1/09/2008

Impulse Response Tutorial

Impulse responses allow you to 'sample' your room and use it as a reverb in whichever program supports it. I personally love impulse responses for their 'set' characteristics, though they're not as versatile as some reverb plug-ins I find these limitations inspiring. Let's assume you are not stupid, you have Leopard and Logic 8. I am going to explain how to do this as quickly as possible using only a macbook with its built in microphone and horrid speakers. Of course you can substitute the hardware with something proper or you can even substitute this tutorial with a proper one from a 'professional' website, you know, a website that doesn't use the word 'pood' when describing low quality audio files. Anyways, I'm getting started:
You can find the program here: /Applications/Utilities/Impulse Response Utility.app. This is the general space of the program. If your screen does not look like this, you have opened Guitar Hero.



1. After choosing to work in Mono, I only have to press 'R' and the 'S' will automatically light up. 'R' puts the program in record mode and makes the 'Sweep' option available.


2. Before I press 'Sweep', I make sure my 'Monitor Mute' box is checked. I usually enjoy feedback but not through the already piercing 'speakers' on the macbook. I'll be sure to experiment and pull this off with the feedback on one day. So I Sweeped it!


After I have sweeped the frequencies my recorded waveform looked like this. Basically horrible low and high frequency responses because my microphone and speakers eat it.


3. At the bottom left, I will want to press 'Deconvolve' then 'Create Space Designer Setting...', I name my file accordingly and jump into Logic 8. I could 'audition' my reverb but I rather skip this entirely and go straight to logic because I get confused and discouraged at what I am hearing.


4. In Logic 8, I open space designer and see my impulse response staring right at me in its own little custom section. My impulse response looks like this. The volume is not decaying rather it is gating and sounds like pood.

This is easily customizable by grabbing the points and adjusting it to your liking. I prefer my turd room to trail like this.


You can hear quick examples of the IR's I created. Apparently my room has walls made of metal.
My loop dry
My loop wet
One shot dry
One shot wet

Or using the preset in Ultrabeat. Listen to the accentuation of the hi-hats.
Ultrabeat sample dry
Ultrabeat sample wet

To recap.
1: Press 'R'.
2. Press 'Sweep'
3. Press 'Deconvolve' and 'Create Space Designer Setting' (not sure why this is one step)
4. Get in Logic and fix your jacked volume settings in space designer
Now go get your platinum record. You know, the one you are reminded to achieve every time you open logic 8......

8 comments:

bolig said...

any sugggestions for good places in the room to attempt "sweeping" in theory I know some spots I'm just curious if you have any leap ahead tips for faithful trash audio readers.

surachai said...

I had to consult the manual on this one because I simply didn't know the answer and it seems like they don't have one either.
Here is what they suggest, and they're missing a word in the last sentence:

"Speaker and Microphone Placement Suggestions
Where you place your speakers and microphones in the recording space will have a large impact on the specific echoes and reflections that your impulse response captures. It goes without saying that creativity is key, and experimentation the name of
the game! "

Benn said...

Really bizarre to see that it has a sweep generator. Are you sweeping with your Macbook speakers?
I've always used balloons and a lighter, then cropped the tail of the waveform, and imported into IR1 or audition. For warmer impulses I've used a small "blip" alert signal thing on a megaphone.

I imagine that your result from sweeping with speakers (or even monitors) would be overly warm. Have you noticed this?

Eric said...

Do you know the IRU beyond the manual?
I am trying to find out if the red line on the deconvolved IR is in fact a clip.
It seems no matter how low I record the sweep, it always clips... after I deconvolve. Do you see this?
Is there a function/step I am missing? Or is this common?
When I've done some testing, they seem to be a little crunchy .. if I don't back off the 100% wet slider in Space Designer. ** dry is set to 0% in this case.

I am recording some IRs of some old mics to use as a low-fi smear on audio recordings.

I also need to understand better, how much of the beginning & tail to cut & fade, since these are not used as time based reverb IRs. How does a small amount of silence affect a track it's being played back on.
It seems on some "vinyl IRs" I've found on the web seem to almost act as a slap back, due to some silence/space at the top of the IR wav.
I made a short movie to see the clipping red line on the waveform.
http://www.generate-music.com/QT/testpeak6.mov

adrjork said...

Hi, I read your tutorial on Logic's IR (http://trashaudio.blogspot.com/2008/01/impulse-response-tutorial.html) and I have a question:
I need to sample the reverb of 8 rooms in a hospital because my dad is died there, and I want to give a piano-recital using the 8-reverbs like audio filters in real-time.
Naturally, nobody in the hospital can gives to me the authorization to enter the rooms and do the work... So I must secretly do the work. I can't enter the rooms with big and expensive audio-card or speakers etc. I must find a solution to have a good result using little instruments.
My idea was to use a macbookpro + zoom-H4 + mediavox (http://www.bespeco.it/detail.php?mac=6&cat=114&det=2311&lingua=en):
with this set I have a better microfone than the built-in mic of the mac, and a better speaker (freq. response 35hz-19khz) than the built-in speakers of the mac.
But my question is: also my set is far from a good set! so... in your opinion, could be possible that my set is not so far from yours (all built-in) then I could really use your set instead?
Another set could be: macbookpro + zoom-H4 + built-in-mac-speakers. In your opinion there's really a difference between this last set and the former set wih mediavox?
Please, give me an advice.
Thanks.

adrjork said...

A pair of questions...
In a 5*4 meters room, I want to get IR mono to 5 channel: 5 mics in the center of the room, and a speaker toward the ceiling.
My question is: using the sine sweep, what is the minimum watt rms for the speaker to get a decent IR in a 5*4 room?
Another question is: if I save a IR 5-channel set for SpaceDesigner, and later I should use this set with a simple stereo system, could I "convert" the 5-channel IR that I saved into a stereo IR set?
Thanks

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