Logic Pro X: Use impulse responses

Use impulse responses

Space Designer can use either recorded impulse response files or synthesized impulse responses. The Sampled IR and Synthesized IR buttons above the main display switch the impulse response mode.

Important: To convolve audio in real time, Space Designer must first calculate any parameter adjustments to the impulse response. This requires a moment or two following parameter edits and is indicated by waveform changes in the main display.

Figure. Sampled IR mode, showing full interface.

Impulse response parameters

  • Sampled IR button: Switch to Sampled IR mode. In this mode, an impulse response sample is used to generate reverberation.

    When you first click the Sampled IR button, a Load impulse response window opens. Choose an impulse response file from any folder.

  • Synthesized IR button: Turn on Synthesized IR mode. A new synthesized impulse response is generated, derived from the values of the Length, Envelope, Filter, EQ, and Spread parameters.

  • IR Sample pop-up menu: Shows the name of the loaded impulse response. Click to choose an IR sample command. The format of the impulse response is shown to the right of the pop-up menu. Also see Space Designer global parameters overview.

    • Load IR: Loads an impulse response sample without changing the envelopes.

    • Load IR & Init: Loads an impulse response sample and initializes all envelopes.

    • Show in Finder: Opens a Finder window that shows the location of the current impulse response.

    • Open IR Utility: Opens the Impulse Response Utility window. This application enables you to create your own impulse response files.

Several related parameters are shown below the main display.

  • Quality pop-up menu: Choose the sample rate. Lo-Fi produces a grainy reverb. Low halves the host application sample rate. Medium matches the host application sample rate. High is smooth and clean sounding.

  • Reverse button: Reverse the impulse response and envelopes. When the impulse response is reversed, you are effectively using the tail rather than the front end of the sample. You may need to adjust the Predelay and other parameter values when reversing. See Space Designer global parameters overview.

  • Length knob and field: Adjust the length of the impulse response. This control works in conjunction with the Size knob.

  • Size knob and field: Adjust the sample rate of the loaded impulse response file, thereby changing the perceived size of the reverb by widening or narrowing the room. Size can also be used to preserve the original length of the impulse response when changing the sample rate with the Quality pop-up menu.

    The Size knob value has an impact on the decay because it is multiplied with the Length knob value. To explain, a Length knob value of 100% and a Size knob value of 100% result in a decay that is the full length of the loaded impulse response.

Use Sampled IR mode

In Sampled IR mode, Space Designer loads and uses an impulse response recording of an acoustic environment. This is convolved with the incoming audio signal to place it in the acoustic space provided by the impulse response.

Figure. IR Sample pop-up menu and Display mode bar buttons.
  1. Click the Sampled IR button above the main display.

    When you first click the Sampled IR button, a Load impulse response window opens.

  2. Choose an impulse response file from any folder.

Manage the loaded impulse response file

  • Click the IR Sample pop-up menu to use any of the following commands:

    • Load IR: Loads an impulse response sample without changing the envelopes.

    • Load IR & Init: Loads an impulse response sample and initializes the envelopes.

    • Show in Finder: Opens a Finder window that shows the location of the current impulse response.

    • Open IR Utility: Opens the Impulse Response Utility window.

      This application enables you to create your own impulse response files. It provides recording, editing, and processing facilities designed specifically for this task. See Impulse Response Utility Help in the Impulse Response Utility application for details.

All factory impulse responses are installed in the /Library/Audio/Impulse Responses/Apple folder. Deconvolution files have an .sdir file extension.

Any mono or stereo AIFF, SDII, or WAV file can be used as an impulse response. In addition, surround formats up to 7.1, discreet audio files, and B-format audio files that consist of a single surround impulse response can be used.

Use Synthesized IR mode

In Synthesized IR mode, Space Designer generates a synthesized impulse response based on the values of the Length, Envelope, Filter, EQ, and Spread parameters.

Note: You can switch between a loaded impulse response sample and a synthesized impulse response without losing the settings of the other.

Figure. Synthesized IR Display mode bar.
  • Click the Synthesized IR button above the main display.

    Repeated clicks of the Synthesized IR button randomly generate new impulse responses with slightly different reflection patterns. The current impulse response state is saved with the setting file, including parameter and other values that represent the IR reflection patterns and characteristics.

    Note: Clicking the Synthesized IR button while you’re in Sampled IR mode switches to the synthesized impulse response stored with the setting.

Set the impulse response sample rate and preserve length

Changing the sample rate upward increases—or changing it downward decreases—the frequency response (and length) of the impulse response, and to a degree the overall sound quality of the reverb. Upward sample rate changes are of benefit only if the original impulse response sample actually contains higher frequencies. When reducing the sample rate, use your ears to decide if the sonic quality meets your needs.

Note: Natural room surfaces—except concrete and tiles—tend to have minimal reflections in higher frequency ranges, making half-rate and full-rate impulse responses sound almost identical.

Impulse response parameters in the global controls section.
  • To set the sample rate of an impulse response: Choose one of the following from the Quality pop-up menu:

    • Lo-Fi: This setting divides the sample rate by four. If the project sample rate is 96 kHz, the impulse response sample rate is converted to 24 kHz. If the project sample rate is 44.1 kHz, the impulse response sample rate is converted to 11.025 kHz, and so on.

    • Low: This setting effectively halves the sample rate. If the project sample rate is 96 kHz, the impulse response sample rate is converted to 48 kHz. If the project sample rate is 44.1 kHz, the impulse response sample rate is converted to 22.05 kHz, and so on.

      When you select a half sample rate, the impulse response becomes twice as long. The highest frequency that can be reverberated is halved. This results in a behavior that is much like doubling every dimension of a virtual room—multiplying the volume of a room by eight. The Low (and Lo-Fi) setting can also be used for interesting tempo, pitch, and retro digital effects. Another benefit of reducing the sample rate is that processing requirements drop significantly, making the lower quality settings useful for large, open spaces.

      This behavior also applies when you choose Lo-Fi, but the sample rate is divided by four and the impulse response is multiplied in length four times.

    • Medium: Space Designer uses the current project sample rate. The sample rate of a loaded impulse response is automatically converted to match the current project sample rate, if necessary. For example, this allows you to load a 44.1 kHz impulse response into a project running at 96 kHz, and vice versa.

    • High: Space Designer uses the highest possible sample rate.

  • To retain the original length of the impulse response when the sample rate is changed: Adjust the Size knob value. Using this parameter with your Quality pop-up menu choice can lead to interesting results.

    If you’re running Space Designer in a project that uses a higher sample rate than the impulse response, you may also want to reduce the impulse response sample rate. Adjust the Size knob value to reduce CPU processing time without compromising reverb quality.

    Tip: You can make similar adjustments while running in Synthesized IR mode. Most typical reverb sounds don’t contain an excessive amount of high frequency content. If your project is running at 96 kHz, for example, you would need to use lowpass filtering to obtain the mellow frequency response characteristics of many reverb sounds. A better approach would be to first reduce the high frequencies by choosing a lower rate from the Quality pop-up menu, followed by using the lowpass filter, thus conserving significant CPU resources. It is also worth noting that longer impulse responses (sampled or synthesized) place a higher strain on the CPU.

Set impulse response lengths

Impulse response parameters in the global controls section.
  • Rotate the Length knob to set the length of the impulse response—sampled or synthesized.

    The Length knob setting changes the decay value, depending on the current Size knob value. To clarify, a Length value of 100% and a Size value of 100% result in a decay that is the full length of the loaded impulse response.

    All envelopes are automatically calculated as a percentage of the overall length. Changes to the Length or Size parameter values result in envelope curves stretching or shrinking to fit the new impulse response length.

    Note: When you’re using a sampled impulse response file, the combined Length (and Size) parameter values cannot exceed the length of the underlying impulse response sample.

Published Date: Aug 9, 2019
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