Understanding Convolution Reverb

Question

This question is more for me to understand the uses of convolution reverb, how to perform it, and clear up any misconceptions I have.

As I understand it, to create a convolution impulse you:

• Set up a speaker and measurement mic in the space you wish to model
• Play an impulse from the speaker and record the result. I was told to use a sound file that is a click followed by a sine sweep, followed by another click.
• Load the original file and the recorded file into a deconvolver and create an output file.
• Use the output file with your convolution reverb plugin

Convolution reverb should be the ideal way to process your ADR, but I've never been on a project that used it. I've only played with the built in profiles that come with the plug-in.

My questions are:

• What kind of equipment do you use to record in the field?
• What deconvolver program do you use?
• What kinds of applications do you profile for?

Thanks!

Answer 1

You have a pretty thorough understanding of the process. To answer your questions:

• What kind of equipment do you use to record in the field?

There are no hard and fast rules but you should try to use as professional a product as you can get your hands on. A powered speaker is a must, so be sure you have a source of power wherever you are planning on recording. Re. microphones, many folks use DPAs or Earthworks, but Schoeps, ATs and Neumanns are all great choices. If you don't have access to any of those, use what you do have! You also have a choice as to how many channels you are going to record, ie. mono, stereo, quad or higher. What configuration you choose will depend on your recorder, how much time you allot for setup, and of course your application. If you are preparing for strictly ADR purposes then stereo would more than suffice.

• What deconvolver program do you use?

Altiverb is the most popular in film post-production (at least here in LA). Another choice is TL Space. Still another is IR-L from Waves (although I've not used it). Here's an excellent article from Sound on Sound regarding the various apps and how they compare.

• What kinds of applications do you [use] the profile for?

Totally depends on what you want to use if for! You could stay strictly within the constraints of treating ADR to match dialog (or whatever space you are trying to match into), or use it on music or sound effects to simulate a desired real-space. I often use Altiverb to change the original file altogether, to design it into something new. Also, remember that convolution reverbs do not have to use only impulse responses from real spaces; they can use any sound sample that you throw into it to convolve against something else, ie. an anvil hit used as an impulse response will impart a metallic characteristic into whatever sound you process.

Experiment and have fun!

PS. There are many 3rd party impulse response libraries out there for users of convolution reverbs. Here's one that I've enjoyed using from time to time.

Answer 2

I also had the impression that one could use just a static click for the impulse sound. I was thinking of using a start pistol for recording convolution impulses.

Now that I think of the click-sweep-click-theory, it could have the idea of gathering attack - pre delay - frequency response of the reflections - and lastly decay information.

Would be nice to know more on this approach. Anyone?

Answer 3

"I was told to use a sound file that is a click followed by a sine sweep, followed by another click."

Hmmm... Who told you that? If you record the response to a click, then that is the impulse response. It's literally the response of a room to an impulsive noise, like a click. No deconvolution or other processing necessary. You then convolve your music with the impulse response, and it will sound as if you had played the music in the space instead of the click.

You probably want to use a dedicated clicker, though, rather than a speaker. You want the sound as close to an ideal impulse as possible, not filtered through a speaker's frequency and phase response. You want it to sound as if your performer is actually in the cathedral, not like you're playing a boombox of their CD inside a cathedral. You also want it as loud as possible (without clipping), so that the signal-to-noise ratio of your recording is high.

You can also derive the impulse response from a sine sweep or maximum-length sequence or other signal by deconvolving first. This improves the signal to noise ratio, but (ideally) it's going to produce the exact same thing as the straight impulse response. Practically, one method might produce better results than the other. See Wikipedia

See here and here for the use of maximum-length sequences to measure impulse responses. These work like a chirp, but better.

And remember you can record from two microphones at once to get a stereo image of the response.

Answer 4

Be sure to use as flat a mic and speaker set as you can in terms of frequency response so that you capture as accurate an impulse response as possible.