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RecordingImpulse response of a room


slemaker

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Hey all,

 

I've just been asked to make an impulse response recording of a motion capture stage I work in a lot.  After reading a little about it, makes perfect sense.  Not sure how to do it though.  Do you place an omni mic in the middle of the room?  Do you play the audible frequency range through a speaker?  If so where would the speaker be placed?

 

Thanks for any info.

 

Charlie

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Hey all,

I've just been asked to make an impulse response recording of a motion capture stage I work in a lot. After reading a little about it, makes perfect sense. Not sure how to do it though. Do you place an omni mic in the middle of the room? Do you play the audible frequency range through a speaker? If so where would the speaker be placed?

Thanks for any info.

Charlie

Place the mic wherever the "listening" position would be. Place the speaker (or whatever) where the sound source would be - use multiple positions for... well, different positions. You can use an omni mic or a cardioid or whatever. Depends on the sound you're hoping to achieve. You can use a stereo/surround setup, too.

As the the source, the sinewave sweep is probably the most precise method. Theoretically though, you would need to compensate for the speaker's own tuning. A starter's pistol works quite well, too. Check out AudioEase's website, they have a lot of info on creating impulse responses.

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Keeping in mind this is a motion capture stage where talent and mic positions will be in a virtually infinite number of positions.  Post mixes will be incorporating virtual scenarios such as walls, cars, oceans, and anything else under the sun that could effect reverb in an environment.  I can't very well test pings from every inch of the stage itself.  Using altiverb's playuback file may be the right path, but where to place the source and mics I'm still not sure about...

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Anyone do motion capture audio?  My other thought was to put the reflective markers on a slate and on the mics.  Place markers on the mics as well and then place the mics in the center of the room.  Record stereo with one mic facing one direction and vice versa.  We could roll the mocap cameras and then render the whole thing knowing the markers on the mics and slate would be logged in the space exactly to the millimeter.  Clap the slate from maybe 20 different locations in the space...

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I recorded a lot of IR for my last feature. I used sweep tones from the Altiverb site, a pair of wooden 'clap sticks', and placed the speaker in three different positions, and the mic in three different positions in the room. In smaller spaces, i reduced the number of iterations.

 

I would get as many iterations as possible - with changes in both speaker and mic positions.

 

The main character in the film was a 12 year old girl who lost her voice before the shoot, and pretty much went through six weeks barely able to speak. She was ADR-ed for the entire film and the spot house at San Francisco used my IR recordings with excellent results, I was told.

 

-vin  

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Keeping in mind this is a motion capture stage where talent and mic positions will be in a virtually infinite number of positions. Post mixes will be incorporating virtual scenarios such as walls, cars, oceans, and anything else under the sun that could effect reverb in an environment. I can't very well test pings from every inch of the stage itself. Using altiverb's playuback file may be the right path, but where to place the source and mics I'm still not sure about...

You will only ever be able to capture the IR of the studio, however relevant that may be. You don't have to do one for every camera position, especially in this case. To get the IRs for the various places you mentioned, you will have to go to these placed.
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" Post mixes will be incorporating virtual scenarios such as walls, cars, oceans, and anything else under the sun that could effect reverb in an environment. "

is post trying to recreate the sound of the MoCap stage, or to create the sound of the resulting (finished) environment ??

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They want to capture an impulse to use in their impulse response reverbs.  What you need is a starter pistol or large clap slate.

The quality of the clap you use will change the effect the impulse has on the shape of the reverb.  The sharper and louder the better it will work and I am sure they want a clean colorless impulse.  For instance in the last Superman movie; Superman Returns they used a metal spoon to hit a glass vase for the impulse response.  This gave their reverb a glassy emtallic sound to enhance the the effect of supermans Fortress of Solitude.  Let us know what they actually do with it, generally you would record the dialogue on stage then record the impulse response in the actual location you are supposed to be in.

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I always carry a loudspeaker and I take an impulse of the room with a equalized sweep (Equalized for a flat response of my loudspeaker), usually I use a pair of omnis on diferent positions (I do a photo of the different positions), or sometimes I use a Soundfield SPS200. Principally I use the Waves IR1, and  usually I do a full stereo IR (L+R on two mics) or multichannel

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Also: if you are at all handy with max/msp, there is an external called IRmeasure~ as part of the HISStools collection. This object gives you some flexibility in the type of impulse you can use, either an ESS (exponentially swept sine wave), a burst of white/pink noise etc., with complete control over the various aspects of the impulse - attack, length, spectral range, decay. The impulse is measured using a speaker(s) and microphone(s). Once you have a IR of a space you could ostensibly use it in other IR verbs or use with other HISStools objects like their multiconvolve~, a zero latency deconvolution reverb. It is truly nice! And free!

Here is a link to the Huddersfield website: http://www.thehiss.org/

Here is a link to a web-post about how to use the external: http://m.youtube.com/watch?v=0waUi6qpf-k

I have used this object to take impulse responses of auditoriums, stairwells, a long hallway, or less conventionally for creative equalization to approximate the sound of being inside a closet while listening to something in another or room. Other fun things to try were pieces of analogue gear, tape machines, digital delays, effects pedals, a channel strip... Really anything you can put a signal through has an IR!

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I have found Apple's "Impulse Response Utility" application the easiest way to do this. It has everything you need to record quality IRs and you can even preview them without having to load up your DAW. It's made for use with Logic's "Space Designer" but I use it with TL Space & Altiverb.

 

I'd do at least 3 positions with your mic(s): close, middle, & far. I prefer the 10 seconds sine sweep method.

 

 

John

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