Speaker Layouts In Immersive Audio Systems
What is an immersive audio system?
Immersive audio, sometimes referred to as 3D Audio, is the name given to content that envelops the listener by utilising multiple speakers to create the feeling of being physically present in an artificial environment. The immersive audio system comprises the equipment designed to deliver that experience in a manner as close as possible to the intention of the content creator.
Typical surround sound systems include five or more speakers configured on roughly the same level as the listener(s), with all front and surround speakers ideally aligned along the same horizontal plane. This layer is called the listener-level layer.
Immersive audio systems add the crucial third dimension of height. These height speakers are typically found high on walls, and/or on the ceiling, and are directed towards the listeners. The wall height speakers, and ceiling speaker layers can be utilised as separately, or processed jointly as the upper layer.
Why is adding height speakers so groundbreaking?
Interestingly, height speakers aren’t a new concept. Dolby produced a special master of We Were Soldiers way back in 2002, which included an entire overhead soundtrack designed for systems with height channels.
What’s really changed is surround sound mixing and reproduction technology, which now allow sound engineers to create an object-based mix, as opposed to the traditional channel-based mix.
Channel-based audio has all sounds allocated to specific speakers, that then reproduce the sounds in the assumed correct locations in your environment. This relies heavily on your system closely matching the layout of the sound mixer for optimal surround sound.
Object-based audio instead allocates sounds to a location in three-dimensional space, and has no regard for speaker locations or quantities. One of the biggest benefits of object-based audio mixes, is that the reproduction isn’t constrained to a fixed number of speaker channels in the exact layout the sound engineer utilised to create the mix. As such, object-based audio mixes are perfectly scalable to suit your unique environment.
An object-based audio mix can be reproduced from as little as three front speakers (Left, Centre, Right), two height speakers, and one subwoofer for the low frequency effects (LFE). This can scale to as many as 34 speakers. In systems without these large numbers of speakers, sounds simply get redistributed to be created in space by the nearest applicable speakers, based on the three-dimensional coordinates and your surround sound processor’s notion of where your speakers are located in your room.
So how many speakers do I need then?
The unfortunate answer is… It depends. Ultimately it boils down to three questions:
How big is your room?
How many rows of seats do you need?
How close of an experience do you want to the intention of the content creator?
Level 1 - Conveys basic artistic intent.
Level 2 - More accurately conveys artistic intent.
Level 3 - Meets or exceeds (typically exceeds) reference commercial cinema exhibition standards.
Level 4 - The maximum level of achievable performance across every parameter.
The first two are relatively easy to figure out so we’ll focus on question 3.
How accurate your experience is in your environment is dictated by a number of aspects, but with regards to the speaker quantity we are going to look at three key attributes in the spatial resolution category: (CTA-RP22 ref included for technical deep dive)
Maximum allowable horizontal angle between adjacent surround speakers (CTA-RP22, 5.6.2.1)
Maximum allowable horizontal deviation from median angle for wide speakers (CTA-RP22, 5.7)
Maximum allowable vertical angle between adjacent height speakers (CTA-RP22, 5.8.2)
If you’ve reached this far, and your still not sure how many speakers you need, that is completely understandable; there’s quite a bit of technical know how required to navigate designing an immersive audio system.
To keep things as simple as possible, it is best to co-ordinate your design with an experienced home cinema designer.
It’s also worth noting that it’s rare for a project to perfectly marry up to a single performance level as described in the CTA-RP22 guidelines; everyone has a unique space, and unique requirements. Knowing where to make compromises to keep your project within budget, while still meeting your expectations, is where experience will shine in the creation of your unique immersive audio environment.