Master Class: Audio Alchemy

Apr 1, 2008 12:00 PM, By Dennis Miller

Learn how to use convolution to transform your sound files.

BONUS MATERIAL
Additional Resources: Click for a list of good programs that support convolution.
Web Clips: Listen to audio examples that demonstrate the many things you can do with convolution.

Tweaking Time

If you aren't happy with the initial results you're getting, you can do lots of things to the IR to get extra mileage from it. But before looking at those options, remember that one of the great things about convolution is that the same IR can produce different results depending on what source you pair it with. Even if you don't get good results the first time around, there's no reason not to try the same IR again with a different audio file.

As Burgan notes, “The effect a particular IR will have cannot usually be fully predicted because it is dependent on both the IR and the signal it is being convolved with. IRs based on true white noise or IRs that contain only one frequency are an exception. This makes it difficult to have a favorite IR, because an IR that sounds good on one signal may produce useless effects upon another signal.”

One way to tweak your results is to explore the parameters of your convolution program. For instance, as mentioned previously, many programs have an envelope function that lets you determine how much of the IR you want to apply to your sound and how the IR's amplitude changes over time. Web Clip 12 illustrates the use of the envelope feature in SIR2. SIR2 even has a feature to generate a few repeating envelope shapes automatically (see Fig. 4). The 16-segment sawtooth, for example, will create a pulsing effect on any sustained sound.

According to Burgan, “The envelope of the IR is imposed on the carrier file when the two files are convolved. For example, if an IR has a very abrupt ending, the convolution ‘tail’ that you get when you convolve a signal with it will have the same abrupt ending. One of my favorite tricks is to create an IR that has a very long, slow fade-in over multiple seconds, then an abrupt ending — the convolution tail will have the same envelope. This can be useful for creating near-real-time ‘time reversal’ effects. Try putting a signal containing someone talking through such an IR, and the result will sound almost as if you reversed the signal that had the person talking — but you can do it while streaming live!” See Web Clip 13 for an example of this approach.

Many convolution programs also let you swap channels, reverse, or filter or EQ an IR. You should also expect to find a bypass switch and wet/dry mix feature. Programs designed primarily for reverb will also have predelay, room size, tail length, and other options, which offer even more resources for exploration. Altiverb's Decay parameter, for example, imposes an exponentially decaying envelope over the loaded IR. This can produce unexpected results, depending on how well the program can track the amplitude curve of the IR. Altiverb's Size (room) parameter is also useful for shifting the pitch of the carrier (see Fig. 5). Listen to Web Clips 14 and 15 to hear the impact of using different room sizes on a single low piano note being convolved by a drum loop.

Modify the IR

Rather than adjusting an IR in your convolution program or using an unedited audio file as an IR, you can modify the IR file in a variety of ways using your audio editor to make even more unique sounds. For instance, you can cross-fade two IRs to impose a modulation between two keys, or you can reverse or time-stretch an IR (see Web Clips 16 and 17). Lowering the bit depth of an IR also has an impact on the resulting file.

According to Burgan, equalization really helps improve the results of the audio convolution. He notes, “It's very important to balance the spectrum, especially if there is too much energy (resonance) in a narrow range of frequencies. Moving a medium/narrow bell at +10 dB through the entire frequency range of the IR is an effective and fast way to search for undesired or disturbing frequencies. When you find the unwanted frequency, try to adjust the bell Q and lower the gain until that frequency disappears [see Web Clip 18].” Burgan reminds readers to lower the main volume when starting this procedure, or you might burn your speakers (and ears) with unexpected resonances.

Many times, convolution results in a dull sound with little or no high end. Camnasio makes these suggestions: “When the resulting sound is too dull, I try to equalize the IR in the low range with a low shelf or a highpass filter. This comes from a method that is well known to mixing engineers, used when you wish to add a touch of reverb to the mix and don't want to lose definition in the low frequencies. You can also try to boost the high frequencies with a high shelf on the source material before using it for the convolution.”

Camnasio adds, “When your source sound has a narrow spectrum, the result of the convolution process will depend very much on the pitches it contains. For instance, take a piano melody and try to convolve it with different IRs. You should notice that some perform well, and others less so; some produce a balanced spectrum, while others generate strong resonances that you may want to remove with equalization. If you stretch the IR, the resonances are going to change. So this could be a very interesting method to refine the IR to suit your specific source material.”

Postprocessing

Even after you've performed the convolution, there are still some techniques that can make your files more useful. Burgan describes some ways to process your final convolved files to improve their sound. “Because of the nature of convolution,” he says, “resonances can occur very easily. For example, let's say a frequency of 600 Hz is strong in both the IR and in the signal being processed. The convolved result will have a very strong signal at 600 Hz, because the two signals resonate with each other at that frequency. There is no way to avoid this, nor can one predict in advance where resonances will occur, because it depends both on the IR and on the signal being processed. Therefore, it is a good idea to always follow a convolution engine with a good parametric EQ so that you can tame the resonances that will inevitably appear. In this case, I'd use a steep notch filter at 600 Hz to reduce the amplitude at that frequency, assuming you don't find the resonance useful.”

Burgan concludes, “People who have really mastered the art of convolution processing will create IRs specifically for the signal they wish to process with a goal in mind, applying the notions of spectrum, envelope, and duration to craft an IR that accomplishes the desired goal.”

No doubt making your own IRs is the “power user” approach, and if you have the time and inclination, it may be the best way to ensure that you'll get the results you're after. But more than likely, you already have many files on hand that you can use to explore convolution, and the collections from Spirit Canyon Audio, Virtuasonic, and others offer endless additional material for experimentation.

Convolution has vast potential for sound-design and compositional purposes and is one of the most underrated resources around. No matter what type of music you produce or which tools you use for that purpose, the files sitting on your hard drive could be just right for creating new, unique, and colorful sounds. Give convolution a try and see what types of happy accidents you come up with.

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Associate Editor Dennis Miller uses convolution in the soundtracks of many of his mixed-media works. Listen to them at www.dennismiller.neu.edu. He wishes to thank Alessandro Camnasio and Darrell Burgan for their help with this article.

Dedicated Collections

Though it's possible to match up any two files for convolving, there are several excellent collections that provide a massive number of IRs created specifically for sound-design purposes. These collections are organized in ways that make it easy to find an IR at a certain tempo or one that produces a desired quality in your sound. Note that none of these files come in split-mono format, so you'll need some software that can do the conversions for you if your host program requires it. On the Mac, Monkey Tools Sound Grinder (http://monkey-tools.com) is an inexpensive option;on the PC, several audio editors (Sound Forge and WaveLab, for example) include batch-processing features.

Spirit Canyon Audio
www.spiritcanyonaudio.com
Spirit Canyon's three collections are Kaleidoskopy ($34.99; 2,000-plus IRs), Spectral Relativity ($24.99; 850-plus IRs), and Sanitarium ($34.99; 500-plus IRs). Each collection comes both on CD-ROM and by download, and each is well organized in categories covering specific tempos and timbres (Colors, Cosmic, and Industrial, for instance). The IRs come only as WAV files and are 24-bit, 44.1 kHz (Spectral Relativity is 16-bit), with durations ranging from under a second to a few seconds. You'll find lots of audio examples and free IRs from the different collections at the manufacturer's Web site.

Virtuasonic
www.virtuasonic.com/synesthesia.htm
Virtuasonic produces a single IR library called Synaesthesis ($64.95) that consists of more than 1,000 24-bit, 44.1 kHz IRs in WAV or AIF format. The files are grouped into folders listed by letter (A, B, EF, GH, and so on), so it's a little tricky to track down a file if you have a specific purpose in mind. But given that experimentation is the name of the game with convolution, there's no reason not to start at the top and work your way down the alphabet. The manufacturer also includes a searchable HTML-based list of all the IRs, so clicking on categories such as Swirling or Metallic can point you in the right direction.

Emmanuel Deruty
www.1-1-1-1.net/pages/impulses/index.htm
Sound designer and composer Emmanuel Deruty has put together a unique set of IRs collected from his research activities and other sources. Some use Lorenz functions to generate spectra that are then synthesized as audio files for use as IRs, and others are sampled from acoustic sources such as musical instruments. The entire collection is offered as donationware.

Additional Resources

Convolutional empiricism: a comparative study of different IRs on source files

Convolution theory: more on convolution

BONUS MATERIAL
Additional Resources: Click for a list of good programs that support convolution.
Web Clips: Listen to audio examples that demonstrate the many things you can do with convolution.