Skip to main content

Using Surround Propagation Modes In LiquidSonics Reverbs

By 18th February 2024No Comments

LiquidSonics reverbs are renowned amongst professionals in the creative industries for their superior surround capabilities. Not just for the captivating quality of the reverberation itself, but also for their flexible workflow capabilities such as the surround propagation modes.

This article sheds some light on these advanced facilities, equipping you with extra techniques to control the reverberant space with surgical precision and creative flair helping you on your journey towards mastering the art of surround music production and score mixing.

Professional Workflow Propagation Requirements

For immersive surround mixes, the ability to differentiate the reverb treatments for different positions including fronts, mids, and rear signals proves highly valuable. Often, contrasting lengths or pre-delays between front and back are preferred, with a lusher or longer decay in the surrounds enhancing the surround space. In the past this was commonly adopted by many engineers using utilising multiple stereo reverbs on dedicated busses for front, rear, and (in 7.1) side channels.

When originally designing Cinematic Rooms, we consulted with Scott Michael Smith about how he uses reverbs in surround mixes. Scott shed some light on the mix process for us with respect to how existing multi-stereo workflows needed to be translated more fully into the surround reverb space.

He explained that the use of multiple stereo reverbs in surround productions not only offers him a level of control often absent in dedicated surround reverb plugins, there are also inherent differences in the way sound transits the space in a stereo vs a surround reverb. This is because surround reverbs, regardless of panning or bussing, inherently send back a full surround reverb (reflections and late tail) to all discrete channels. While impressive and appropriate for certain applications like solos or mono sources, it can become cumbersome or overwhelming when dealing with orchestral or rhythmic elements, where specific mic placements call for a more tailored spatial treatment.

He gave the example of an orchestral setup with four ambient/surround mics. An engineer may wish to have these mics routed to their own dedicated reverb, with the resulting tails solely impacting the surround or elevated channels. The ability to pan and control reverb returns individually within a surround mix is invaluable and we may need to prevent any sound or reverb from the rear mics from reaching the front channels which is difficult with a traditional surround reverb. Of course this is the effect when using when using stereo reverbs, but it can lead to poor decorrelation across stereo channel sets requiring the reverbs to be adjusted as a means of bringing them out of correlation; this can be time consuming to manage especially when auditioning and adjusting presets across the set. We discussed with Scott about ways to bring the best of both worlds together in Cinematic Rooms.

Users of some DAWs are familiar with using non-surround plugins in a “multi mono” mode. Most LiquidSonics surround reverbs can be used with an internally configured “multi true-stereo” propagation modes available. This means they provide the ability to control which stereo channel planes the processed tails are returning through, enabling independent routing (and in some cases editing) of each reverb plane within the plugin.

In practical terms, consider a 7.0 surround mix using Cinematic Rooms. With a multi-stereo propagation mode, you could send a panned back-left mic to the rear processing plane of the reverb, with the processed signal returning exclusively to the back left and right channels with appropriate cross-feed properties in the right side. Meanwhile, the front channel is isolated from this and handles its own independent reverb sends and processing. This approach can provide the isolation between channels that some mix situations require and can save the number of required reverbs for a standard surround mix too, all whilst ensuring a cohesive space and a well decorrelated set of reverb channels as we would expect from surround reverbs.

Following our discussions with Scott, we built these multi-stereo capabilities into the reverb. The front, side, rear and elevated stereo planes all behave as independent reverbs when appropriate settings are chosen, but can still be managed as one. Since then even more propagation modes have been built into our surround reverbs including Seventh Heaven Professional, Cinematic Rooms Professional, Tai Chi and Lustrous Plates Surround.

Recently Added Modes & Capabilities

While some of the propagation modes have been available since release, recently two additional modes have been added following workflow discussions with Martin Wrang, a post-professional working with production sound, front of house mixing and podcast editing. Martin explained that while the true surround plane propagation mode was very helpful he had a need to treat the fronts as a special case. His workflow called for the ability to treat the front channels as an LCR (left, centre, right) reverb rather than as a stereo pair with an isolated mono centre channel (as is the case in the true stereo propagation mode). Following discussions, a new propagation mode has been introduced known as “Stereo Surround Planes + LCR”.

“One of the things I really like about Cinematic Rooms Professional in my workflow, is that I can set up any configuration I like inside the plugin itself while keeping it on a 5.0 track. I can set it up as multi-mono, front to back stereo pairs, or a full surround reverb without changing track/bus/plugin widths in Pro Tools. I often like to use stereo reverbs pairs for the front and rear channels of a 5.1 setup to add width and space to sounds, while keeping the front to back separation intact.

The new ‘true-stereo with LCR propagation’ mode includes the centre channel in the front LR reverb. This allows me to add the same sense of space to centre channel sounds as to LR sounds, and it lets me pan freely across the LCR while having the reverb respond in a predictable and natural way. If the reverb clutters up the centre channel to much, I can simply turn down the reverb for that channel, or mute it entirely, using the surround plane editing.”

This is not the only recently added propagation mode. Following conversations in the summer of 2023 with John Michael Caldwell about his specific workflow needs, we also added the capability to prevent the reverb from washing between the left and right channels. This mode known as “All Except L/R Propagation”, allows reverb to transit forward to back (and vice versa), and between elevations channels, but keeps the reverb to the side of the room into which it is panned.

Surround Propagation Modes

The available propagation modes are:

  • Full surround propagation – sound entering one channel will cause decorrelated reverberation to propagate into in all other channels
  • Stereo-stereo plane isolation – left/right propagation for all planes, but no front/back or elevation propagation (all channels remain fully decorrelated)
  • Stereo-stereo plane isolation with LCR – as above, but with the front plane treated as LCR rather than stereo left/right + mono centre
  • All except L/R Propagation – propagation throughout the space except between the left/right/centre (i.e. nothing along the x axis)
  • Mono channel isolation – no propagation between any channels, every channel is in effect a de-correlated mono reverb in multi-mono mode

To give a more visual representation of the modes consider the screenshots below where different modes are active in a Pro Tools session. The send position of the panner shows which discrete channel audio is presented to the reverb from, and the mixer tool clearly illustrates the channels reverb is produced for. In all cases the crossfeed level has been deliberately set lower than usual, so it is clear which channel is processing primary reverberation (i.e. on the loudest channel shown in the mixer meters), versus which are secondary cross-feeds (i.e. the lower levels on the meters), versus which are silent due to a complete lack of propagation.

1. Full surround propagation

We see that sound entering the right channel creates reverberation on the primary channel, and propagating reverb into the other channels with 6 dB of attenuation. This is similar to the way many traditional surround reverbs behave where the design prohibits discrete control over the crossfeed elements.

2. True-stereo plane isolation

When engaging the true stereo plane isolation mode (without LCR) the reverb appears on the front right channel in response to a front right dry source and propagates only across the front L/R stereo plane meaning that we observe reverb at a lower level on the front left channel. There is no other crossfeed throughout the system, and the centre behaves as an isolated mono reverb.

A: Audio panned to front.

No propagation occurs into the side, rear or elevated planes as it is restricted to the front plane only.

B. Audio panned to left side channel

Panning audio into the left side channel, we observe that reverb is only present in the left/right side plane. Hence the fronts and sides are logically controlled with the same reverb settings and have different correlation properties, but are being processed independently of one another.

3. True-stereo plane isolation with LCR

Engaging the stereo planes with LCR propagation, we observe that reverb additionally bleeds into the centre when audio is presented on the front left/right channels. We also note that when the centre channel receives audio, reverb will be processed into the front left and right channels.

A: Audio panned to front right – shows behaviour as though the fronts are acting as a 3-channel LCR reverb with audio crossfeeding to the centre and left channel.

B: Audio panned to centre – reverb crossfeeds into the left and right from the centre but no further.

C: Audio panned to rear – displays true-stereo propagation without any front-back propagation illustrating that the rears are fully isolated from the other discrete channels.

Additional notes – you can mute reverb processing in the centre channel with “Mute Centre Reverb” and “Mute Centre Reflections” – therefore it is possible to have this LCR style reverb set up so that the centre stays dry, but any audio received in the centre will throw reverb to the front left and right channels but no further.

4. All except L/R Propagation

Audio panned into the right side channel propagates into the right front, right rear, and right elevated channel. No reverb propagates across the x axis (i.e. across left/right boundaries or into the centre).

5. Mono channel isolation

Reverb will only present on channels that are receiving an audio signal. This is equivalent to the “multi-mono” mode that some hosts provide, but  benefits include producing fully decorrelated reverb outputs and using less CPU/memory.

Note that these propagation modes are not available in HD Cart due to the heritage of the algorithm itself. The other LiquidSonics reverbs discussed here were designed from the ground up to support these capabilities.

Superior Pan Tracking

Once you have configured the propagation mode required, the level of the crossfed audio can be tuned using the appropriate cross-feed level controls. These facilities control how closely the reverb tracks the source as it pans around the space.

In Cinematic Rooms Professional you also have independent control over reflection and reverb propagation level, delay and roll-off filtering.

Lustrous Plates Surround provides some control over bloom and low cut for the cross-feed signals. Dialling in bloom for the cross feeds helps create a greater sense of size and depth across the stereo field.

Tai Chi and HD Cart do not provide dedicated control over pan tracking level.

Try LiquidSonics Reverbs For Yourself

All of the LiquidSonics reverbs are available to try for free for 14 days, just head to our demos page to drop a code into your license manager and pick up the installers from the downloads page.