Impulse Response

Convolution Reverbs are the next generation of room simulation. Convolution is a much different process than the traditional synthesized reverbs. Convolution starts with a sample of a room, where traditional digital reverbs synthesize the sound of a room. It's very much like comparing sampling drum machines to synthesized drum machines. The sampled drum sounded more realistic, while the synthesized one had more flexibility. A sampled reverb is naturally going to sound more realistic, as it is a recording of a real room, and over the last few years, convolution reverbs have added the ability to modify the samples, much like their synthesized counterpart.

Audio Ease's Altiverb 6XL pictured above

The Difference Algorithm (synthesized) reverbs are very flexible as they are creating reverbs with mathematical calculations. Convolution reverbs sample, or record, the sound of an actual room (impulse response) and add the sound of the room to each and every sample traveling through. The longer the reverb time, the more DSP it takes to process. Basically, a 3 second reverb (convolution) is processed 44100 times a second. A 96k impulse would be processed 96,000 times per second times the duration of the reverb time, and so on. Now, stereo is obviously twice as hard to process. It has taken a while for computers and hardware to catch up with being able to smoothly run convolution reverbs, as it is very CPU intensive. Many modern Reverbs are now offering 5.1 convolutions which are obviously 6 times as hard to process as their mono counterparts.

How an Impulse Response is created

An impulse response is created by playing a starter sound into a room and recording that sound with microphones in the room. The starting sound is specific to the convolution reverb. It is typically created with one of two common sound sources, a pistol shot, or a frequency sweep, that come with the application. The pistol shot is fast and good for on the fly sampling of room. The frequency sweep method is more accurate, but more time consuming. The recorded sound is then loaded into a secondary application that removes, or strips, the starter sound from the recording, leaving you with the "impulse response." The impulse response is then loaded into a folder that can be copied in with your other impulses to become another preset. You can load multiple impulses into one folder, along with pictures if you like, and add the folder to your plug-in settings folder. The title of the folder becomes the "bank name" for the collection of impulses, and jpg pictures dropped into the same folder show up in the plug-in window. Some convolution reverbs also allow for quicktime movies, or flash, to be also loaded into the folder for display.

A common question I get about creating impulse responses is "What speakers and microphones are you supposed use to create an impulse response?" Well, there is no set configuration to use. It's the same principle as setting up a chamber reverb. Whatever speakers you use will color the sound of the reverb, as will the microphone and preamp used. I try to choose a speaker that has a fairly flat frequency response, and use a variety of microphones to record the room with. If I had a choir in the particular room, I might choose a specific microphone to record with that has a character I like for the sound source. It's OK to record the room with a series of microphones. Just label them properly, like "Carnegie Hall, 10M, U67," "Carnegie Hall, 10M, tc30k," etc.

I have found the feature of capturing my own impulses invaluable. I am currently working on a film doing all of the post production. I showed the person recording all of the production audio how to capture impulses with the given frequency sweeps, and now I have already loaded them in, stripped the original tone, and created impulses for all of the rooms used in the film. It has allowed me to match the ADR(automatic dialog replacement) tracks I've re-recorded to the actual sound of the rooms they were filmed in. I had them use the same microphones they used to capture the production audio with so that the impulse would match the tone captured. Another great feature of creating impulses is that you don't have to actually use speakers and microphones. You can run the starter tones through outboard gear, and record the output. Most Convolution reverbs come with a series of presets, including sampled plate reverbs, spring reverbs, and such, along with actual rooms. I have already gone through my personal reverb units and sampled my favorite settings. I don't have the flexibility of getting the sound of changing, say the, diffusion of that particular unit, but I get the diffusion of my convolution reverb instead. The advantage of having multiple reverb units "in the box," to me, far outweighs having to keep patching in and recalling different outboard reverbs. Recall is now just simply opening up the session.

Pictured above are Digidesign's ReVibe and Trillium Lane's TL|Space

Surround Impulse Responses

Audio Ease's newest offering, Altiverb 6XL, which is finally TDM, can now capture 5.1 impulses at up to 384k sample rate. I would not plan on running to many of those presets in one session, as each would probably come close to chewing up an entire Accel card. At 48k, a stereo in-out convolution uses up one chip per second of reverb time so a 3 second stereo reverb impulse would use 3 chips on an Accel card(9 total per card). Going to 96k doubles the dsp required, so the same impulse recorded at 96k (3 sec) would use up 6 chips (ouch!). I haven't used or recorded any 5.1 impulses yet, as the Quad impulses are usually what I'm looking for anyway. I don't know about putting reverb in the center channel or LFE anyway. A quad impulse at 48k still uses 2 chips per second of reverb time. These can be run native on an LE system as well, but as you can see, they can still be pretty intense, even for a quad core machine.

Sharing the wealth

Altiverb, as well as others, have additional impulses that can be downloaded. Teams of engineers, as well as individuals, are constantly out sampling new rooms to be used. There are even user groups that share each others impulses, so the amount of impulse presets is unlimited, very much like a sampler. You can never run out of impulse responses. I have been trading impulse responses for Altiverb now for about 4 years, so my library of presets has become invaluable. The ability to capture new rooms makes the possibilities endless.