Use impulse responses in MainStage

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.

1. In MainStage, click the Sampled IR button above the main display.

2. Click the Sample IR pop-up menu and choose Load IR.

3. Choose an impulse response file from any folder.

• In MainStage, 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 lets you 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 supplied impulse responses are installed in the /Library/Audio/Impulse Responses/Apple folder. Deconvolution files have an .sdir file extension.

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

In Synthesized IR mode, Space Designer generates a synthesized impulse response based on the values of parameters such as Length, Filter, Spread, and various envelope values.

• In MainStage, 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: Click the Synthesized IR button while you’re in Sampled IR mode to switch to the synthesized impulse response stored with the setting.

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.

• In MainStage, choose one of the following from the Quality pop-up menu to set the sample rate of an impulse response:

  • 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.

• In MainStage, 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 maximum Length value 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.