FAQs

Impulseresponse.org answers your FAQs:

1. What is an impulse response?

In musical applications, an impulse response (IR) is the response of an acoustic system to an impulse of energy. In the case of a concert hall and musical applications, an IR is the hall’s response to an impulse of energy (sometimes an actual gunshot). How the sound of the gunshot decays in the hall depends on how the sound bounces around and is absorbed by the hall. We call the decay of the gunshot the room’s “reverberation”. Since the gunshot supposedly contains all frequencies at the same level, we can safely assume then that the decaying reverb of the hall contains the sonic fingerprint of that hall complete with all its reflections, resonances and timbre. If we digitally record this decay, we have an impulse response. The impulse is the gunshot, the room’s reverb is the response.

2. Can I get impulse responses of things other than real rooms?

Yes. The process is basically the same: inject audio energy (an impulse) into the object, digitally record the objects response to the impulse. You can inject an impulse of energy into lots of different electronic things: hardware reverb units, software reverb units, microphones, speakers, EQ’s, compressors and limiters. Acoustically, other things like piano sounding boards, violin bodies, etc. yield interesting IR’s. Basically, if you can put energy into a system of some kind, acoustic or otherwise, and digitally record how that system reacts to the energy, you can get an impulse response.

3. What does an IR file look like?

It’s simply a digital audio file like a .wav or .aiff file as it is really and truly audio. If you open the IR of a concert hall in an audio editor, it looks like a nearly full scale spike of energy early that decays over the course of the IR. Since audio files are typically large, most of the time IR’s are compressed as .zip or .rar files to make downloading over the web more time efficient.

4. What does an IR sound like?

If the impulse was a gunshot recorded in a concert hall, the recorded response sounds like a gunshot recorded in a concert hall because that’s what it IS. The interesting thing is what you can do with an IR file…

5. What can I do with an IR file?

You can use IR’s for a number of things. If you’re an acoustician, you can gain a great deal of information about the acoustics of a room by statistically analyzing the IR. Things like reverb time, distance to the nearest reflecting surfaces, initial time delay gap (a measure of the hall’s perceived acoustical intimacy), interaural cross correlation (measures envelopment) and other esoteric things are all contained in the gunshot recording and can be extracted with the appropriate software.

The real magic for the rest of us is that a recording engineer can use IR files to add an acoustic space to otherwise dry recordings by loading the IR file of a desired room into a convolution processor plugin and convolving the dry recording with the room IR. If you don't understand this just keep reading...

6. What is convolution?

Convolution is a mathematically intense process whereby one file is multiplied by another. Desktop and laptop computers have only become powerful enough in the last 4 or 5 years to do real time convolution. Early in the game there were a few dedicated hardware devices like the Sony DRE777 that did convolution, but since general purpose computers have become powerful enough, these devices are more or less obsolete.

In the case of adding reverb to a digital recording, the recording is convolved with an IR file and the resulting file has the characteristics of BOTH orginal files. For example, if you had a recording of yourself in a closet (very dry) and an IR of Carnegie Hall (reverberant hall), convolution of the two files would result in a recording of you being in Carnegie hall. Not a recording like being in Carnegie Hall, but an exact mathmatically computed version of you in Carnegie Hall. It will sound exactly like you made the recording with the mic setup the IR was captured with. The realness of the result needs to be heard to be believed.

7. Is it expensive?

No. Everything you need is either free or very inexpensive on the web. There are free Digital Audio Workstation (DAW) programs (Computer Music magazine has one), free convolution plugins (SIR for example) and inexpensive or free IR files from this website and others.

8. How do I use convolution to add reverb to my recordings?

The process is relatively simple. You’ll need an IR file (obtainable from this website and others), a convolution plugin (most are VST compatible and some are free on the web) and a VST host DAW or audio editor (Cubase, Logic, Audition, etc.) To add reverb to your recording, do the following:

1. record a dry (no reverb) wave file into your DAW (VST compatible).

2. load a convolution plugin (SIR, Pristine Space, etc.) into the channel that contains the recorded file. Alternatively, you can place the convolution plugin across the software mixer output to add reverb to all tracks at once.

3. Load an IR file into the convolution plugin.

4. At this point, the convolution plugin should be processing the audio real time so that you can hear the results when you hit play in your DAW software. When you mix down to an audio file in your DAW, the convolution effects will be added to the file permanently.

5. Note that all convolution plugins that I know of allow you to adjust many parameters such as wet/dry balance, EQ, stretching and compressing of the IR, etc.

9. What else can I do with IR’s and convolution?

1. Be creative. In IR file is nothing but audio, so any audio file can be convolved with any other audio file (within reason). Some convolution processors have a length limit on the IR they can load (measured in seconds.) Also, the longer your IR file, the more you tax your computer. Beyond that, anything you can do to an audio file you can do to an IR (EQ, reverse, chop up, process with other plugins, etc.)

2. There are IR’s of rooms, concert halls, giant arenas, coffee cans, expensive reverb boxes, vintage EQ’s and compressor/limiters, esoteric mic preamps, etc. You can add the sonics of any of these spaces or pieces of gear to your file without actually having to own them! The quality of your results depends on the quality of the initial IR.

3. There are also inverse IR’s. These can effectively subtract the sonic effects of a device. For example, I’ve seen inverse IR’s of a Shure SM57 mic and a regular IR of a Neumann U87 mic. (Brand names are copyright their respective owners.) This would allow you to record something with an inexpensive SM57, use an inverse IR to remove the coloration added by the SM57, the convolve the result with an IR of a U87 to add the qualities of that mic. I’m not sure it will sound exactly the same as if you’d recorded initially with the U87, but it sure is a lot cheaper!

Voxengo’s Pristine Space convolution processor is particularly well suited to this kind of processing as it has 8 channels (4 stereo pairs) of processing with numerous routing options. Using one instance of Pristine Space, you could remove the effects of the SM57, add the sound of the U87, and then subsequently add an expensive mic pre and plate reverb all in one step!

4. Make a surround recording from stereo. Using either Ambisonic IR’s or front and back stereo pairs of IR’s of one hall, you can convincingly create a surround recording from simple stereo. The results are spectacular from an ambience perspective.