Waves IR-1 Parametric Convolution Reverb

The next step in sampled reverb

By John Krogh

For the last 25 years or so, the method for creating artificial reverb (i.e., simulated ambience) has been to synthesize room characteristics such as early reflections, room size and shape, etc.

But thanks to huge leaps in processor technology, today’s consumer-grade computers are capable of serious number crunching — the kind of math that makes a different kind of reverb processing possible. It’s no surprise, then, that plug-in developers have started capitalizing on this, resulting in a new type of reverb based on convolution.

Simply put, convolution reverb blends together an input signal with another signal called an impulse response. An impulse response (IR for short) is a recording, or “snapshot,” of a real space or hardware processor. In the case of Waves’ IR-1, an impressive collection of acoustic spaces and high-end hardware units were recorded at 24-bit/96kHz resolution, resulting in the single largest, most comprehensive commercially released IR library to date.

Tipping the scales at just over 1GB, IR-1’s library includes top-flight recording studios, night clubs, concert halls, churches, and more from all around the globe, as well as a choice sampling of presets from what is likely a high-end “L”-brand reverb unit (I’m guessing here, but with names such as LX 48L Concert Hall, what else could it be?). But there’s more to IR-1’s story than its rich library, so let’s get to it.

There are advantages and disadvantages to convolution reverb. Because it relies on samples of acoustic spaces, convolution sounds much more natural and convincing compared to synthesized ’verb. On the flip side, you can’t change the characteristics of a sampled space the way you can with digital reverbs. Early reflections, density, size — it’s all part of the IR recording. What Waves has done is combine the best of both worlds: IR-1 is a hybrid processor that uses convolution as the basis of its reverbs, but also offers the ability to tailor the IRs in a similar way to working with synthesized digital reverb.

To make this possible, impulse responses were captured in a multi-channel format, which IR-1 divides it into three components — direct sound, early reflections, and reverb tail. Each of these can be engaged independently and have its own gain and pre-delay. Direct is the portion of the IR that was recorded with a microphone placed in a straight line from the speaker used to drive the space.

It’s not the same as the Dry signal — an important distinction, as Direct-processed signals will include characteristics of the IR.

By itself, having this kind of control is powerful. For example, you’re free to use just the early reflections of a large hall to create a sense of space without causing the track to sound cluttered from the reverb tail.

But it gets better. Gain envelopes can be created and applied to the IR shape, allowing you to redesign the decay and other aspects of the recorded sound. For creative applications, using an envelope can produce wild results. Or you can use an envelope to do something as simple as make a gated reverb.

There are other, more familiar reverb controls, such as damping, reverb time, density, and so on. As parameters are adjusted, the display updates to show how the IR source signal has been changed — it’s pretty cool, and adds a certain “wow factor.” Any change requires a recalculation of the IR, though, which is an offline process that can take a second or two to complete. On my 1GHz dual-processor G4, for example, changes to density, decorrelation, resonance, and other controls along the lower half of the plug-in were made almost instantly, even when audio was being processed. Tweaks to low- and high-frequency damping and EQ took a few seconds longer.

It might go without saying, but all this processing magic comes at a price — IR-1 is more of a processor hog than your average host-based reverb. Fortunately, Waves has provided less CPU-intensive options. Stereo IRs can be used in different channel configurations: Efficient and Full. Efficient mode processes signals as dual mono sources, with a control for summing left and right sides of an input. It takes less CPU power, since only two convolution processes are required (one for each channel). Full mode uses four channels, left input to stereo and right input to stereo, which takes twice as much processing juice. You can choose the mode for any stereo IR when the plug-in is instantiated.

Additionally, from the plug-in interface you can choose between Full and Low CPU modes; the resolution of convolution processing is lower with Low, which saves anywhere from 20–45%, according to Waves. In practice, I couldn’t hear any difference between the two modes in the context of a mix, so it’s conceivable you could operate in Low CPU mode entirely without any audible compromises.

This was the first Waves product I’ve tested that uses a USB iLok dongle for copy protection. Installation and authorization on the dual-processor G4 running OS 10.2.6 went off without a hitch. In minutes after installation I was up and running within Logic 6.3.3. Set-up on my 1GHz Titanium was a different story — I could install the plug-in, but Logic refused to see it. According to Waves, this was due to a known bug in OS X 10.3.x; contact Waves for a workaround if you encounter similar difficulties.

After auditioning a cross-section of presets ranging from ambiences and rooms to halls and Lexicon-like samples, I was convinced: This is a remarkably real and complex-sounding reverb. “The sound is very natural,” commented EQ’s Mitch Gallagher. “There’s an organic quality to the ambience that sits very well around acoustic instruments and vocals. Even when using IR-1 on a delicate solo instrument such as nylon-string classical guitar, the reverb doesn’t seem artificial.”

I agreed. I applied IR-1 to bone-dry cello, rock vocals, close-miked and overhead drums, steel-string acoustic guitar, and even solo Tuvan singers. IR-1 delivered in every case, whether it was a smaller room or cavernous sound I was looking for.

Waves has a hit on their hands. IR-1 raises the bar for convolution reverb plug-ins. It’s not the only game on the market, but Waves’ IR-1 is the best for several reasons. It’s available for Mac and PC. It’s unmatched in its programmability. And the library covers more ground than other reverbs in its class. I’d have to nitpick to find fault, and even then I can’t really make a case. If you’re looking to add a convolution reverb to your plug-in arsenal, this is the one to get.