Capturing Impulse Responses of Spaces
For a comprehensive overview, please refer to the supplementary material.
- A laptop with an IR utility software installed
- An audio interface
- Microphones (preferably omni-directional condensers)
- Speaker/s (if you are using a sweep)
Things to consider:
- Watch signal to noise ratio while recording and adjust the preamp gains with care
- Play the sweep at a decent level but watch for unwanted vibrations
- Allow some headroom (around 4 – 6 dB)
- A stereo recording technique (A-B) is a common method but you can use any number of channels
- Experiment with distance and try to capture a variety of perspectives from a single space/object
- Stay quiet while recording!
Capturing Impulse Responses in Reaper (ReaVerb)
Reaper offers a variety of tools for convolution. The process of capturing IRs is fairly straightforward and can be broken down into three steps. We will look at how to do this using the ReaVerb plug-in:
Step 1 : Create the tracks and generate a test tone
Create three new audio tracks. Name them IR, test tone and demo respectively. Remember to follow the naming convention and include all the necessary details in the final files. Naming the tracks in this way can help you avoid confusion and speed up the rendering process. Add the ReaVerb plug in as an effect onto the IR track. Click the ‘add’ button and choose ‘File’. This is where you would open your IRs for processing sounds, but you can hit cancel since we haven’t generated them yet. This action opens up a new panel on the right, where you can generate test tones and deconvolve recordings. Click on ‘Generate test tone’ and choose a desired duration based on your location/object and save it to your project’s directory. It’s important that the sample rate of your test tone is the same as your recording. Now you have a test tone that you can play back from the speakers. Drag the file onto to the test tone track you generated earlier.
Step 2 : Record the test tone using the microphones
Record enable your track and choose the correct hardware inputs. Make sure you aim for a healthy signal with minimum amount of noise. If the space is busy, you’ll have to choose a quieter time or work around it somehow. Stay silent during the recording. You generally want your recorded file to the the same length as the test tone. While you can edit the file later, a better approach is to create a time selection around your item (in this case test tone) and modify the transport control so that it limits the recording to that time selection. You can create a time selection around a media item by adding a keyboard shortcut from the action menu. Just open the ‘Actions’ menu and search for ’set time selection to items’ and add your desired shortcut. Once you create a time selection around your test tone, you have to right click on the record button from the transport bar and choose ‘time selection auto punch’. You are ready to record the test tone in your space! It’s a good idea to render the track from the time selection at this point rather than using the recorded file.
Step 3 : Deconvolve
Now that you have a recording of the sine sweep in the space, you need to deconvolve it. Add ReaVerb to your demo track and choose ‘Add -> File’. This time click on ‘Deconvolve’. You’ll be presented with a window that will ask for the location of two files. The order matters here. The first one is the recorded sweep and the second one is your test tone. Make sure you use the correct test tone for the correct IR if you are using different test tones with varied lengths. After you choose the corresponding files, you’ll have to name your IR and save it. That’s it, you’ve captured your IR! This will also add the IR to ReaVerb for instant convolution.
Note that if you are using the transient method, you can import the IRs directly into ReaVerb after editing.