Create Your Own Impulse Responses with Apple's Space Designer and Impulse Response Utility
When doing sound design and mixing, I almost exclusively reach for Logic's Space Designer plug-in for my reverberation and signal processing needs. Its utilization of convolution technology to map the characteristics of recorded acoustic spaces and loudspeakers (called impulse responses, or IRs) onto any audio signal adds a special organic ingredient, giving sounds character and complexity.
In fact, the more I delved into Space Designer, the more interested I became in creating and using my own IRs. While I frequently continue to turn to Logic's excellent library for its pristine recordings of concert halls and rooms, I've found that my own IR recordings of spaces and loudspeakers make excellent fodder for compositions, sound design projects, and post-production work. The right IR can make tracks sound real or even downright mysterious!
While most advanced users are knowledgeable about Space Designer's capabilities, few are aware of how to go about creating their own IRs. In this article, we'll explore how to go beyond Space Designer's stock library by rolling your own IRs with Apple's Impulse Response Utility.
Accessing the Impulse Response Utility
Let's get started by launching the Impulse Response Utility. Unlike normal installed apps, the software is part of Logic's package contents, in a folder hidden away within Logic's support files. The quickest and by far the most convenient way to open the Impulse Response Utility is from within Space Designer itself. In the Input side of the plug-in window, click the disclosure arrow immediately to the right of IR Sample (see Figure 1) and select Open IR Utility.
Creating a Speaker Impulse Response
A great way to get your feet wet is by creating an IR of an available amplified loudspeaker. The more interesting the speaker, the better, as we're looking for something that transforms and colors sound signals. (Studio monitors and home stereo speakers aren't useful for this task, as they're designed to be transparent.) My favorites are tiny, cheap, battery-powered portable speakers that distort and color the sound in interesting ways. If you get ambitious, speakers in toys can also be rigged up with a home stereo amplifier to achieve nice results.
Start by cabling the output of your audio interface (or 1/8" audio out of your MacBook or Mac Pro) to the speaker, and cabling a microphone to an available input preamp. Choice of mic will greatly affect the sound of the recorded IR, as you're capturing not only the speaker/amp, but also the entire signal chain (preamp and audio interface included). Don't worry about it too much, however, as the result we're going for is pretty lo-fi. I usually use a common dynamic mic for this application, and I like to think I'm micing up a little guitar cabinet when I'm doing this, placing the mic in close to the speaker to capture its character, as shown in Figure 2. Again, don't sweat the details of placement—remember, you're after creative results, not traditional ones!
Setting Up Impulse Response Utility
When your speaker/mic setup is complete, it's time to set up Impulse Response Utility to capture the recording. Unless you've used it before, the software will launch with a prompt asking you to select the track layout for your recording, as shown in Figure 3. (You can also access this option by selecting New Project from the File menu.)
The choices in this dialog support mono, stereo, and surround recording of IRs, whose format is dictated by the number of mics (recording) and speakers (playback) utilized. For the speaker/mic setup I've described here, Mono configuration is perfect.
After choosing the IR recording format, you need to set the appropriate audio hardware used by the software, working at the top of the window in the Hardware I/O Assignment area. Figure 4 shows my selections for this example.
Generating the Sweep
The process of creating an IR entails both exciting the source (speaker, room, device, and so on), and recording its response. I find that using a sine sweep works best for the excitation task, because you more accurately capture how the source reacts to all frequencies over time. Impulse Response Utility will both generate and remove the sine sweep signal for you (more on removal later), which is really convenient. Within the Sweep Channel flip menu, located on the left side of the interface within the Monitor and Sweep Generator section, choose the channel on your audio interface that will output the sweep signal (see Figure 5). This setting should correspond to the physical output where you have the speaker connected.
You can also change the length of time the sweep will play (10 or 50 seconds), although I've found that 10 seconds does the job nicely for speakers. The Reverb setting governs how much reverberation "tail" to allow in your recording. Since we're recording a speaker and not an acoustic space, it makes sense to capture only a minimum amount of reverberation (1 second).
Next, set the input channel for recording within the Inputs/Tracks section of the interface under the Input column for the channel (there should only be one in a mono recording), as shown in Figure 6. Your setting should correspond to the physical input where your mic is plugged into your audio interface.
Impulse Response Utility will generate test tones to aid in setting the preamp gain on your mic channel. Do this by clicking the checkbox next to "on" within the Monitor and Sweep Generator section, which will cause the speaker to output at the selected frequency (1 kHz by default). Ideally, the signal should be pretty hot, and you can gauge this result by observing the Level meter for the channel. You can also try setting the Test Tone output to 100 Hz, 5 kHz, and 10 kHz via the flip menu, to ensure that clipping won't occur at different frequencies of the sine sweep.
Recording the Impulse Response
Now that levels are set, you're ready to record the IR. In the Inputs/Track section, click the R (record) button in the same channel row where you chose the input channel and set the levels, record-enabling the channel. You'll notice that the Sweep button in the Monitor and Sweep Generator section will no longer be grayed out, so go ahead and click it!
The sweep will play back while recording the results, stopping automatically after the elapsed time you designated in the Reverb menu. After the process is finished, you'll be prompted to save the project, and you should see a waveform of your recording displayed in the Editing area of the interface. Figure 7 shows the Impulse Response as recorded, sine sweep and all.
Here's where the magic takes place. If you were to use the audio as is, you would hear the sine sweep loud and clear. (Click the play button at the top right to verify this.) In order to use it as an IR, you need to deconvolve the recording, taking out the sine sweep signal from the recording, and leaving only the speaker's response. Do this by clicking the Deconvolve button under the Process area within the Monitor and Sweep Generator section (see Figure 8).
After deconvolution, the audio waveform will look and sound drastically different, consisting of a short transient or "pop," as shown in Figure 9. (Try playing this to verify.)
Once the IR has been deconvolved, you can perform simple editing tasks to optimize its use in Space Designer. This often entails trimming "tops and tails," which you can do by selecting the area and clicking the Cut or Fade buttons shown in Figure 10. Considering that we're creating a speaker IR, it's a good idea to trim any silence at the beginning of the recording, which could produce a tiny but significant delay. (This silence might be desirable if you're working with an IR of an acoustic space.)
Using the IR Within Space Designer
Now comes the fun part: Using your newly created IR within Space Designer. Impulse Response Utility's Create Space Designer Setting feature makes this process a snap. Click this button at the bottom left of the interface to save a copy of the IR to Logic's default location (~/Music/Audio Music Apps/Impulse Responses), as shown in Figure 11. In the process, Impulse Response Utility also automatically creates a Space Designer preset for easy access from within the plug-in window.
Now you can access your work via any Space Designer plug-in preset menu (see Figure 12). When using a speaker emulation, it makes sense to do this on a Space Designer plug-in instantiated as a channel insert instead of a send effect (series instead of parallel processing). To hear the result of the entire signal sent through your "speaker," turn the Dry control to 0, adjusting the Wet control to get the appropriate gain level.
Try listening to results on an available audio track with recorded material, and observe the interesting timbral characteristics the speaker IR imparts. You can even rock out with your speaker replacing the cabinet in Amp Designer. Do this by inserting an Amp Designer plug-in first in the channel, followed by a mono Space Designer plug-in with the speaker IR loaded (again, 100% wet). Within Amp Designer, select your amp as normal through the plug-in settings menu, and defeat the matched cabinet by selecting Direct within the Cabinet flip menu (see Figure 13).
Now that you're familiar with the technique, you can apply what you've learned to other sources for creative convolution. Try setting up a studio monitor in a garage, closet, open washing machine, or even a grand piano to output the sine sweep. Capture the results with whatever mics you have handy. Afterwards, try moving up to multiple speakers and multiple mics (all accessible under the Track Layout dialog when starting a new project). You'll soon find that you have your own unique library of IRs to impart that special "something" to your recordings.