Apple Impulse Response Utility User Manual

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.

The Impulse Response Utility interface is divided into four areas:

Â Hardware I/O Assignment area: Contains pop-up menus that allow you to select the

audio input and audio output.

Â Monitor and Sweep Generator section: Offers monitoring and sine sweep generation

parameters, plus buttons to deconvolve your audio tracks into an impulse response and audition your IR.

Â Inputs/Track section: Lists all audio tracks used for your impulse response recordings. Â Editing area: Displays the audio information of the currently selected audio track.

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.

Deconvolution and Impulse Response Utility

Space Designer combines, or convolves , an impulse response of an acoustic space with an audio signal. The end result is that your audio is placed “inside” the acoustic space, represented by that impulse response. Think of an impulse response as the total echoes (reflections) in a given physical space, following an initial signal spike. The impulse response file is simply an audio file recording made in a space that contains these echoes.

Making an Impulse Response Recording

There are two practical methods used for impulse response recording.

Using a Transient Recording

The first method involves the recording of an impulse in a space, using a device such as a starter pistol. The gunshot is the impulse, and the audio file you record captures both the impulse (gunshot) and the room response.

The advantage of this method is that the recorded audio file can be used in Space Designer without any further processing. You can simply use Impulse Response Utility to record your starter pistol impulses, and then combine them into a Space Designer Impulse Response (.sdir) file (and Space Designer setting file).

The disadvantage of this method is that it’s extremely difficult to make a perfect, undistorted recording of a starter pistol shot. This is due to the extremely loud nature of the initial transient of the shot. A further issue is that starter pistol shots contain very little high or bass frequency information—which, in turn, limits the usable frequency range of the convolved reverb.

Using a Broadband Sine Sweep

The second (and preferred) method for creating impulse responses involves playing a broadband audio sine sweep into a space, and recording the sweep (and space) at optimal recording levels. A sine sweep that covers the entire audible frequency range is used, resulting in a broad-range, and often higher quality, impulse response.

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 .

When you use a sine sweep, Impulse Response Utility uses a process called

deconvolution to time align and level align all recorded reflections—that are present

over the entire recorded sine sweep—into the very beginning of the file. This results in an impulse response that Space Designer can use to combine, or convolve, with your audio signal. Impulse Response Utility can then generate an .sdir setting from the impulse response.

Setting Up Your Impulse Response Recording System

Prior to using Impulse Response Utility to create an impulse response, you need to set up your recording hardware—allowing playback and recording of the sine sweep that Impulse Response Utility will generate.

Ideally, you will have:

Â One speaker for each (speaker) location required by the chosen format, if you are

going to be generating sine wave sweeps. As examples: If you are recording a true stereo impulse response, you will need two speakers; if you are recording a six channel surround impulse response, you will want six speakers, and so on. If you are not generating sine wave sweeps, you don’t need speakers to play them back (the sound is generated by the starter pistol).

Note: In many cases, it is sufficient to record one speaker position (but all

microphone positions required by the chosen impulse response format), and create a mono/omni discrete impulse response. This will result in a less CPU-intensive impulse response in the required format. See “Recording a Mono to Omni 5 Channel Impulse

Response” on page 32.

Â One or more microphones to record the resulting audio from each speaker (or your

gunshot) combined with its echo reflections. The number of microphones you will need depends on the chosen impulse response format. As examples; if you are recording a six channel surround impulse response, there are six microphone positions, so ideally you would have six microphones; if you are recording a six channel B-Format encoded impulse response—where only one microphone position and four tracks are required—you would need fewer microphones, and so on. Please see “B-Format Surround Encoding” on page 28 for more information.

Â An audio interface that offers enough outputs to send a broadband audio sine

sweep into each speaker (if using sine sweeps), and enough inputs to record the resulting audio from each position.

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

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. As you’ll only be recording one sine sweep at a time, you can set up as many speakers as you have, record them into Impulse Response Utility, then move the speakers to new positions, and repeat the process. To give you an example, imagine you only have a stereo pair of speakers, but want to create a quadraphonic impulse response, which requires the recording of four tracks in four different speaker positions. You could set up your pair of monitors, record their output, then move them to record the other pair of tracks in the other two speaker positions.

This method of creating impulse responses is obviously more time consuming, and not having all your speakers set up at once makes optimal monitor placement—relative to each other—difficult. With time and careful planning, however, you can still achieve the same results.

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Tip: You can also create multi channel impulse responses in Impulse Response Utility

through use of the Mono to Omni Discrete impulse response format. This only requires the recording of one speaker by as many “channel” microphones as needed (for the chosen format). The resulting impulse response places a lower strain on your CPU. See “Recording a Mono to Omni 5 Channel Impulse Response” on page 32.

If you have enough monitors, but not enough microphones, you can set up the required speakers, and simply move your available microphones between them. This makes optimal microphone placement—relative to other microphones—difficult, but you still have the advantage of optimal speaker placement (and let’s face it, it’s way easier to move microphones than speakers!).

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.

A more detailed discussion of speakers and microphones that are suitable for creating impulse responses, as well as suggestions on speaker and microphone placement, is found in “Microphone and Speaker Suggestions” on page 23.

Creating Impulse Responses in Impulse Response Utility

Creating an impulse response in Impulse Response Utility consists of the following steps:

Step 1: Creating a new impulse response project

Configure your Impulse Response Utility project for the number of tracks your impulse response format requires.

Step 2: Setting Impulse Response Utility parameters

Set the hardware I/O, monitor, and sweep parameters for your audio track recordings.

Step 3: Recording

Use Impulse Response Utility to record the audio tracks needed for your impulse response.

Step 4: Deconvolving your audio recordings

Assuming you have recorded sine sweeps, click Deconvolve after all audio tracks have been recorded. Impulse Response Utility will generate an impulse response from your tracks.

Step 5: Editing and Auditioning

Cut silent portions, apply fades, and so on, to optimize your impulse response.

Step 6: Creating a Space Designer setting

Click Create Space Designer setting when finished with the steps above. This adds the newly created impulse response to your list of available Space Designer settings.

The following sub-sections describe each step in detail.

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).

Impulse Response Utility offers formats ranging from conventional stereo impulse responses, true stereo (where each channel is processed discretely) and quadraphonic formats, to five, six, or seven channel surround formats. These latter formats can be in either: traditional multi-track, or B-Format encoding. Please see “B-Format Surround

Encoding” on page 28 for more information.

You can also create multi channel impulse responses in Impulse Response Utility by using the Mono to Omni Discrete impulse response format. This only requires the recording of one speaker by as many “channel” microphones as needed (for the chosen format). The resulting impulse response places a lower strain on your CPU. See “Recording a Mono to Omni 5 Channel Impulse Response” on page 32.

To create a new impulse response project:

1 Choose the desired impulse response format from the Configuration menu.

In the window below the Configuration menu, the dialog displays the number of speaker and microphone positions required by the selected format.

Click OK

This exits the New Impulse Response Project dialog, and the required number of tracks (determined by the format you chose) are shown in the Inputs/Track section.

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. The audio output selection determines the audio device used to monitor the recorded signal, and to play back the generated sine sweep.

To select an audio input device:

m In the Audio Input menu, choose the desired audio interface driver.

To select an audio output device:

m In the Audio Output menu, choose the desired audio interface driver

Changing the Sample Rate

The Hardware I/O Assignments area also displays the current sample rate. You cannot change the sample rate of your impulse response project in Impulse Response Utility. If you want to record your audio tracks at a different sample rate, you will need to change it in the Audio MIDI Setup utility, or in the software for your audio interface (if applicable). Consult the documentation for your audio interface if you’re unsure.

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.

Do one of the following:

Â Drag left (or down) to reduce the volume, or right (or up) to raise the volume. Â Type a numeric value for the monitor level (any value above 0 dB will set the monitor

level at 0 dB.)

Important: To avoid audio feedback when recording, be sure to mute the monitor

output.

To mute the monitor output:

Enable the Monitor Mute checkbox.

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