In my previous article on Ableton's Convolution Reverb I gave a very rough example on how to create an impulse response. I went into a big room, and simply recorded the sound of myself clicking. This is the transient method to recording impulse responses. You record an audio file that captures the initial transient of the click, and this audio file can be pulled into the Convolution Reverb.
A better way is to use the included IR Measurement Tool. This measures the room by sending out a sweeping sine wave through a speaker. The sine wave uses an exponential sweep of the audio spectrum - going through all the frequencies in the audio range. Capturing the room this way gives you a much more accurate impulse response of the room. Let's take a look at how to set this up.
The IR Measurement Tool is a Max device that can be used to create your own IRs, which you can then pull into the Convolution Reverb. This means that you will need to have the Ableton Suite with Max For Live. The device can be located in Max for Live > Max Audio Effect. Drag this onto an Audio Track. This tool will measure the space and then you can save out the impulse as an audio file.
You'll need a speaker (for a mono file), or two (for stereo) to send out the sine wave, and you'll need a microphone to pick up the audio sine sweep signal. A good quality condenser or field recording microphone is ideal, but any microphone that picks up a good signal will work. Try setup the speaker(s) in the middle of the room. But you can really set them up anywhere you like. Walk around the room and use your ears to pick up where you think the best spot is. Clap your hands and hear the reflections and decide where you think is best. You can even record multiple times with the speaker set up in different locations. I'll leave this up to you. Now the place the microphone somewhere near to the speaker so that it can pick up the signal of the speaker when it sends out the sweeping sine wave.
Make sure you have se tup your Audio Input Device in Ableton so that it will record in the audio signal. On the audio track with the IR Measurement Tool set it to Monitor Mode to In, and Record Arm the track.
On the IR Measurement Tool, leave it set with the default values. The only thing I changed was the Chan from True Stereo to Mono to Stereo as I was only using one speaker. On the right of the plug-in, click on Input. Now press the Sweep button to send out the sweeping sine wave. The exponential sweep will go through all the frequencies. The microphone will capture this giving a very accurate impression of the impulse across the whole frequency range. A progress bar is at the bottom of the IR Measurement tool showing you the progress of the sine sweep.
When it is done, the progress bar will read '˜Measurement Done'
Now click on Save, to save out the impulse response. A dialog box will come up asking you where to save the file. Choose a location. This will save the impulse out as an audio file. Maybe save all your impulse responses in one location so that you can easily browse through them.
You can also click Trim and Normalize to trim the silence out of the IR and normalize the audio level.
If you need to you can go back and change some of the parameters such as the IR Time for longer or shorter IR lengths, and the Amp if you require more amplification, but I found the default values to be perfect for recording the impulses.
These impulse audio files can now be brought into Ableton's Convolution Reverb.
Here is my dry Snare sample.
And here is the snare sample with the impulse response of my room:
I set up the speaker and microphone in my lounge. It's a small to medium sized room with tiled floors. So I could use this impulse response if I wanted a small room with a hard surfaces sound.
Now you can build up a collection of high quality impulse responses for the Convolution Reverb. Giving you a choice of different '˜real' spaces to use in your recording. And when you invest in higher quality field recorders, microphones, and AD/DA converts you will really start getting top-notch impulse responses. But you can really start off with any gear and upgrade as you go.
This method of sending an exponential sweep of the sine wave over the frequency range can also be used with analog equipment such as tape echo and spring reverb devices. Unfortunately I wish I had an analog device such as a tape echo to test this out, but all my equipment is '˜in-the-box' now. It works in much the same way. Instead of using a speaker and a microphone, you simply connect the device to your audio system. Consult your audio interface or mixer's manual on how to connect devices such as delays or reverbs to your equipment. Then send the sine wave test to it and the impulse audio file will be created. If you want to measure any digital devices such as digital reverbs use the Impulse button instead of the Sweep button.
This method will give you a very accurate impulse response of your room or analog device. And it doesn't have to be limited to rooms and devices. You can get out into nature away from the studio and record some interesting spaces. Think of the possibilities! Tunnels, caves, canyons, etc... Gosh if there was a way to build a housing for your microphones and speaker you could go for a swim and record the sounds underwater. But I don't think sending a sine sound wave underwater is a good idea. You may attract the attention of some unhappy sea life such as sharks! So maybe stick to above water. Have fun creating these impulses and bringing them into the Convolution Reverb.