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Convolution Processing With Impulse Responses

How It Works
Published April 2005
By Martin Walker

Convolution Processing with Impulse Responses

Although convolution is often associated with high-end reverb processing, this technology makes many other new sounds available to you once you understand how it works.

Convolution or 'sampling' reverbs are now extremely popular, and it's not hard to see why. They provide the sounds of real acoustic spaces that are far more realistic than most reverb effects (although these remain incredibly useful when you want more flexibility), and can also give your recordings the sound of being passed through an exotic mic, preamp, compressor, guitar amp, or indeed any other effect, both hardware and software. Each of these spaces or devices is encapsulated into what is called an 'impulse response' file that can be loaded into a compatible convolution playback device. Such devices range from freeware PC and Mac plug-ins to dedicated hardware such as Focusrite's Liquid Channel or Sony's DRE S777 reverb processor.

As with most other technologies, many people will be content to use the IRs (impulse responses) bundled with their particular playback device, and to download other free ones from the Internet. (I provided a list of suitable sites for convolution reverb plug-in owners in my recent round-up of PC Freeware in SOS July 2004). While capturing your own impulse responses to the standard of commercial libraries may be daunting, it's not particularly difficult to get respectable results with a little gear, and you don't have to restrict yourself to the usual halls, churches, and studios either.

Effect specialists in the film, TV, and video-games fields are finding it extremely useful to capture IRs on location for use later on when placing overdubbed dialogue or effects into a real acoustic space. Moreover, not many people have cottoned on to the fact that you don't have to restrict yourself to IRs of existing spaces and gear — once you abandon realism and start convolving other audio files there's a bizarre new world of treatments out there for your audio tracks.

Convolution Input Signals

To capture the sound of a real space, plug-in, or hardware processor, you need a suitable test signal. The most obvious approach is to play back a short impulse containing all frequencies, so that you can record the resulting response across the complete audio range — a single 0dB-level pulse lasting a single sample could be used (and I've recommended this in the past for listening to the smoothness of reverb tails). If you don't want to cart loudspeakers about to generate this pulse in the space you want to capture, real-world alternatives include firing a starting pistol, popping a balloon, or in an emergency even clapping your hands — although judging by the results I've achieved, this latter option is of very limited use.

There are three main sources of convolution impulse responses. The easiest thing to sample is a software plug-in chain, since this can be captured entirely within a single software application, as shown here using Wavelab as an example.There are three main sources of convolution impulse responses. The easiest thing to sample is a software plug-in chain, since this can be captured entirely within a single software application, as shown here using Wavelab as an example.Capturing the sound of a hardware device is only slightly more difficult, requiring a soundcard send-return loop to be set up.Capturing the sound of a hardware device is only slightly more difficult, requiring a soundcard send-return loop to be set up.Capturing the sound of a real acoustic space generally requires a loudspeaker and microphone combination, although some acoustic noise sources can also be used in place of the loudspeaker.Capturing the sound of a real acoustic space generally requires a loudspeaker and microphone combination, although some acoustic noise sources can also be used in place of the loudspeaker.

However, there's a disadvantage in all these signals — their shortness results in a relatively poor signal-to-noise ratio. In more practical terms, this means that the background noise in your IRs will be high. The best way round this limitation is to extend the test signal, which is why most serious experimenters use some sort of swept sine wave signal typically lasting twelve seconds or so. To capture particularly noisy environments such as those inside factories, on board ships/planes, and inside other vehicles as cleanly as possible, you can double this to 24 seconds, and of course swept tones are also the easiest way to capture the sound of software plug-ins, mic preamps, guitar amp/speaker combos, and so on.

Some software convolution playback devices offer a function to convert the swept-tone recordings you've captured into IRs. These include the Altiverb IR Pre-Processor from Audio Ease, the latest version of Waves IR1, and the one that started it all on the PC some six years ago, Sonic Foundry's Acoustic Modeler (now Sony Media's Acoustic Mirror). Suitable test signals may be bundled with the software, or are available as downloadable files.

They can even be generated by some utilities. For instance, for PC users there's Voxengo's Deconvolver (reviewed in SOS October 2004), which is a generic $23 utility which can generate swept-tone test signals that you can transfer to any audio playback device for use in the field. You can then use any DAT or other audio recorder to capture suitable response recordings, and Deconvolver can convert these recordings back into impulse responses. Since most people will end up with loads of test recordings, there are batch-processing options in the software to convert them all in one hit.

Lateral Thinking

Apart from the obvious acoustic spaces, you can already find quite a few impulse responses captured from mic preamps, guitar amps, loudspeakers, speaker simulators, tape machines, and so on. These can be a great way to add sonic interest to an otherwise bland electronic sound, and their typically short IRs often act like complex EQs with responses far more complex than most resonator plug-ins.

However, a little lateral thinking reveals that there are plenty of other sounds that can be pressed into service to provide very musical results. You could for instance capture the unique collection of resonances belonging to the body of an acoustic guitar or violin, or the sympathetic resonances of a piano soundboard, and apply them to other instruments or sounds. Since these resonances 'ring' slightly longer than the original sound, this will also impart some 'time-smearing' to whatever you're treating, like a very short reverb. While this won't convert an electric guitar/piano sound into an acoustic one, the result will certainly have some of the acoustic instrument's characteristics, as well as adding extra life and interest.

For more realistic sampled instrument libraries, Tascam's Gigastudio 3 uses its Gigapulse convolution engine to provide Pedal Up and Down resonances for its latest Gigapiano II library. You can also use the same convolution technology to model different mic responses, as shown here.For more realistic sampled instrument libraries, Tascam's Gigastudio 3 uses its Gigapulse convolution engine to provide Pedal Up and Down resonances for its latest Gigapiano II library. You can also use the same convolution technology to model different mic responses, as shown here.

Other possible sources for IR sampling include telephones and human speech in general, so you can give one sound some of the characteristics of another — this can be a very fruitful area for anyone working on film or game soundtracks, or indeed musicians in search of new and exciting sounds.

However, you don't have to have an end result in mind, and some of the most interesting results can be achieved if you completely throw caution to the wind and load any WAV file into a convolution plug-in to act as an IR. Of course the results will be unpredictable, but for the sonic experimenter this is all part of the excitement — drum loops and synth arpeggios can be turned into unusual echo units; pad sounds result in drones, ambient washes, and background textures; and short files provide exotic comb filtering.

Conversely, you can also try loading impulse responses into various other plug-ins to produce yet more outlandish results. Candidates include vocoders that can load audio files for use as carrier waves (such as the one in Propellerhead Reason, Prosoniq Orange Vocoder, or the Waves Morphoder), additive software synths like Camel Audio Cameleon 5000 and White Noise Additive, or other 'morphing' plug-ins such as the Prosoniq Morph VST plug-in. All of these use the characteristics of one sound as the basis for treating another — it's not convolution, but it's the results that count.

Convolution Engines

There are now quite a few convolution plug-ins supporting different formats and platforms, and there are also convolution functions built into various stand-alone applications like Adobe Audition, Bias Peak, Steinberg Nuendo, and Magix Samplitude. There seems to be a far wider range available for PCs than for Macs, and some of the best are only available on one format or the other. For instance, Audio Ease' Altiverb and Apple Space Designer are only available to Mac users, while the highly regarded Voxengo Pristine Space and Tascam Gigapulse (and of course the freeware SIR) are PC only.

Pro Tools users will no doubt be interested in looking at Trillium Labs TL Space (which is available for both Mac and PC), while Waves IR1 also covers both Mac and PC, including RTAS and MAS options. Prosoniq Rayverb is also cross platform, but as mentioned in the main test is rather different from a convolution reverb. There's even a convolution engine available for Linux and Unix (BruteFIR), and a few innovative plug-ins, such as Delay Dots' Spectrumworx and Spectral Morpher, offer convolution alongside other frequency-domain effects.

It seems that almost every month a new convolution plug-in of some description is released, as well as new updates for some of the existing ones. An up-to-date list that I've found useful can be seen at Spirit Canyon Audio's web site, which describes all the latest contenders and what platforms they each support. You can read it at www.spiritcanyonaudio.com/convolvers.php.

Third-party IR Files

Although the more expensive IR hosts like Wave IR1 are bundled with world-class libraries of reverb responses, others (like Voxengo's Pristine Space and the freeware PC utility SIR, for instance) come with none, to keep both costs and download times down. However, most users are interested in adding to their IR collection, and there are now quite a few web sites offering freely downloadable IR files created by users — many of the commercial convolution plug-in developers host uploads offered by their users, and there are also sites like Noisevault maintained by enthusiastic users.

Since capturing the essence of real spaces is quite an art to do to a high standard, the majority of such files tend to be captures of hardware devices such as reverbs and amplifiers, and there are some gems to be found if you've got a fast modem and plenty of time. However, free IRs tend to be of wildly varying quality, so there are a few things to listen out for when auditioning them if you want to pick out and use the best.

First, the vast majority of files tend to be 16-bit, and all of these compromise the dynamic range of 24-bit signals being treated. Second, and rather more obvious in many cases, I've noticed many signs of poor set up, with high levels of background noise, missing reverb tail ends, or still vaguely audible test-tone whines (again most obvious near the end).You don't have to limit yourself to reverb when using convolution plug-ins. Any audio files can be loaded in as an impulse response, providing a vast range of complex effects. Here you can see some of the 863 effects of the Spectral Relativity library (reviewed in this article) loaded into the freeware SIR plug-in.You don't have to limit yourself to reverb when using convolution plug-ins. Any audio files can be loaded in as an impulse response, providing a vast range of complex effects. Here you can see some of the 863 effects of the Spectral Relativity library (reviewed in this article) loaded into the freeware SIR plug-in.

Commercial IR Collections

Although they took a long time to appear, there are now also some excellent commercial third-party IR libraries, and I'll be looking at a few of these for the remainder of this article. However, before you think of investing in any of these you should be aware of various potential restrictions to their use if you want to avoid frustration.

For example, while the majority of us would ideally like a universal audio-file format to be used for IRs, it's not surprising that commercial developers who bundle huge IR libraries with their convolution products tend to use a variety of proprietary formats for them — after all, such libraries are a huge investment of time, expertise, and money, and developers don't want them passed around the globe for all and sundry to use with other products.

Fortunately, most convolution plug-ins that use their own IR file format can also accept more standard ones, which makes it fairly easy to use them with commercial libraries. The two most widely adopted IR import formats are WAV files for PC and AIFF files for Mac. Since many Mac/PC audio applications accept both of these file types it's generally fairly easy to convert libraries from one to the other if necessary, to load them into a particular convolution plug-in. Audio Ease Altiverb is slightly more tricky, since it only accepts a left-right pair of SDII (Sound Designer II format) files, although once again Mac DAW software such as Bias Peak, MOTU Digital Performer, and Apple Logic should be able to perform the conversion for you.

If you're considering buying IR libraries, not all convolution playback plug-ins may be totally suitable. Waves IR1 for instance only supports responses up to six seconds in length, and any library response longer than this will be truncated as shown.If you're considering buying IR libraries, not all convolution playback plug-ins may be totally suitable. Waves IR1 for instance only supports responses up to six seconds in length, and any library response longer than this will be truncated as shown.

However, a few convolution plug-ins might yet give unexpected results with commercial libraries. For instance, while Waves' IR1 has an excellent reputation for its audio quality and world-class library, it currently restricts its IRs to a maximum of six seconds in length, and simply truncates any longer responses at this point. Waves told me this was because many hosts don't allow re-allocation of memory later on, so IR1 grabs six second's worth when it's initially loaded — when 96kHz stereo IRs are supported, as well as multiple instances of the IR1 plug-in, this was chosen as a reasonable compromise.Convolution Processing with Impulse Responses

Unfortunately, this restriction will prevent you from using many of the excellent responses from the commercial IR libraries reviewed here, except those in the deliberately shorter collection provided by Kaleidoskopy. Nuendo's Acoustic Stamp convolution engine similarly has an arbitrary 12-second limit, although this has far fewer library implications. Another thing to look out for is that the Reverse function of Trillium Lane's TL Space truncates any IR longer than five seconds.

Prosoniq's Rayverb is slightly unusual in not being a standard convolution player. Instead, it uses 'inverse raytracing' to model real acoustic spaces, and provides comprehensive control over its room model. This makes it extremely versatile for reverb purposes, and you can still import and analyse third-party library files (although both Mac and PC versions only accept AIFF files), but they will be interpreted by Rayverb 's algorithms, and will probably sound completely different from what their developers originally intended.

Convolution Software Reviews In SOS

  • Audio Ease Altiverb: May 2002.

www.soundonsound.com/sos/may02/articles/altiverb.asp

  • Delay Dots Spectrum Worx: August 2004.

www.soundonsound.com/sos/aug04/articles/delaydotsspectworx.htm

  • Delay Dots Spektral Bundle: Plug-in Folder June 2002.

www.soundonsound.com/sos/jun02/articles/plug-in0602.asp

  • Emagic Space Designer: December 2003.

www.soundonsound.com/sos/dec03/articles/spacedesigner.htm

  • Prosoniq Rayverb: October 2004.

www.soundonsound.com/sos/Oct04/articles/prosoniqrayverb.htm

  • SIR (Super Impulse Response): PC Musician June 2004.

www.soundonsound.com/sos/jul04/articles/pcmusician.htm

  • Sony Media Software Acoustic Mirror: January 1998.

www.soundonsound.com/sos/jan98/articles/acoustic.htm

  • Tascam Gigapulse: in Tascam Gigastudio 3 review January 2005.

www.soundonsound.com/sos/jan05/articles/gigastudio3.htm

  • Voxengo Pristine Space: September 2004.

www.soundonsound.com/sos/sep04/articles/voxengo.htm

  • Waves IR1: May 2004.

www.soundonsound.com/sos/may04/articles/wavesir1.htm

Pure Space IR Libraries

The first of the third-party IR libraries I'm going to look at comes from Ernest Cholakis, who will already be well known to many musicians for his creation of the DNA Groove Templates. He has been exploring reverberation impulse responses for the last ten years, and has already released sample CD's using this extensive knowledge, including the acclaimed Drone Archeology (reviewed in SOS December 2000).

Each Pure Space-series CD-ROM contains a set of impulse-response files available in 16-bit at 44.1kHz or 48kHz sample rates, or in 24-bit at 44.1, 48, 88.2, 96, 176.4, and 192kHz — for professionals this immediately places the potential audio quality above and beyond that of most other offerings from third-part developers. Available file formats are WAV for the PC and SDII for the MAC.

Using his own extensive recordings of spaces such as concert halls, cathedrals, monasteries, and even the ambience inside the Great Pyramid Kufu at Giza, Ernest has re-synthesised the impulse responses in the Pure Space libraries, which means that the results suffer from none of the limitations of real-world recordings — for instance, because of the way Ernest generates his responses, the 24-bit versions offer a dynamic range of more than 140dB, while their frequency responses are virtually flat, so you can EQ your sources to taste before convolving them.Convolution Processing with Impulse Responses

There are currently two libraries available — Classical & Mystical and Film & Sacred — and each contains 55 impulse responses cleverly organised in order of acoustic 'size'. Each response has three further numbers associated with it, that compare its low-, mid-, and high-frequency decay times to the average for that set. When you're looking for a suitable reverberation impulse for a recording, this approach can make a lot more sense than wading through names like Large Hall or St Luke's Church Row 42.

After all this careful preparation, the IRs themselves certainly don't disappoint, and don't run away with the impression that since they are re-synthesised they might sound unnatural or lack character. These are among the cleanest and most silky-smooth reverbs I've ever heard, with none of the grittiness, lumpiness, or metallic overtones that characterise many synthesised reverbs. There are some gorgeous tails in there with oodles of character, and they encompass a huge range of decay 'timbres'.

Convolution Processing with Impulse ResponsesI tried out these libraries with close-miked solo instruments, vocals, and even anechoically-recorded orchestral snippets, and without exception they sounded very natural indeed. There are no small rooms (the smallest space in the Classical series has a two-second tail) but there are chambers, halls, churches, cathedrals, and magical spaces galore.

These are extremely professional libraries containing work created over ten years, and with price tags to match, ranging from $299 (16-bit), to $399 (for the most popular 24-bit versions), to $599 for the 24-bit/192kHz version. This is still far cheaper than buying a decent hardware reverb (that still probably won't sound as good), but if you want to buy just one of the two libraries I would personally opt for the more versatile Classical & Mystical, since this contains the shortest and longest reverbs (plus loads in between), ranging from two to 10 seconds, and each one sounds quite different.

Since Ernest sells his CDs directly to the end user www.numericalsound.com, he can give them an individual registration number and 'installer lock code', which prevents the casual copier from unpacking and installing the files. Each file on the CD also employs his unique Sonic Trace digital watermark protection system, which encrypts the user's name into each and every reverb impulse, so it can be traced to the original owner even when used in multi-track audio recordings. Finally, there's also an 11-point license agreement to sign before use, whose terms may dismay a few potential users, so check it out before buying. Other than that, the Pure Space libraries are superb resources for professionals, and won't disappoint.

Spectral Relativity IR Library

Described as 'exotic ambient impulses for electronic musicians', Spectral Relativity is a vast 635MB collection of over 800 16-bit/44.1kHz files. They are deliberately not intended to provide realistic reverbs, and although some can provide strange and often ethereal reverb treatments when used as impulse responses, most create sometimes bizarre and sometimes very beautiful tonal colours, drones, and ambient washes.

The first folder contains Colors — 88 extremely short files ranging from nine samples to 64mS. These involve precious little time-smearing, instead offering a huge range of EQs, but with far more exotic responses than simple comb filters. Like most of the other impulses in this collection they are best used 100 percent wet for maximum effect, although there are no rules.

Convolution Processing with Impulse ResponsesThe next folder is named Cosmic, and here we enter a weird and wonderful world of impulses that last anywhere up to 16 seconds. Some, like the deep and doom-laden Black Sun, the plummeting metallic drones of Spiral Descent, and haunting echoes of Near Miss, could be described as weird sci-fi reverbs, while with some of the longer ones the time-smearing becomes so great that the output signals become barely recognisable.

The results are perfect for ambient washes and background textures, and I found vocal recordings to be particularly effective sources, as you can generate haunting human-like drones. For instance, Heliopause and Lunar Monolith provide inspiring 2001-style angelic choirs from almost any source signal, while Planetary Nebula creates pitched metallic drones.

The Fantasy & Fiction folder evokes images from literature and the media, with plenty of sci-fi and fantasy films and books making an appearance. I particularly enjoyed the short metallic smears of Gom Jabbar, the multiple gated repeats of Pickled Peppers, and the reverse flanged drones of Skynet. Horror & Paranormal gives us, among many others, the whispering Angels & Demons, the eerie hollowness of Burning Man, the long moaning of Heart Of Darkness, and the menacing Hiding In The Attic.

The contents of the Impressions folder are designed to evoke an image in the listener's mind. I found the falling pitches of Bombs Away quite haunting when treating complete songs, as were the even weirder Cetacean Mating Call and the high-pass animations of Dust In The Groove. Those with darker leanings should love the Industrial folder, including the pulsing metallic Armor Plate, the gated noise of Cracked Piston, and the power of Feel It Through The Floor. I simply haven't got enough space to cover the remaining Miscellaneous folders, the 102 weird objects in the Periodic Table collection, the 83 high-tech washes and filters of the Science & Technology section, and the 167 combined inhabitants of Weird and Weirder Still.

I absolutely loved this collection, and it's obviously been a real labour of love for its creator Darren Burgan, even down to the file names. The descriptions of each treatment in the PDF manual are absolutely invaluable in guiding you to suitable candidates, although you'll never know exactly how they sound with a particular source signal until you try it. It's great if you're not feeling particularly inspired — I found it almost impossible not to become catapulted into creativity within a few minutes every time I used this library. It's also incredible value for money at just $24.99, and anyone who owns a compatible convolution reverb plug-in should make it their duty to buy it immediately!

Web Resources

  • Acoustics.net is the new and official source of IRs for the Waves IR1 range, with lots of info about how Waves capture and create their own libraries, plenty of IR1 impulses to download, plus a user forum.

www.acoustics.net

  • Audio Ease provide a similar resource for Altiverb users.

www.audioease.com

  • Echo Chamber is a German site with free IR downloads.

www.memi.com/echochamber/responses

  • Noisevault is the biggest freeware IR download source. It also has articles on creating your own, and very helpful forums.

http://noisevault.com

  • CKSDE has both commercial libraries and free downloads.

www.cksde.com

  • Pure Space ambient impulses.

www.numericalsound.com/purespac.html

  • Spirit Canyon Audio's web site is home to the IR libraries reviewed in this feature, plus a range of fascinating free Reaktor v4 ensembles used to provide some of their textures.

www.spiritcanyonaudio.com

  • Fokke van Saane's site has lots of IRs of everyday objects such as telephones, buckets, factories, domestic environments, and music hardware.

www.xs4all.nl/~fokkie/IR.htm

Kaleidoskopy IR Library

This second IR Library from Spirit Canyon Audio is a slightly more subtle collection than Spectral Relativity, intended to be useful in a wider range of musical styles. It manages this by restricting its file lengths so that the end results aren't so 'smeared out', which means that percussive sources like drums and most loops don't lose their sense of timing. This time all the files are 24-bit, which will please audio purists who want to maintain the maximum dynamic range in their music, and there are 2025 of them totalling 684MB.

A huge selection of files are specifically intended for use at certain tempos, providing exotic beat-sync'ed effects. The IRs are grouped into folders according to tempo, covering 80-150bpm, each folder containing around 90 IRs. To provide the maximum rhythmic compatibility, there's some overlap between the folder contents, with quite a few effects provided in multiple versions to sync with multiple tempos. If you want an even closer match for a specific tempo, you can import the closest version into your favourite audio editor and time-stretch it to fit exactly.Convolution Processing with Impulse Responses

Of this huge section of the library I particularly liked the feedback cluster echoes of Large Print Edition, the tinkling chimes lines of Binkertell, and the guitar histrionics of Heavy On The Harmonics. The beauty of all these treatments is that while you can use them 100 percent wet for unbelievable transformations, adding a little processing under a bland drum loop, for example, can turn it into something really unique.

There are five folders devoted to single-shot effects, and in each one the contents have been subdivided into short, medium, and long IRs, indicating the amount of time-smearing. The Modulators are the shortest, and I particularly liked the machine-gun burst of 60Hz Massage, the swelling chordal drones of Bad Year Blimp, the cartoon cough and drilling of Don't Look Now, the bouncing judders of Logistical Logic, the telephonic echoes of Fondling The Leaded Crystal, and the shimmering metallic overtones of Tungsten Glow.

The medium and long Reverb folders contain such gems as the rising metallic pitches of I'm Hunting Wabbit, the distant ringing tones of Stainless Steel Balls, and the piano clusters of Aluminium Diminished. The Rooms contain very short tails and even some vaguely normal reverb treatments such as the chime decays of Messages In The Ground Loop Hum, the distant slapback of Hubcap Wallpaper, and the twin tones of Twelve Story Atrium.

The time-smearing of the Weird folders is still generally less than one second, but the treatments are more extreme, with more pitch-shifting and panning effects. I particularly liked the timepiece chiming of At The Tone It Will Be Two O'Clock, the haunting echoes of Brain Stem Dysfunction, and the distant caverns of Floating City.

Overall, while Spectral Relativity excels in creating longer drones and treatments that are often a springboard for creativity, Kaleidoskopy seems to work better at jump-starting existing songs that need rejuvenating, since you can generally still recognise the original material beneath, under its new suit of clothes. Kaleidoskopy is just $34.99, or you can buy it as a bundle with Spectral Relativity for just $49.99. Together they are a tour de force of inventive beauty and weirdness, and should be owned by anyone who wants to explore new sonic territory.

Hints & Tips

The most important general piece of advice I can give you about using IR plug-ins and their libraries is that the CPU overhead is nearly always directly related to the length of the response being used. This makes sense, since each sample of incoming audio must be processed for the full length of the convolution 'tail' — the longer this is, the more calculations are being carried out at any point in time. So if you don't want to run out of processing power in your songs, try to keep your reverb decay times within reason.Always bear in mind that when using convolution plug-ins, their CPU overhead is directly related to the length of the impulse response — here you can see Voxengo's Pristine Space taking just two percent of my Pentium IV 2.8GHz processor with a 100mS impulse, but nine percent when running a 10-second one.Always bear in mind that when using convolution plug-ins, their CPU overhead is directly related to the length of the impulse response — here you can see Voxengo's Pristine Space taking just two percent of my Pentium IV 2.8GHz processor with a 100mS impulse, but nine percent when running a 10-second one.

If you fancy having a go at capturing your own impulses, each developer that offers a suitable convolution test signal will also generally provide step-by-step instructions on how best to use it, and there are also other more general guides on the Internet. It should go without saying that to capture an acoustic space accurately you need to use high-quality mics, preamps, amplifiers, and speakers with a flat frequency response. Omnidirectional or cardioid mic responses give the best results, although any polar response should work reasonably well.

In general, the speaker that plays your sweep signal (or the starting pistol, or balloon, or whatever) should be placed where the performer would be, while the mic capturing the acoustic response should be positioned in the audience, although obviously they are your impulse responses so you can point things where you like within reason! If you've got a multitrack recorder and plenty of mics you can even try capturing IR surround reverb by simultaneously recording the sweep tone at various different positions, and importing them into suitable surround convolution plug-ins like Waves IR360, Voxengo Pristine Space, or Tascam Gigapulse.

Impulse Power!

Finally, if you're searching for exotic transformations, don't forget that, just like any other plug-in, you can chain those with convolution functions. A single convolution plug-in can provide results that are either extremely realistic or totally bizarre, so imagine what two or three of them can sound like in series!

Published April 2005