One thing which can be challenging to create artificially is a believable sounding outdoor reverb effect. Having spoken to a number of audio post professionals about this topic, it’s evident that this is something which a lot of people find difficult. Just how do you create an outdoor acoustic using reverb plug-ins? What are the settings to consider and understand?

If there’s one person who can provide some insight into this, it’s Michael Carnes. A classically trained musician and composer, Michael was also principal engineer for Lexicon from 1980 until 2012, developing many of their superb studio reverberation processors, including the groundbreaking 960L and PCM96. He was also responsible for Lexicon’s plug-ins. In 2012 he founded Exponential Audio, with the first product being Phoenixverb, a natural sounding reverb plug-in designed to help place sound sources convincingly into acoustic spaces. Other reverb plug-ins followed, including R2, R4, NIMBUS, Excalibur, Stratus and Symphony. Exponential Audio software became a part of hundreds of films, including many popular major releases. The company was acquired by iZotope in late 2018 and Michael went back to composing. The reverbs developed by Michael Carnes are among the absolute best in the world. With extensive knowledge on the subject, there is perhaps nobody better placed to give us some ideas and suggestions about creating outdoor reverbs. Michael, over to you…

Michael Carnes

In my time as a reverb designer, I don’t think I ever tried to duplicate an environment—whether a concert hall or an exterior.  I was always more interested in a flexible generalized approach than a specific one.  That probably comes from my time as a composer (training those ears) and my time hanging the mics recording classical concerts.  I’ve spent time in a lot of halls, both good and bad, and have always listened hard to what a room is doing.  I think that same curiosity was in play when I wanted to make exterior reverbs.

At some point in my life (my wife says forever) I began clapping my hands, whooping and whistling whenever I found myself in an interesting environment.  This might be a school parking lot, a box canyon in southern Utah, a dry sea-cave on Kau’i or a stand of Aspen in 20 feet of snow. It eventually led to some of my reverbs, but it could also serve a user of nearly any reverb. It’s a matter of eyes as well as ears. Here’s what I was looking for.

What’s the rough geometry in the entire 360-degree field? Are there surfaces reflecting back at you? Reflecting at an angle to you? Bouncing off a wall behind you?  How far away from you (and each other) are the primary surfaces?  That’s going to give you primary and secondary reflection times (a foot is a bit less than a millisecond and a meter is about four). Approximate numbers are just fine.  From this you can come up with a rough ray-trace of how a signal bounces around.

What are the surfaces like?  Are they brick walls (slappy), jagged cliffs (slappy, but less so) or trees in a forest (surprisingly diffuse).

What are the materials you see?  Stone and brick will reflect a lot of high-frequency energy.  Trees will reflect less in the way of HF. Leaf-litter or underbrush will pull away even more highs.

There’s a little science (very little) in the way you use your body to generate test signals.  If you’re a good whistler you can generate sweep tones of an octave or two in the upper midrange.  A big “whoop” will give you coverage over a lower range. Both will give you a general sense of how frequencies decay.  Clapping hands so that your palms smack together will give you a fairly high pop. Cupping your hands will bring the frequency down.  Claps are great to tell you about pre-delay and let you hear the first reflections.  After a while you’ll have a good idea of how 50 milliseconds sounds as opposed to 100 or 200.  Don’t be shy about doing this:  you’re in audio so people already think you’re crazy.

First thing you listen for will be prominent reflections (handclap with palms coming together).  If you hear a rat-a-tat-tat before the smoother part of the reverb, those are reflections.  In a hall, you might be concerned with the first 100-150 milliseconds. But in an exterior they might take longer.  Are the reflections crisp or are they smeared?  Do the best you can to guess the time interval between your handclap and then between prominent reflections. Think musical rhythms.  Remember that you’re not aiming for perfection—just a reasonable guess.  That’s going to drive you to the early reflection and diffusion part of your reverb.  If you don’t have those, you might need to go straight to the reverb section. In some reverbs you can turn decay time all the way down and find some nice reflections.

The other part is the reverb itself, when there’s a smooth tail.  In some cases this might be quite long (box canyon, for example) and in others it will be short (in a forest).  You might spend some time learning to hear this.  Do the lows fall away quickly? Or is it the highs that drop off?  This is a place where you’ll want to play with the spectral part of your reverb.

I’ve been giving you general ideas in the sort of spaces you can visit and do these simple tests. This is important in giving you the tools you need. You’ll need to guess what would work in an environment where you don’t really have pops or sweeps of the real space.  If you’re in post (and chances are that you are), it would be unusual to have available pops to give you a true sense of the scene.  You’ll have plenty of boom and lavaliere, but neither of those is very helpful.  If you happen to have live mics on set, things will probably be too noisy anyway.  So you’ll have to create an appropriate reverb yourself.  Those little trips onto city streets at a rare quiet time, or football fields, or mountain valleys, will give you a better sense of that category of acoustic space. As long as the ears provide a reasonable match for what the eyes see, your job will be done.

Reverb Parameters

Lots of reverbs are capable of giving you satisfactory exteriors, some better than others.  But you absolutely must get deep under the hood and learn what every parameter does. You might be able to isolate parts of the reverb (turn off early reflections to hear the tail ‘tank’, and do the opposite to hear the reflections).  Be aware that parameter names are rarely fully descriptive of what the parameter can do. You may well find that what you need isn’t where you’d expect it to be. One you have that level of knowledge about your ‘verbs, you can combine that with what you’ve heard in the exterior—or guessed from photographs.  Like anything else it’s a skill: if you work at it you’ll get better.

Final Thoughts

This approach might seem a little blasé, but here’s an important underlying truth. We’re the product of evolution and evolution is parsimonious.  Our ancestors in deep time had an interest in finding lunch without becoming lunch. As long as you could hear the direction of that snapping branch, that’s really all you needed.  It didn’t have to sound like Boston Symphony Cave before you’d pay attention.  I feel confident in suggesting that there’s a huge amount of data reduction that goes on between our eardrums and our brains. As professionals we hear with a little more accuracy than most people, but still we don’t hear a full impulse response.  We hear some sort of derived effect.  This is a very roundabout way of suggesting you do not sweat it.  A little science, a little intuition and a lot of practice will get you close enough.

Back to you, Paul

Paul Maunder

Thanks Michael.

So there you have it, some words of wisdom from a reverb expert, Michael Carnes. The key points to take from this are to use your eyes and ears, along with a deep dive into whatever reverbs you have. Of course, there are plenty of reverb plug-ins to choose from, but one which I’d like to mention with regards to exterior reverbs is Exponential Audio Stratus. This includes a powerful early reflection area where you can choose from several reflection patterns and adjust them as needed. Channel distribution (center-only, center + LR, all-around, etc) is also controllable. Get familiar with Stratus, and you‘ll be able to create great sounding outdoor (and indoor!) reverb effects.

The Exponential Audio reverbs, Stratus and Symphony, are available from iZotope.

Photos by Johannes Plenio and Mike Bird