Waves IR-1 Parametric Convolution Reverb Plug-InIMPULSE RESPONSES FROM OUTSTANDING SPACES 12/01/2004 7:00 AM Eastern
When I first heard about Waves' IR Parametric Convolution Reverb, the letters IR and the word “convolution” puzzled me. After some research, I figured out what was under the hood in this powerful new plug-in.
Convolution, simply stated, is a mathematical operation in which a source signal is processed by passing it through an FIR (Finite Impulse Response) filter, which then generates a third signal. IR (Impulse Response) plays a central role in all digital reverbs. Impulse Response convolution means taking a sound and multiplying it with an impulse response sound file of an aural space (natural or synthetic) so that the signal sounds as if it were played in that aural space.
The IR-1 is a sampled-acoustics convolution reverb plug-in that uses sampled Impulse Response files to generate output. These IR files are responsible for the reverb's sound signature. Waves ships more than a Gigabyte of meticulously recorded IR files with the plug-in. Traditionally, convolution reverbs haven't allowed much control over parameters, but Waves has figured it out and has delivered a very useful, highly adjustable, excellent-sounding reverb plug-in.
The IR-1 provides complex, incredibly smooth sampled-acoustic reverbs and allows total control of all familiar reverb settings plus a few new controls. Familiar controls include Reverb Time, Size, Reverb Density, Independent Gain and Pre-Delay of the three components of the IR's output (Direct, Early Reflections and Reverb Tail), Wet/Dry, Reverb Damping and EQ. The gain and pre-delay controls can be linked so that they can be adjusted as a group, and the EQ (4-band) and Damping settings are the same click-and-drag type found in Waves' Renaissance plug-ins. Unfamiliar controls include Convolution Length, Reverb Resonance, Decorrelation Control, Direct Control, ER/TR-X Control and some unusual Pre-Delay settings. The Convolution Length parameter controls the length of the process filter. This means that the IR-1 will provide up to six seconds of reverb based on the Impulse Response file. Most of the supplied IRs are less than four seconds, but if an IR is longer than six seconds, it will be cut off. This results in a “gated” reverb sound, which can be smoothed out by using the Envelope Gain. Setting the convolution length shorter than the RT60 Reverb time will save some CPU load.
Reverb Resonance magnifies or reduces resonances in the original IR while Decorrelation Control is a stereo-widening control. The IR-1's “direct” signal is not the same as a “direct” signal on most other reverb units. In the IR-1, the direct signal is actually a convolved signal (like the other reverb components), but it is convolved with the first reflection from the impulse response. In other words, if you were sitting in the 10th row of a hall, it would be the very first sound you would hear coming directly off the stage without any reflections. If you want your recording to sound as if it were coming from that 10th row (or wherever the IR was recorded), then you would turn this control on and set the Wet/Dry slider to fully wet. This setting defaults to off as most of the time you would be using the IR-1 as a reverb and the “direct” is not reverb.
The ER/TR-X control allows you to move the point where the early reflections stop and the tail reverb begins. Pre-Delay settings in the IR-1 are unusual because they can be set as high as 500 ms and the Reverb Tail pre-delay can be set to -100 ms. I found this minus predelay setting useful when I encountered some latency issues on a particularly distant-sounding impulse response.
The ability to shape the sound of the IR-1 is the engine of this reverb. Equally important is the gas that you put into the engine, and Waves' gas is high-octane. Waves partnered with Angelo Farina at the University of Parma, Italy, to create a set of extended-frequency, extremely low-noise 32-bit impulse responses. The 2-CD library offers more than 120 IRs that range from huge to small, with lots of in-between spaces. Some of the more famous acoustic spaces include the Sydney Opera House, Ryman Auditorium and St. Johns Church, while some of the less-famous small spaces include car interiors and the Waves bathroom. In addition to real acoustic spaces, Waves also created IRs from classic hardware devices including multiple IR samples of a Lexicon 480L. One of the strengths of IR reverbs is that they are not limited to a finite number of IR files. In fact, Waves offers a Website for sharing IR files at www.acoustics.net. Waves IRs are in a proprietary format (i.e., can't be read by other processors), but the IR-1 does allow you to import third party IRs in .WAV format, many of which can be found for free on the Internet.
Most IR convolution reverbs are very computation-intensive. When I received my evaluation copy of the IR-1, I found that I didn't have enough horsepower in any of my computers to run the plug-in. To remedy this, I purchased a Powerbook 1.5GHz laptop, and for I/O and a host, I used a Metric Halo Mobile I/O 2882 and Apple Logic Pro. I set up this rig as an external reverb processor so that the only task on the laptop's CPU was the IR-1 reverb.
IR-1 has many ways of dealing with CPU demands. When you instantiate the plug-in, there are four choices: mono, mono-to-stereo, efficient stereo and full stereo. Full stereo uses four convolution engines, while mono-to-stereo and efficient both use two and mono uses one. Increasing host buffer size can reduce CPU consumption, but higher buffer settings also have more latency. A CPU mode switch offers settings: Low can save up to 45 percent of the CPU cycles, with a slight reduction in convolution resolution. Shortening the convolution length will free up some CPU power. Higher sampling rates also create more CPU demands.
I set up the IR-1 on a stereo aux send while mixing George Duke's latest album. Most of the time, I was looking for reverb with some length, rather than short ambient spaces such as bars and rooms. I tried it on drums, vocals, strings, keyboards and guitars. It sounded smooth and detailed, with one exception: I couldn't get it to feel right on a vintage Rhodes. The Rhodes had a bright “bell-y” sound that was accentuated in the reverb's reflections. I was using one of the seven Santa Cecelia Concert Hall IRs. Turning down the early reflections and setting the tail pre-delay to -100 ms helped, but I was already committed to using it on other instruments in the mix so I didn't want to change it much. In that case, a Yamaha REV5 did the trick. If I had enough processing power, I would have instantiated the IR-1 once more with a different IR and dedicated it to the Rhodes. One of the IRs that I particularly liked was the Todd/AO Scoring stage. I had recently cut some tracks there for Disney's Mr. 3000 film, so the room sound was still fresh in my memory. Adding IR-1 to my tracks suddenly put them on that scoring stage. The IR-1 could be a very useful tool when you want your tracks to sound as if they were actually recorded in a particular environment. I didn't have time to audition all of the IRs, but I must say that I fell in love with the Sydney Opera House. One of the interesting effects of using the IR-1 was that it sounded so good, I found myself pushing the reverb more than normal. Trimming the returns brought the mix back into perspective.
There were a few minor drawbacks: When you make a parameter change in IR-1, the output mutes and the display shows “calculating…” The calculations usually take less than a second, but it is still a bit annoying. Also, you can't automate its parameters. You aren't able to use decay times longer than six seconds. And the IR-1 is pricey — $800 for RTAS/HTDM/MAS/AudioUnits/AudioSuite/VST/DirectX and $1,200 for TDM. But the biggest issue to keep in mind is processing power. Naturally, the first thing I tried to do was redline the software by setting up Logic and the Mobile I/O to run at 96k. I loaded a four-convolution engine set to Hi CPU. At 96k, the IR-1 absolutely sings. However, on my external laptop system, the IR-1 sometimes hiccupped, especially if I increased the reverb time. Adjusting Logic's buffer size helped a little. Using fewer convolution engines or switching the CPU mode from Hi to Low did slightly degrade the reverb quality, but I want to emphasize the word slightly. At 48k, everything worked fine. What this means is that the IR-1 is a CPU-eating beast. With enough processing power, however, it works and sounds great. Bottom line: As long as you have the CPU horsepower, you will love this plug-in.
Waves, 865/909-9200, www.waves.com.
Erik Zobler is an L.A.-based mixer.