With the proliferation of surround thanks to Apple’s Spatial Audio, many engineers are having to start working in surround. It’s certainly not an easy thing to do and the majority of the content being released reflects the inexperience that most engineers have in this area. Mixing in surround requires a learning curve that has been well established thanks to the film industry and research done in this field for many decades. Of course there’s no “proper” way to do it but there are many things which can poorly affect most listeners’ experience. In this post I’ll try to outline a few tips that should hopefully help anyone who is looking to transition from working in stereo to surround.
The biggest deal breaker that I hear many Spatial Audio releases is distracting content that isn’t in the front. How often do you hear a lead vocal that’s hard panned to the right in stereo? Putting a lead vocalist behind the listener is even worse in terms of breaking the listener’s experience. This largely has to do with how we perceive sounds from different directions. Sounds from behind us will usually grab our attention in an aggressive way since we need to watch out for a tiger that could be ready to lunge at us.
In general, panning direct sounds anywhere other than the front should be done carefully as it will easily grab your attention in a way that breaks your connection to the music. In the film industry this is often referred to as the “exit sign effect” referring to the exit signs above the doors at the back of a theater. When something is awkwardly placed back there viewers will often turn around to try to find what it was that made that sound. Percussive sounds and lead elements tend to be more distracting.
Most of the time I’d recommend using everything beyond the front stereo field as a way to increase immersion through a sense of space. Feeling like you’re in the concert hall or in a stadium. Where I think it works well to send direct signals all around is when there’s an element in the mix that’s layered. It could be background vocals panned all around the listener or stacked guitars. Even a circle of 12 drummers as Hans Zimmer did for Man of Steel.
When it comes to generating “immersing” material for other speakers, the biggest mistake you can make is to just send the same signal to multiple speakers. With surround, decorrelation is key. When a similar signal is sent around, all that you’re really going to get is phase issues. Every playback system will be different and if it’s being played back in a large system like a theater, there will be people sitting all around the room at different distance ratios to the various speakers. The result will be comb filtering and not being able to discern the directionality of elements in the mix. In extreme cases there can be audible flamming.
There are many techniques to generate decorrelated material for making something more immersive. Two popular ways are with upmixing plugins and reverb. When it comes to reverb, there are some Atmos reverb plugins available but you can also create your own by using stereo plugins on different sets of speakers and having slightly different settings for crafting the space and decorrelation. When you want to keep a sound from losing directionality, a stereo reverb could be used with the L going to the left surround and the R going to the left speaker.. This keeps the reverb for a certain element in the mix confined to one side of the room. If you’re mixing an orchestra you could send some violin spot mics exclusively to the left side reverb and celli spots exclusively to the right. This keeps some separation between them instead of all becoming a single wash of reverb in the entire room.
When it comes to moving a signal around in the room, special care must be taken. I’d recommend keeping signals at the perimeter of the room instead of pulling anything “into” the room since that’ll lead to more speakers playing back the same signal and issues with correlation. If moving a sound from the front into the sides or rear, there can be issues because of the way that our hearing works. For more info on this, look into the cone of confusion and spectral splitting. Our hearing is not good at discerning the directionality of sounds directly to our sides (thanks to how our pinnae are shaped) which leads to weird effects when a sound moves into or through that region. The other issue is that on systems up to 7.1.4, there’s a large gap between the front speakers and the surrounds so panning will never sound precise. It’s really only at 9.1.6 and larger systems where you can get decent panning resolution at the sides.
The LFE channel presents many challenges. Many downmixed formats will ignore the LFE channel so whatever you send there needs to be something that the mix can live without. The level that subs are set at also vary widely (even in movie theaters) so there can easily be a 10dB window which that signal will be played back at. Lastly, sending a correlated signal can once again mess with the signal being sent to other speakers. As a result, I recommend using the LFE channel sparingly. If you feel that the mix could benefit from some use of the LFE, I’d recommend generating entirely new material for it. This is usually done by either transposing down a sound or by using a subharmonic generator plugin like Lowender or LoAir. It’s also important to low pass the LFE signal. Most LFE subs don’t sound great playing 120Hz. Please don’t send a full range signal.
The center channel can also be tricky to deal with. A phantom center signal sounds different than the same signal sent to the center channel so it’s not simply a matter of sending something to the center channel. On top of that, the Atmos binaural settings also treat centered material differently. I’d recommend experimenting on both speakers and binaurally to find what’s best for your mix.
Of course all of the “rules” I’ve mentioned can be broken. An example would be to send the same signal to the LCR. In a large room where people are sitting from the extreme left to the extreme right of the system this can help make that sound appear from the front rather than having someone on the extreme left hearing something coming out of the center speaker as being to their right. For surround music where you’re primarily targeting small playback systems I’d avoid using such techniques.
When it comes to recording in surround again decorrelation is king. There’s a paper discussing this on my website and there are a number of great AES papers regarding this such as those by Richard King. For more information on working in surround I’d recommend Alan Meyerson’s Mix with the Masters videos and Tomlinson Holman’s book Surround Sound: Up and Running.