Impulsive Thoughts on Convolving Reverbs

For this column, I wanted to talk about Impulse Response Reverbs; what they are, how they work and how they can make your mixes better. The focus will be on the popular Altiverb and Waves IR-1 plug-ins, although the technology behind Impulse Response sampling applies to all the currently available products. First and foremost, the existence of plug-in based IR reverbs have only come about due to the massive increases in computing power. Impulse Responses are not models of rooms, halls, churches or digital gear. They are actual samples of each, reproduced using a technology called convolution – hence the term ‘convolution engine’. Simply put, either a ‘spike’ of sound (a cap gun for example), or a full sweep tone is sent into the space you want to capture. Using a microphone array, the IR or Impulse Response of a space, is recorded. By using a convolution engine combined with an Impulse Response, you have ‘real’ reverb at your fingertips. The mathematical calculations involved in creating good convolution-based Impulse Responses are massive. Lucky for you, Audio Ease and Waves have already done all the work, and both have plug-ins with a huge library of top-notch impulse responses. We’ll take a brief look at both of them below, but make sure to check the manufacturers websites for in-depth info.

Audio Ease Altiverb

Audio Ease Altiverb Altiverb was the first IR plug in on the block, and has been out for a few years already. They recently released a new Version 4, offering tone control, parameter automation, and more importantly, better performance to lighten the CPU load. Another cool thing about Altiverb is their Impulse Response library, posted not only by the company, but also by a legion of devoted users. I’ve gotten some kick ass IR’s off of www.audioease.com/IR. You can even use the search function to type in "Studios" for example, and a listing of the currently available Studio IR's pops up. How do Cello, Westlake and several Bill Putnam Rooms sound to begin with? Actually, as I sit here, I just downloaded one I didn’t have yet; the Wiener Konzerthaus in Austria. Within just a few minutes time, I now have mono to stereo and stereo-to-stereo impulses responses, as well as mono to-quad and stereo-to quad IR’s. Think about it…what we have at our disposal, the ability to just go online and download a world-class acoustic space and use it immediately, are amazing. Also, with Altiverb, you can sample your own acoustic spaces, or even some of your outboard gear. I downloaded a nice EMT 250 Plate IR that I use all the time on vocals. Altiverb runs on the G4 and G5 Macs and works with MAS, HTDM, RTAS, VST and Audio Units.

Waves IR-1

WAVES IR-1 Waves recently release their Impulse Response plug-in for the PC and MAC platforms, running with RTAS, HTDM (Mac), AudioSuite, VST, Direct X (Win) and Audio Units (Mac). Users have control over key reverb parameters including Size, Decay Envelope, Density, Resonance and EQ, among others. There is also a cool Reverse button that can be used for some great sounding effects. The IR-1 comes with pre-loaded Factory Presets, but also has 2 CD’s full of Impulses; CD#1 is called Sampled Acoustics and features some beautiful Opera Houses, Concerts Halls, Cathedrals, Studios and other spaces. CD#2 is called Virtual Acoustics, and contains Sampled IR’s from synthetically generated and pre-processed IR’s, as well as a nice set of classic reference reverb hardware sounds such as the Lex 480L. The Waves team traveled to locations around the world to gather these IR’s - using a custom recording setup/method developed in conjunction with Professor Angelo Farina of the University of Parma.

Concert Halls • Kirishima • Uhara • Rome Santa Cecilia Auditorium – Small Hall • Rome Santa Cecilia Auditorium – Medium Hall • Rome Santa Cecilia Auditorium – Large Hall • Clairmont Hall • Herbert Zipper Hall • Ingram Hall • Sydney Opera House – Concert Hall Auditoriums • Atherton • Parma Auditorium • Ryman Auditorium (Grand Ole Opry) Theatres • Noh Drama Theater • Rishon Letzion Heichal Hatarbut • Wells Fargo Pavilion Churches • Belle Meade Methodist Church • Presidio Chapel • St. John Church • St. Paul’s Chapel • Stanford Memorial Church • Trinity Church Recording Studios • Mishkenot Shaananim Music Center • Hamon Studios • Cello Studio 1 • Masterfonics - The Tracking Room • Schnee Studios • Signet Sound Studio A • Sound Emporium Studio B • Stockton Studio C • Westlake Audio Studio D Opera Houses • Teatro Valli • Sydney Opera House – Opera Theatre Amphitheaters • Siracusa Greek Theater • Taormina Greek Theater Scoring Stages • Todd-AO Clubs • Birdland • Bluebird Cafe • The Bottom Line • The Kitchen • The Knitting Factory • TLV Medium Halls • Azrieli Hall • Knights Halls - Medium • Knights Halls - Refectorium • Sydney Opera House – The Studio Small Rooms • Knights Halls - Small • Hamon Studios – Small Concrete Room • JamSync Bathroom • JamSync Florida Room Stairwells • Skyscraper Stairwell Car Interiors • Ford Econoline 150 • Lincoln Navigator Stadiums • Gaylord Entertainment Center Outdoors • Nekarot Cavern • Luzit Cave - Medium • Luzit Cave - Large • Knights Halls - Courtyard Devices • Classic Hardware Devices Above: the library of impulse-generated acoustic spaces included as presets in the Waves IR-1.

How They Make Your Mix Sound Good Impulse response reverbs are like no others. There is nothing quite like putting your vocals into a world-class studio, or your strings into a fine European concert hall. It’s especially gratifying when the musicians you are mixing notice the sonic difference and ask what you’re using on them. It’s gotten to the point now where repeat clients are requesting to be put in certain IR rooms. That still blows my mind! I don’t have a personal preference for one or the other, as I use them both. To me, it’s similar to using EQ - I have many to choose from, and I work with each if I feel it will get the job done. There are variations within each plug-in, but again, it depends on your current needs. It’s personal taste. Yes, the processing power requirements are there for IR plug-ins, but I have found without question that they are worth the trade off. As computing power grows, and manufacturers continue to tweak their products, this will become a non-issue. Until then, try out an IR and take your mix to the next level.

SOME OTHER IR PRODUCTS Above, from top: Emagic Space Designer (plug-in), Yamaha SREV1 (hardware), Sony DRE-S777 (hardware).