How to do Impulse Responses

What is an impulse response?

An impulse response is for the normal people a recording of the reverberation that is caused by an acoustic space (could be: let’s say a room) when a short sound is played. The sound can be an electric spark, starter pistol shot, a swipe or the bursting of a balloon, for instance.  A recording of this approximated ideal impulse may be used directly as an impulse response in a convolution reverb like Space Designer or Altiverb. Provided that the pulse is short enough compared to the impulse response, the result will be close to the true, theoretical, impulse response.

How to make them

Impulse responses are actually very simple to record, but attention to detail is very important. There are two methods of going about it. One uses a short explosive sound (“spiking”) and the other a frequency-swept sine wave (“sweeping”). As I mostly record my IR-recordings at places with no power I use the first.

A few principles apply no matter what method you prefer. First: noise! Minimizing noise is always key, but ambient noise is particularly troublesome when recording IRs. However they can be quite hard to remove if the ambience is made of animals, traffic, etc. If you happens to be recording in a room with background noise as air-conditioning, heating, road noise, machinery, and wind you are in trouble. Even when using very high-quality equipment, unwanted ambience noise might easily drown out the quiet tail you need in a good IR recording.

So. Before starting. Take a few minutes to listen for any ambient noise and take all possible steps to reduce it. If the ventilation can be shut off, do it. Noisy appliances? Unplug them. You get the picture!

Time to setup the gear. There are cheap stuff you can use that actually works very well. Zoom has a very handy stereo recorder that I have tested myself with great result. I suppose other manufacturers have cheap alternatives as well. No need to use a lot of money on this.

For the making the IR-explosions you also have a few options. What you need to make is a short and loud explosions. A starter pistol is ideal but you can go the budget way and use a balloon.

When the gear is in place start the recording. Make sure you have enough gain to grab the tail. PUT ON SOME PROTECTION FOR THE EARS. You don’t wan’t to damage your ears. After starting the recording wait for silence before making the explosion. Make sure you (and your helpers) don’t move, fart or talks. Then make the explosion. Make sure you wait long enough after the tail so you are sure you have everything before you start to move, fart and talk again.

Thats it! Time to get home and into the studio.

In the studio.

There are a few available convolution reverbs out there. Altiverb from Audioease is the leading in the industry but you have some alternatives like Space Designer from Apple (a part of Logic pro) and IR-1 from Waves. I am using Space Designer.

Before importing the IR-recording into the convolution reverb you need to trim the audio file and remove everything before the peak in the “explosion” and after the tail. Make sure you get the complete tail though. You can use audacity or simply your DAW of choice for this.

Importing into the convolution reverb is usually easy. In space designer you simply drag the trimmed sample into the window and Space Designer fix the rest.

Thats it. You now have a close to the true, theoretical, impulse response of the room you recorded in and anything you play through the Convolution Reverb will sound like you recorded it in that room.

Example of use

I have prepared an audio file where I demonstrate the use of the Convolution Reverb. In this example I recorded the IR of an old nazi bunker which was halfway filled with water. Quite a special reverb as you may imagine. Added a picture of the hall so it is possible to see the room.

In the demonstration I have played the same recordings with and without the IR’s in the convolution reverb (Space Designer) to let you hear the difference. I have included a clarinet, some drums, a cowbell and a guitar to let you hear how it sounds with different instruments. Here goes:

Why use them?

Do you believe in magic? Probably not. But the rational reason to why use them is to make it sound like everything in your sound production is recorded in the same room. The not so rational reason to use them is to “capture the feel” of a room. I use this in my dark ambient music. This may or may not work in the result but it works in my process of making it. If you wan’t to listen you should check my album Screaming Silently

Thanx for reading this very simplified howto on how to do response impulses. I know this can be explained a LOT more detailed but this is the easy way to get started. Now go out there!

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