terms to help you to be able to navigate this world of 3D sound better. 

What Is Spatial Audio?

There are a lot of new layouts and formats for you to take advantage of and we will give you the basics so you can start mastering and diving into the immersive sound world.

What Is Channel-Based Audio?

To start, we begin with the one format we all are familiar with - the stereo format, with two speakers, one left and one right. Here, a sound is sent to both speakers, and it will be perceived as if it was coming from the centre, even though there's no speaker there. Besides this, it's also possible to send more signals to one speaker and then to the other so that the sound is perceived to move from the centre to one of the speakers more in one direction than the other. 

it's important to note that when working with stereo, we need a 60-degree angle to reach the sweet spot, where one should be placed to get the spatial sound resolution. 

LCR

At some point, a new speaker was added in between the left and right one -  the centre speakers. 

And why do we need another speaker? Well, the world is not a perfect place, and in spatial audio, that means that not everyone is in the sweet spot. If you go to the cinema and you're lucky to sit in the sweet spot - perfect! But if not - for example, if you are too close to the screen or to the left, that breaks the sound experience because you won't get the same experience as if you were in the sweet spot. 

Centre speakers work like anchors and allow you to continue having a good sound experience. 

5.1 Surround

Another format came around, the surround sound, which improved the cinema experience, by having sounds coming to the sides and from the back, like 5.1 formats. When you go to a movie theatre, there isn't one right surround speaker and one left surround speaker but an array of speakers on the sides and in the back. 

In the image above, imagine that you are in the blue position: you would get sound from the back and right side, and you would get great sound, or even if you are on the last row and everything is happening in front of you, you would still get a good experience, but now imagine being surrounded by an array of speakers, like the second image, you would get a much better experience.

What’s The .1 All About?

Later on, the movie industry also wanted to increase the impact of sounds with lower frequencies on effects such as explosions or earthquakes, and that was when the LFE and Sub-woofer made their appearance.

To achieve this, they added larger and more speakers. But the problem is that, at the time, people were using analog sound, and that means a very limited dynamic range - if we amplify everything, the background noise will go up, and it will affect the dialog. 

The best solution was to create a channel only for low-frequencies effects reproduced by a sub-woofer to be played much louder than the other channels to get much more feeling and without affecting dialogs, etc. 

Immersive Audio

The last format is immersive audio (also known as 3D audio or spatial audio) - surround with a height component; not only do you have speakers around, in front, in the sides and in the back, but you have speakers above you allowing you to explore sounds coming from above.

Of course, there are other formats, like the one below - the first number is the number of speakers around us, the second number refers to LFEs - sub-woofers and the third number is the number of the channels above. 

Using immersive formats will provide the sense that sounds are coming from everywhere. For example, the 7.1.2 format, which is basically the traditional 7.1 with 3 speakers on the front, 4 surround speakers but with 2 additional channels above, one left and one right, to give the sense of sound coming there. 

And that's all for channel-based audio, you can watch the video below to get a more detailed explanation! 

Now that you know everything about channel-based audio, it's time to get to the next topic: object-based audio, which includes formats like Dolby Atmos, which you probably already heard about.

What Is Object-Based Audio?

In this type of audio, instead of mixing sounds to the output channels that we want, as in channel-based audio, we keep each channel completely independent, add some data about their position (metadata) - like the 3D position, the size or movement - and finally, we are going to distribute these independent channels (objects). Then, during reproduction, based on these settings, the render will decide what to play in each speaker - everything in real-time.

Object-based audio started to get some attention with the release of Dolby Atmos. Instead of mixing all sounds in the studio and distributing the final mix, object-based audio uses a different approach by distributing all sounds independently, which are mixed only during reproduction.

In the studio, you will still use a panner to position your sound, but you don’t apply that panning information to the sound – you simply say where you want your sounds to be positioned. That information is distributed, and during reproduction, depending on your actual reproduction system, that panning information is actually applied to sound.

One of the most well-known systems using object-based audio is Dolby Atmos, but there are other object-based audio formats, such as MPEG-H, AuroMax, and DTS:X.

Dolby Atmos supports both object-based audio (objects) and channel-based audio (often called beds). An object will have its audio (a mono audio clip) and the corresponding metadata, but Dolby Atmos also supports channel-based audio by using 7.1.2 beds (7.1 + 2 height channels).

Dolby Atmos supports 128 independent channels, which means that we may have up to 118 audio objects plus 10 channels reserved for a 7.1.2 bed (more info on beds later on). During reproduction, Dolby Atmos supports up to 64 independent speakers, which means that a movie theatre may have up to 64 speakers which are controlled independently, meaning that each speaker can reproduce audio that is different from all other speakers. The room may even have more than 64 speakers, as long as some of them share the same audio signal.

A movie theatre with Dolby Atmos support will be slightly different from a regular 5.1/7.1 room.

During the production of a movie, some audio content may have already been pre-mixed. For instance, the music score could be delivered to the re-recording mixers already mixed, or at least pre-mixed in stems (7.1 strings, 7.1 brass, etc.); or imagine a sound effect ambience that was already recorded or edited in 5.1.

Besides objects, Dolby Atmos also supports channel-based content, which is called audio beds. This means we can also include “regular” channel-based audio. Dolby Atmos uses 7.1.2 beds, which correspond to a traditional 7.1 (3 front + 4 surround channels + LFE) with 2 additional channels for the ceiling (one on the left side of the ceiling and another on the right side of the ceiling). For instance, all existing 5.1 and 7.1 tracks can be directly mixed to a 7.1.2 bed. And even without using objects, a mixer can still explore the height component by sending audio to the overhead speakers.

With objects, each sound must be independent as a way to control its panning, but with the beds, that is not an issue since we use “regular” panning to place the sounds around. As such, the re-recording mixer could use objects for the most important or relevant sounds, where accurate localization is more critical, and mixed secondary sounds in beds.

And so we finish today's topic, our video series about the basics of 3D audio continues, and we suggest you watch the following video about object-based audio to get a more insightful explanation about it and also a couple of curiosities about immersive audio that you probably didn't know 😉.

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