My name is Oscar Björk and I am the project leader and initiator of the audio and cultural heritage project: Echo of our time which is financed by Kulturbryggan in Sweden. If I'm not out on location and capturing the reverb of some historical building I work as sound/mix engineer mainly focused on the music genre rock and metal. I also play guitar and produce the music for the band Nocean.
Hi! I'm glad that you found this page and that you want to know more about it. I have worked with capturing convolution reverb of various places in Sweden such as: Warship Vasa, Uppsala Cathedral (National church), Stockholm City Hall, and The Royal Dramatic Theatre (Dramaten)
It takes some time and knowledge to create files with a good result, but it is usually much fun and you often have to be very creative. Plus the files you create can be really useful for audio related productions.
What is a Impulse Response and a Convolution Reverb
To fully understand what convolution reverb is, we first need to understand what an impulse response is and how to create and use these.
A simple example to understand an impulse response is to imagine that you are recording the bang of a balloon in a church. The bang itself (the direct sound) becomes the ”impulse” and the ”roomsound” that lingers after the bang is the ”response”
Although it is possible to create an impulse response (convolution reverb) with the "Balloon method", this approach has disadvantages, apart from the fact that you probably scare the shit out of people nearby, the files are not optimal neutral in the frequency reproduction. You usually get decent results but not much more than that.
I recommend a more developed method that provides more accurate reproduction, and you also get less background noise in the files (SNR) this method is called the exponential sweep method.
With the exponential sweep method, you use a speaker instead of something like a balloon/starting pistol. The speaker you set up plays an audio file with a sweep for som time, often about 30 to 50 seconds, where the sweep playbacks different frequencies at a time.
The sweep file that is played should preferably go from 20Hz to 20,000KHz (The entire audible frequency register for human hearing)
This audio sweep needs to be recorded with one or more microphones, and the recorded result have to be converted later in something that’s called deconvultion.
In the deconvolution process a software in the computer removes the actual sweep file that was played back and what is left are all the reflections (everything in the recording that is not the actual sweep file) This is an extra step if you compare with the "Balloon method" but is not so complicated if you have the right software and the resulting files will almost always be much superior.
Se the guide of how to perform the whole process including deconvolution here
During deconvolution, the recorded material is also compressed into a short audio file. A Impulse response file is (for the most part) converted in the standard format wav.
This impulse response wav file can be played in a regular audio player where the files will sound like a click with room sound coming after it. However to use the file in a correct way, it needs to be loaded in a convolver plugin in a professionally audio software where the files become like a filter for other audio files to pass trough.
What you need to create your own files
Do you want tips of gear to use? See this page
When you record and create impulse responses, it becomes a audio print of the following set of components:
The physical room/enviroment your record in.
2. The speaker
3. The microphones
4. The audio interface
Location and the distance between microphones and speakers will greatly affect the the result.
If you want to create an Impulse response for the purpose of using it as a convolution reverb, your goal should be to capture as much of the surrounding room/environment as possible and as little as possible the character of the technical equipment you use.
You also need some type of software to create impulse responses (convolution reverb). If you use a mac and the audio software Logic Pro, a plugin called Space Designer and Apple Impulse Response Utility (AIRU) is included, which contains everything you need to create and later use the impulse responses in Logic Pro X. Other alternative softwares is: Sparta suite, Voxengo convolver, and Waves IR-1.
A software plugin that is very popular is called Altiverb, it has a large impulse response bank and is easy to use but the downside is that it is expensive and unfortunately it has it’s own closed system. Impulse responses you record in Altiverb can not be used in other software.
And yes... of course, you also need a computer and cables to connect everything.
Other use of Impulse Reponses
It is worth mentioning that Impulse responses have become very common to use in guitar and bass amplifier modulation such as Fractal - Axe FX, Kemper, and Helix, where Impulse responses are used as saved ”micing positions" of speaker cabinets. This is done with the same technique with impulse responses but has not the same goal as convolution reverb, as you want to capture something else.
For the purpose with guitar cab Impulse responses, you always want to capture as much of the speaker and the microphone character (and often as little of the room as possible) Speakers for guitar amplifiers are "colored" in the voicing and sound with their own character and you want to capture them for this purpose. This is usually done with close micing technique. The microphones used are usually just centimeters away from the guitar speaker element.
The use for Convolution reverb IRs
I use convolution reverb on every mix I make. The advantage over classic algorithmic computer made reverb is that it sounds more natural and authentic. Algorithmic reverb on the other hand has the advantage that it’s often easier to modify after desire.
It is easy to see how this is useful in video production. Both in post-processing where you can fake the acoustics resonance of similar locations afterwards on dialogue but also to record Impulse responses to the actual recording locations in order to be able to add more of the location's resonance afterwards as an extra safety on the actual video location .
We are at a time where we more and more looking for the ultimate immersive experience. In recent years, VR has become increasingly popular and it has made it possible to use new audio formats in a broader way than before. Convolution reverb is used today in game engines. I am convinced that it will be used even more in the future.
Convolution reverb is unfortunately not something that is widely used in cultural heritage as of today. But with the project Echo of our time I hope to influence the future in this mater. Since the method enables the preservation of how a place resonates in a complementary way compared to a traditional audio recording, it is easy to see how the files could be valuable in the future.
I hope the project EOOT can be a start for taking this method more seriously and to use convolution reverb as a standard format for intangible cultural heritage in the future.
I hope that you now have deeper understanding in what convolution reverb and impulse responses are. Check out more on this blog for more extensive information and a full tutorial on how to create your own files from scratch.