Convolution Reverbs

Question

I'm just starting to get in to the magical world of recording my own impulse responses to create my own Convolution reverbs. I'm currently using Waves IR-1 and i have a few questions regarding recording impulses. The setup i have been using is a pair of Röde NT-1s, in a sort of faking binaural setup, and one Genelec 8040. I'm recording and playbacking straight in/out of Pro Tools.

First of IR-1 wants 4 channels, four files of audio, LL LR RL RR. I imagine LL and RR is stereo, hard left and hard right, but what positions are the LR and RL?

Also, when the sweep i recorded is processed it doesn't really sound like the room i recorded it sound sort of tinny and too big. Could this have to do with the playback level of the sweep?

Any general tips/tricks and pitfalls of recording reverbs would be great :D

Answer 1

Recording 4 impulse responses for 2 playback channels is confusing. You could easy use a 2 channel impulse response to create a 2 channel playback signal. If you get a more detailed look, you will realize that you have with a 2 channel IR 'only' signal from hard left and hard right. But in a mix, you have signal from right which goes to the left channel an conversely. So you get a more realistic and more detailed reverb signal if the IR-processor takes 4 IR Samples to create a reverb. Have a look in here: Recording Impulse Responses for mono and stereo playback

Creating your own impulse responses is a very creative thing and can make a lot of fun - if the things work correct. I got a lot of troubles and frustrations when I record my own IR's for my Impulse Response Collection.

The idea about my Impulse Response collection was to create new 5.1 spaces. I thought it's an easy thing, but in the end, it tooks about 4 month to complete it! And the multichannel IR's for Altiverb I have completed some weeks ago because Altiverb does not support multichannel deconvolving yet. I realized soon that creating IR's for more than 2 channel playback is not an easy thing, there are no tools for that and I had to find an own way. Apple's Impulse Response Utility which actually is designed for that has completely failed, it could not deconvolve it's own material!

For recording IR's I can give you this tips: -Play sweeps as loud as you feel it is right for the room. If you play a sweep too loud in a small room, you get too much rumbles you don't want. If you play back a sweep too quiet in a large room, for example a church, you get not enough room response/reverb.

-Avoid distortions under any circumstances! Distortions can happen to single frequencies! Have a look in here in the spectrum of impulse response sweep. If single frequencies clips, it get's some weird an uncontrolled behaviors like negative phases and unstable signal phase in general.

-To find a good spot in a room: listen. Or think about: If there would be musicians in the room, where would you place them to get a nice sound and where would you place the microfones for that.

To mix and adjust levels for IR's I have used TL Space. TL Space is not that smart than other Convolution Reverb Plugins, so you can add an remove IR's whenever you want and TL Space will playback it. But be careful: TL Space has a bug and mixes up channels! Another disaster which costs me a lot of time to find that out.

More infos about recording your own IR's you can find at my tutorial. It's only part 1, if I find time, I'll complete it once. But I don't know when.

Good Luck

Guido

Answer 2

I'm not familiar with reverb specifically, but from my general knowledge of signal processing, since you haven't gotten an answer already:

I expect that “LR” and “RL” are “the sound of an impulse emitted at the left position and heard at the right position” and “the sound of an impulse emitted at the right position and heard at the left position” — which is which, I can't say since it depends on how the notation was chosen.

That is, you need to make four recordings using each of the four possible combinations of the two speaker positions and two microphone positions.

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Actually, I just tried searching with “convolution lr rl” using Google and turned up what looks like the manual for the program you're using, and it has this to say, which agrees with my speculation and explains the labels:

FULL STEREO SESSION:
To create a 4 convolution IR, Record your Left speaker with the Left mic (LL) and Right mic (LR). Then record the Right speaker with the Left mic (RL) and the Right mic (RR).

I also note that the next section of the manual describes several different modes including an “Efficient Stereo” mode which requires only LL and RR recordings. You might as well record and use the full set and see whether it makes a difference.