Convolution Reverb is an effect that digitally simulates how sound would reverberate in a physical space. It uses a recording from the room’s Impulse Responses to generate an echo. Because this type of reverb includes a higher density of reflections than algorithmic reverb, it sounds more realistic. How Does Convolution Reverb Work? At its core, it’s a convolution reverb that uses samples of real acoustic spaces. The library contains everything from famous concert halls and churches to clubs, airports, offices, garages and various small rooms like bedrooms and bathrooms.
Convolution reverb is said to be the go-to tool for realistic artificial reverberation. But what needs to be true so it’s really authentic?
Convolution reverb is an exciting technology that sound designers can use to capture the impulse responses (IRs) of any physical location (such as caves, churches, offices, or hallways) and process audio in real time as though it were being played at that location. Convolution Reverb Basics The general concept behind convolution reverb is to capture the impulse response of an acoustic space. The impulse response is the signal you get if you feed any linear and time-invariant (“LTI”) system with a perfect impulse.
At first sight, convolution reverb technology looks quite fool-proof. In theory, you can practically sample the exact behavior of an acoustic space or reverb algorithm. Or can you? Let’s look a bit closer at how it works.
Convolution Reverb Basics
The general concept behind convolution reverb is to capture the impulse response of an acoustic space. The impulse response is the signal you get if you feed any linear and time-invariant (“LTI”) system with a perfect impulse. In digital signal processing, such an impulse is a single full-scale sample and only zeros otherwise. Please understand that I’d rather not confuse you with the definition of such an impulse in the analog world right now.
If you know the impulse response, you know the exact behavior of an LTI system. At least in the digital world it is obvious why that is. Every digital recording is just a series of many impulses, and an LTI system responds each one in the same way: by outputting a scaled version of it’s impulse response. It’s thus trivial to reproduce this behavior artificially using digital signal processing. It gets a little harder if you want to do it in real-time, latency free and with a reasonable amount of CPU. But the basic technique is trivial.
A practical problem with this approach is also immediately obvious: an impulse response is just a static measurement. Your chances of changing some properties like reverberation time or source position after the fact are very limited, compared to other artificial reverb techniques. To realistically place orchestral instruments in a concert hall, you need lots and lots of impulse response measurements. Also, things like the sound source and microphones used for impulse response measurement affect the outcome. Especially their directivity. However, that’s not what I want to focus on today.
Let’s revisit one sentence from above again: If you know the impulse response, you know the exact behavior of an LTI system. I highlighted the important parts here, where two questions arise:
- Does the assumption of an LTI system hold for reverb?
- Can we exactly determine the impulse response?
Today we’ll focus on the first question. Next week we’ll look at the second one.
Is Reverb Linear and Time-Invariant?
For now I want to focus on the original meaning of a reverb: reverberation in an acoustic space such as a room, a cathedral or whatever place in the real world comes to your mind. It is generally assumed that these environments behave – acoustically – in a linear and time-invariant way.
Linearity means that it doesn’t matter if you reverberate a mixture of several signals together, or reverberate the individual signals one after another and then mix the results. The result will be the same. Similarly, the reverberation is the same – relatively – no matter how loud the signals are.
Time-invariance means that at any instance in time, the result of feeding a signal through a system will be the same, thus characteristics don’t change over time.
So is this true for an acoustic environment?
Generally, sound is rapidly moving air resulting in small local changes in air pressure, which in turn results in other moving air molecules and so on. To get from moving air and air pressure to a set of wave equations describing a linear sound field, some assumptions and prerequisites need to be met:
- Magnitude of sound pressure changes are small compared to the static air pressure.
- Consequently, static air pressure must be constant.
- The sound field must be irrotational, which means air doesn’t move around in circles.
Only if these criteria are true can we assume the sound field to be linear.
Under Pressure
The first criterion is easily met as long as we are talking about non-lethal sound pressure levels on the peak of mount everest and below. At sea level, 120 dB SPL is still around 1/5000th of the static air pressure. Lucky us!
Speaking of static air pressure: we know from the weather report that it is not constant. So there’s that. But at least it changes only very slowly. Nevertheless, we have a problem here, because the Wiener Musikverein might sound different depending on the current weather. Anyone in for a blind listening study?
Similarly, the air in the acoustic space should be generally at rest. Larger air movement will cause drastic changes in sound radiation properties. At every open air concert, when you’re a few 100 meters away from the stage, you’ll notice that during gusts of wind the sound becomes louder or softer for a short time, and high frequencies are damped more or less, depending on wind direction.
So it looks like we already have a problem here. But what about this irrotational thing?
You Spin Me Right ‘Round
In the free field, a sound field with only air and no other obstacles or boundaries, the sound field is indeed irrotational. The same is still true if boundaries and obstacles are present, but large compared to the wavelengths of sound.
In real-life situations however, this is not the case. There are objects, walls and edges of all sizes (which is actually a good thing – perceptually). What happens is that when air moves in the vicinity of sharp edges, it often creates little whirlwinds, which invalidate our irrotationality criterion. You can notice the problem for example with closed loudspeaker designs when the enclosure is not perfectly airtight. Bass frequencies can then sometimes produce subtle wind noises (some kind of pumping white noise).
The same goes for easily moving objects that collide with other objects. A typical example are door panels in your car that start rattling when the subwoofer goes to work.
Of course, these rattling and blowing issues are extreme examples that you don’t want anyway, so there’s no point in whining about the fact that impulse responses cannot accurately capture it. But still there are very subtle nonlinear effects in real acoustic spaces, especially with very long reverb times. The result is a certain amount of chaos and time-variance in real rooms that will get lost in impulse response measurements.
Some Perspective
Unfortunately I don’t know about any studies to quantify how strong the influence of these small chaotic ingredients actually is to a realistic reverb tail. The experimental design would be very difficult anyway. And then it’s still unclear how important this would be perceptually.
The thing is, human perception doesn’t care so much about the finest details of reverberation. In fact, it goes to some lengths to ignore most of this information. We recognize a space by very basic characteristics such as the sequence and coloration of early reflections and the frequency-dependent buildup and decay of the reverb tail. Very fine differences in the exact reflections don’t matter much once they are sufficiently dense.
In non-huge spaces, the rather academic effects described above likely don’t build up sufficiently before the reverb tail decays into inaudibility. As reverberation is all about small effects that are repeated over and over in chaotic feedback loops, it would take some time for such small effects to become dominant. The additional decorrelation however can have an impact on perceived spaciousness and audibility of late reverberation in conjunction with long sustained tones (more on that next week!).
Conclusions
Although acoustic reverberation can be largely considered LTI, there are sources for time-variance and nonlinear effects. The larger and more complex the space, the more these may (subtly) matter. Nevertheless, for the most important audible features, convolution reverb is a great way to convincingly simulate real acoustic spaces. And it’s no doubt the most exact and realistic method available to us.
Next week we’ll start out with something we omitted so far: algorithmic reverbs. And afterwards we’ll have a look at some methods to actually measure impulse responses. And at how the results are affected by noise, time-variance and other disturbances.
Head right on to Part 2!
Convolution Reverb
on Sep 22, 2017 in Recording & Production 0 comments
Convolution reverb is one of the more powerful tools in the sounds designer's arsenal, but one which many desktop musicians find mysterious and confusing, or are simply unaware of. Let's take a look at how convolution reverb works and offer some suggestions for using convolution as a creative sound design tool.
A Brief History of Digital Reverbs
Beginning in the late 1980s with the advent of personal computer-based digital audio processing, one of the Holy Grails of the music software community was realistic digital reverb. Hardware digital reverbs had been around for some time before then, going back to the EMT 250, first released in 1976. But digital reverb software had to wait until personal computers got fast enough, and RAM cheap enough, to make an acceptable digital reverb plug-in possible.
Among the first was Waves' TrueVerb, which provided, at last, an excellent-sounding digital reverb in a not-too-painfully priced package. But like all the hardware reverbs that proceeded it, it was an algorithmic system - it used software formulae, or algorithms, to simulate the way real acoustic reverb worked. That's not to say that algorithmic reverbs sound unrealistic - they can be quite realistic. And at any rate, realism isn't always the goal in music and sound production.
But for those who wanted to recreate real acoustic spaces as accurately as possible, algorithmic reverbs didn't quite measure up. It was only with the release of the first convolution reverb software, Audio Ease's Altiverb, in 2001, that those goals were finally realised.
Altiverb stood alone as the only available convolution reverb for several years, but now there are quite a few contenders in the convolution reverb market, including even some open source and freeware versions. Now that computers are powerful enough to support the technology's heavy processing requirements, convolution reverb offers unprecedented accuracy in the reproduction of real acoustic spaces, along with the creation of all types of artificial and virtual spaces, as well as some creative sound design possibilities previously unavailable.
But having said all that, what exactly is convolution reverb?
What Is Convolution Reverb
Basically, a convolution reverb takes an input signal (the sound to be reverberated) and processes it with the sound of an actual or virtual acoustic space to create the illusion that the input was recorded in that space. The sound of the acoustic space is captured in what is called an impulse response (IR), which often starts as a recording of a short, sharp sound, such as the firing of a starter pistol or the bursting of an inflated balloon (the impulse), in the acoustic space in question. As you can imagine, such a sound excites the reverberation (the response) in the space, and so the impulse response (or at least its initial recording) sounds like an explosion followed by the reverb reflections of the space.
The trigger sound is usually a somewhat broadband sound like a shot or balloon pop, because the impulse response recording should capture the widest range of frequencies in the reverb reflections possible for as accurate a recreation as possible. And, of course, for stereo convolution reverbs, you need to capture a stereo recording; for surround reverbs, you need a surround recording.
Once captured, depending on the convolution reverb software in question, the IR recording may need to be processed to remove the original impulse trigger sound and leave just the response, a process called deconvolution. Many convolution reverbs, however, don't require this step.
Once you have the IR for the space as a file, you then load it into the convolution reverb and input your sound to be processed. At that point, the software convolves the two digital audio signals together to create the output. Convolution itself is a mathematical process that has many applications including statistics, image processing, and electrical engineering as well as audio processing. I'm not going to try to explain the actual formula (mainly because I don't totally understand it myself), but you don't need to know what's happening to hear the results. If you like, you can think of convolution as a kind of multiplication of each sample of the input with each sample of the IR, with the result being that the input takes on the sonic characteristics of the space in which the IR was recorded.
How It Sounds
Here are some examples of convolution reverb in action - I'm going to use the ImpulseVerb plugin bundled with Bias PEAK, a simple convolution reverb which includes a number of real-world and virtual space IRs.
Here's the dry input sound - a single hit on a cowbell (you can never have too much cowbell, right?):
To use ImpulseVerb, you apply it to your audio file and in the dialog choose one of the included IRs - helpfully, ImpulseVerb includes pictures of the space in the IR was recorded, in this case a hallway:
The ImpulseVerb GUI
After application, here's the resulting sound:
Here's the same cowbell, but this time, convolved with an IR recorded in Trinity Church in Boston, a much larger space:
As you can hear, the results are quite realistic. In fact, because this IR was recorded in the actual church, the results of the convolution should be indistinguishable from the cowbell recorded live in the church (assuming the same acoustic conditions in each case.)
In fact, that's one of the important uses of convolution reverb - the ability to recreate the sounds of actual spaces remotely. It's used often in film and video production, where you may have dialog or ambient sound recorded on location that needs to be replaced in post-production - a very common scenario. In that case, location recorders and sound effects engineers will frequently record an impulse response at the location, then use that IR later in post to make the replacement dialog or effects sound as if they were recorded live at the location. It's much easier, and usually more accurate, to recreate the ambience of the location that way instead of trying to simulate it with algorithmic reverbs.
Space Designer
As you saw, the controls you have available in ImpulseVerb are pretty basic, and don't really allow you to adjust the frequency response, room size, or reverb time the way you may be used to in algorithmic reverbs. Many convolution reverbs still work this way - the only way you can really modify the reverb is to modify the IR. But more current convolution reverbs do allow for much greater customization of the final reverb, and one of those is Logic's own Space Designer, an advanced convolution reverb:
Logic Pro's own convolution reverb, Space Designer
Convolution Reverb Pro
We won't go into the details of Space Designer right now, but as you can see, it's much more tweakable than ImpulseVerb - you can adjust the decay and attack of the IR with the Volume Envelope, filter the resulting reverb with a Filter Envelope (note the bezier handles on the filter shape so you can create smooth envelope curves) and much more.
Space Designer is a tweakers paradise.
But what I'd really like to point out here is that Space Designer has the ability to load any uncompressed audio file as an IR, which provides for some powerful and unique sound design possibilities.
Sound Design with Space Designer
Many convolution reverbs only allow files of a specific type to be used as IRs - as I mentioned, they often need to be pre-processed to be usable as IRs. And many convolution reverbs only allow files of a short duration, perhaps only a few seconds, to be loaded in as IRs. But Space Designer not only lets you load in regular AIFF files as IRs, but those files can be of a length limited only by the amount of RAM in your system.
Another important consideration is that the sound you load in as an IR in Space Designer doesn't have to be an actual IR - that is, it doesn't have to be a recording of an actual impulse in a real space. Since Space Designer can accept any audio file, you can use any audio file as your IR, and the results can give you sounds that you can't get any other way.
For example, here's a short bit of me playing the EVP88 Electric Piano softsynth through a standard room reverb in Space Designer:
And here's an ambient pad played on another synth:
And here's the result of loading in the pad as the IR in Space Designer for the same EP track:
It's a completely different sound and feel, but the results still have elements of both the original track and the pad used for the IR.
Here's a completely different sound - one of those plastic tubes that whistle when twirled overhead:
And what happens when it's used as an IR for the EP track:
As you can hear, a completely different sound, but one which still retains elements of both sources. The possibilities when using non-standard sounds as IR files in Space Designer are literally endless.
Well, I hope that's cleared up some of the mysteries of convolution reverb for you - if you're a Logic user, I encourage you to experiment with Space Designer to see where it leads you. If you have access to another convolution reverb, try some of these ideas there too. And as always, have fun.