Impulse Response Utility User Manual
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Using Impulse Response Utility Impulse Response Utility is a multi-track audio recording and deconvolution application. It allows you to create your own mono, stereo, and surround impulse responses for Space Designer, the convolution-based reverb effect of Logic Studio. All parameters and functions that you will need to create impulse responses are contained in a single screen.
Deconvolution is the process used to create impulse responses from (recorded) broadband audio sine sweeps. The following section covers deconvolution and its role in the impulse response creation process. Further sections discuss the hardware necessary for impulse response recording, and use of Impulse Response Utility to make your own Space Designer Impulse Response (.sdir) files.
The recorded sine sweep audio file cannot directly be used as an impulse response. The recorded file contains all the echoes and reflections—in other words, the response—of the space, stretched out over the length of the sine sweep. This is very different to the starter pistol approach, where the response is contained at the beginning of the file in an impulse.
Note: You may also need additional microphone pre-amplifiers if you find that your microphones can’t capture a strong enough signal, or your audio interface doesn’t offer enough mic pre-amp inputs. Example of speakers and microphones set up to capture a 5.1 multi-channel impulse response L Ls C R Rs Surround microphone tree Audio interface Computer It is possible to make discrete multi-track impulse responses—even if you don’t have enough speakers or microphones.
If your audio interface doesn’t have enough inputs and outputs—for the number of speakers and microphones you are using to create your impulse response—you can still set up all mics and speakers. As you record different tracks into Impulse Response Utility, you will need to disconnect/reconnect the speakers and microphones, in turn.
Creating Your Project When you first open Impulse Response Utility, you will be presented with the New Impulse Response Project dialog window. This is the first step in creating a new impulse response project. You can also create a new impulse response project by choosing File > New Project (or pressing Command-N).
Setting Impulse Response Utility Parameters Once you have created a new impulse response project, you will need to assign the audio input and output device, monitor settings, and sweep generator settings. Assigning Input and Output Hardware You need to set up the audio input and output device in the Hardware I/O Assignment area before you can use Impulse Response Utility. The audio input selection determines the audio device used to record audio tracks.
Configuring the Monitor Controls Once you have recorded sine sweep tracks into Impulse Response Utility, you can monitor them through a stereo output of your choice. The monitor controls determine the monitor channel and level. You can also mute the monitor section. To select the stereo monitor output channels: m Choose the desired stereo output pair from the Monitor Channel menu. To adjust the output level of the monitored signal: 1 Click inside the Monitor Level field.
Configuring the Sweep Generator The sweep generator produces a sine wave that sweeps smoothly up from 20 Hz to 20 kHz. The sweep generator parameters set the hardware output, volume, length, reverb time, and pre-roll of the sine sweep. To set the hardware output channel for the sine sweep: m Use the Sweep Channel menu to choose the desired output channel. To set the output level of the sine sweep: 1 Click inside the Sweep Level field.
Reverb Time The Reverb Time parameter determines the maximum recording time for each track. In order to accurately capture the acoustic properties of a space, you should set a value that is long enough to capture the decay phase of all reverberations and echoes. If you are using the sweep generator, the reverb time will start when the sine sweep ends.
Recording Audio Into Impulse Response Utility Once you have selected your hardware assignments, and configured the monitor and sine sweep sections, it’s time to start recording audio tracks. Recording in Impulse Response Utility consists of three basic steps: 1 Select an audio track in the Inputs/Tracks section. 2 Configure the audio track for recording. 3 Repeat this step for each track required by the chosen format. 4 Click the Sweep/Record button to begin recording.
 Mic Position menu: Selects the mic position label for the selected track. Both Impulse Response Utility and Space Designer use these labels to identify, and process, the audio files that comprise a multi-channel stereo or surround impulse response. Impulse Response Utility automatically sets the mic position label for each track, but you can change these labels to any other position (available in the current impulse response format), if desired.  Input menu: Sets the physical input for each track.
VU Meter Impulse Response Utility offers a VU meter that measures either; the signal level present at the (soloed) track input, or the sweep generator output. During playback of recorded audio, the VU Meter displays the level of the selected track. A status message below the VU Meter shows its current operating mode. The VU Meter scale ranges from 0% to 100%, with 100% equalling 0 dB. Recording Once you have configured and record-enabled the desired tracks, click the Sweep/ Record button.
Deconvolving Your Impulse Response If you use the sweep generator, you will need to deconvolve your audio recordings—in order to make them usable as an impulse response. To deconvolve your audio tracks, do one of the following m Click the Deconvolve button. m Choose Edit > Deconvolve (or press Shift-Command-D). Following deconvolution, your audio tracks will be a single impulse response that you can audition, edit, and turn into an .sdir setting.
Editing and Auditioning Your Impulse Response Once you have recorded (or deconvolved your recorded tracks into) impulse responses, you can use the Editing area to do some “touching up” of your impulse response. You can use the Audition IR window to hear how your impulse response sounds when applied to a test audio file.
Transport Buttons The Editing area contains two transport buttons: Â Play: Starts or stops playback of the selected audio track. You can also use the Space bar to start or stop playback. Â Cycle: Engages/disengages Cycle mode. When Cycle mode is engaged (the button is darkened), playback of the selected track repeats continuously. Editing Functions Impulse Response Utility offers three editing functions, accomplished via the two editing buttons at the top left of the Editing window: Cut, Crop, and Fade.
Auditioning Your Impulse Response To really know how your impulse response will sound when used in Space Designer, you need to be able to listen to it in context. Impulse Response Utility includes an Audition window for just this purpose; where the loaded impulse response can convolve test audio files. The Audition window is divided into three sections: the Impulse Response area, the Monitor area, and the Audio Test Files area.
 Monitor Level field: Sets the overall auditioning volume.  Test Audio File buttons: Plays one of the four built-in test source audio files.  Open buttons: Click to load an audio file for use as a test source. Any uncompressed audio file can be used.  Play buttons: Click to play the selected audio file through the enabled impulse response file (or files). B-Format Impulse Responses The appearance of the Audition window changes slightly when using a B-Format impulse response.
Creating a Space Designer Setting Once you are satisfied with your impulse response, the final step is to generate a .sdir file, and a plug-in setting. When done, your newly created impulse response can be used in Space Designer. To create a Space Designer setting, do one of the following m Click the Create Space Designer Setting button. m Choose File > Create Space Designer Setting. Impulse Response Utility will generate an .
Saving and Opening Projects As mentioned earlier, Impulse Response Utility automatically saves your project as soon as you record audio. You can manually save your project at any time, however. You can also open a previously saved project, or revert the current project back to its last saved state. To save your project with its current name and location: m Choose File > Save Project (or press Command-S) to save your IR Project with its current name, in its current location.
Microphone and Speaker Suggestions The impulse response of a space is never just the space’s impulse response. The recorded impulse response is always the response of the complete system—in other words, every piece of equipment you use in the recording process. This includes, in signal-flow order: the digital-to-analog converter, the amplifier and speakers (or powered speakers), the room characteristics, the microphone(s), and finally the microphone pre-amp and analog-to-digital converter.
Below are some specific approaches for you to experiment with when creating impulse responses. Remember, these are just suggestions. There are no rules regarding how far the speakers have to be placed away from the microphone array, the best angles, ideal distances between speakers and microphones, and so on. You may want to try a number of different techniques and placements to find the ideal arrangement.
The Auralization Approach Another common strategy for placing speakers and microphones in a space uses the auralization approach. This technique attempts to capture the reflections from the performer’s perspective. Place one or more speakers at the performer’s location (or another audio source) and place the microphone at the listening position.
The Virtual Echo Chamber Approach A good alternative to the auralization approach is placement of your speakers and microphones in a way that creates a virtual space which resembles a physical echo chamber. To create this kind of impulse response, face the loudspeakers towards the corners, and the microphones towards the opposite corners. This is how microphones in a physical echo chamber would be placed. This eliminates a direct path from the speaker to the microphone, and captures more reflections.
Recording Impulse Responses From Guitar Speakers and Hardware Devices You aren’t limited to creating impulse responses from rooms and spaces. You can make an impulse response from any device that you can play a broadband sine sweep into (and record the results). Two examples are; guitar speaker cabinets and hardware processing devices, such as tube equalizers and reverb effects units.
This scenario requires neither speakers or microphones. You can simply connect the output from your audio interface to the input of your hardware device, and play the sine sweep through it. Connect the output of your hardware device to the input of your audio interface, and record it back into Impulse Response Utility. You can use this method to make impulse responses from classic hardware reverbs, multi-effects units, equalizers, and so on.
Space Designer can decode and convolve these four streams into reverbs, in all supported surround formats. Space Designer offers a comprehensive library of B-Format impulse responses that use all four streams (W, X, Y, and Z). As impulse responses do not make use of Z-axis information, Impulse Response Utility only records and encodes information from the W, X, and Y positions, saving CPU resources and disk space.
Microphone Suggestions for B-Format Recording Perhaps the ultimate B-Format recording microphones are the SoundField single point source microphone series. These microphones contain all the capsules you will need to make a complete B-Format reference recording in one pass, and include hardware to send the separate axis streams to individual Impulse Response Utility tracks. You will only need to swivel the mic once towards each speaker position, and the mic and processing hardware does the rest.
Tutorial This section will walk you through the complete process of creating an impulse response, from project start through to final Space Designer setting. For this tutorial, imagine that you have permission to record in a particularly good-sounding local concert hall. You have decided to record a discrete five channel surround impulse response, as you’ll probably never have this opportunity again.
Repeat this process twenty-three more times, until you have recorded all twenty-five audio tracks needed to capture all possible microphone and speaker position combinations. Step 5: Deconvolving Click the Deconvolve button after you have recorded (and checked) the audio files at each position. At this point, you should manually save the project. Step 6: Editing and Auditioning If you have any unwanted silence at the beginning or end of your impulse response, use the Editing area to remove it.
