Fig. 6. — The setup window for Apple Logic’s Impulse Response Utility.

Making Impulse Responses Using Apple’s Impulse Response Utility
Apple includes the Impulse Response Utility with Logic Pro X. To find it on your drive, perform a Spotlight search or Finder window search for Impulse Response Utility. Once it’s open (Fig. 6), follow these steps:

1) Select File > New (or command+N) and choose Mono (1 Speaker position, 1 Mic position). Click OK.

2) Look at the top of the IR Utility window and confirm that your audio interface is selected for Audio Input and Audio Output.

3) In the Monitor section of the IR Utility window, Mute should be checked.

4) In the Sweep section, make sure Channel is set to the output channel that will run into your guitar amp input, set the level to something like -30 dB or less. This is very important because you don’t want to pummel your amplifier’s input and cause distortion, or perhaps damage your hearing because of an excessively loud sine wave sweep.

5) Run a cable from the interface output you selected in the Sweep section to your guitar amp input, turn your amp volume all the way down, and turn on your amp.

6) In the Sweep section of IR Utility, use the pull-down menu to select Test Tone: 100 Hz, and check the On box.

7) Now, gradually turn up your amplifier until you hear the 100 Hz low-frequency test tone, which is about a low G (6th string, 3rd fret) on your guitar. Without changing the amp volume, unplug the cable from your guitar amp’s input, plug in a guitar with its volume set to full, and play a low G. The subjective volume level of the low G and the 100 Hz test tone should be about the same. If the test tone is too loud or too soft, adjust it using the Level slider in IR Utility. The idea here is to approximately match the test tone with your guitar’s signal. Remember: We don’t want the sine sweep from IR Utility to cause the amp to distort.

8) Set up a microphone as if you were recording your amp. Typically, you’ll place the mic right up on the speaker grille, but if you prefer a roomier sound, move the mic back accordingly. Connect the microphone cable to an input on your audio interface. Set the input level to avoid overloading.

9) Set the Mode to 10 seconds and the Reverb Time to 1.0 second. If you’re recording your amp in a large space or using your amp’s built-in reverb, increase the Reverb Time to 2 seconds or more.

10) Confirm that the correct input is selected in the Input column of IR Utility’s right window, then press the R button to record-enable the software. Now press the Sweep button on the left. You’ll hear a 10-second swept sine wave, which goes from rumbly lows to piercing highs. (You may want to cover your ears or wear earplugs for this.) During the sine sweep, monitor your input recording levels to make sure there is sufficient level, but that it has not overloaded during the sweep. If levels are too low or too high, make adjustments, and start the sweep again. Follow the prompt and name and save the swept sine wave that IR Utility generated.

Fig. 7. — The waveform tail being selected for trimming in IR Utility.

11) Press IR Utility’s Deconvolve button on the lower left. The “blob” of the sine sweep waveform will turn into a spikey impulse. It’s a good idea to trim off the excess of the generated impulse. You do this directly in IR Utility by clicking on the Energy tab, visually finding the approximate point that the impulse signal merges with baseline noise (the grayed right side in Fig. 7) and clicking Cut. Now trim the beginning of the impulse by highlighting the portion just prior to the waveform peak (see Fig. 8), and again click Cut.

Fig. 8. — Using IR Utility to trim the beginning of the waveform.

12) Press the Create Setting button, and name the IR file (i.e., “My amp with close mic 01”).

Fig. 9 — Logic Pro X Space Designer with an IR of a Princeton Reverb amp. Note: Dry fader is set to zero.

You have now created an impulse response profile of your amp/mic/room/reverb that is available in the pull down presets of Logic’s (or GarageBand’s) Space Designer. If you’re using Logic, put Space Designer as a plug-in on a track, call up your preset, and disable the Volume Env, Filter Env, and Output EQ buttons. Most importantly, turn the Dry signal to zero/mute, so you only hear the IR of your amp. See Fig. 9.