Max For Live’s take on convolution reverb is a versatile sound design tool.
When you want to stuff your piano into a phone booth or play your ocarina in Westminster Abbey, a convolution reverb is the tool to use. You’ll find an in‑depth look at how convolution reverbs work in Martin Walker’s article ‘Convolution Processing With Impulse Responses’ in SOS April 2005. This month’s Live column will focus on convolution as a sound‑design tool using the Max For Live Convolution Reverb Live Pack, which is included in Live 10 Suite and available to Live 9 users in the Max For Live Essentials Pack.
Convolution multiplies the frequency spectra of two audio sources. Matching peaks in the constantly changing spectra are emphasised, while a trough in either spectrum suppresses that frequency. With the Max For Live Convolution Reverb audio effect, one of the sources is the audio playing on the track on which Convolution Reverb is inserted, and the other is the Impulse‑Response (IR) audio file that is loaded into the Convolution Reverb plug‑in. IR files for reverb are created by recording a room’s response to a full‑spectrum audio impulse, a noise burst or ascending sine wave, for example. There’s a real art to crafting impulse‑response files and the Convolution Reverb Pack offers an ample supply in its IRs folder. You also get an ‘IR Measurement Device’ audio effect along with instructions for creating your own IR files. But for sound‑design purposes any audio file is fair game for the IR.
To audition Convolution Reverb, place the LD Core Kit Drum Rack from Live’s Core Library Pack on a MIDI track (see Screen 1). It is a standard eight‑pad GM kit with no reverb. Insert Convolution Reverb after the Drum Rack. Near the top of its main window you’ll see tabs labelled ‘IR’ and ‘EQ’. The EQ tab provides a three‑band pre‑reverb EQ for the input. This is critical for sculpting the sound and taming Convolution’s pitch sensitivity. Clicking the name displays the EQ graphic and clicking the square box to its left turns it on and off. The IR tab displays the current IR sample in the main window with Type and IR drop‑down menus below. You use the drop‑down menus or the left and right arrows to their right to change IR types and files. These point to files in the IRs folder of the Convolution Reverb Pack, and you can just as easily drag IRs from there into the device’s sample area. When you drag an audio file over this area a dark‑blue rectangle appears to indicate that you can then drop the file. When you drop a file the Type and IR drop‑downs update to point to that file, and you can then use the left and right buttons to step through files from that point. (Drop‑downs updating doesn’t always work for files outside the Pack.)
One advantage of drag‑and‑drop is that you can preview the IR files in Live’s browser before loading them. Once your IR file is in place, turn the Dry/Wet knob all the way up to hear just what the reverb is contributing — I like to assign the MIDI Mod Wheel to that knob. Note that the Predelay, Width, Gain and Dry/Wet knobs act in real time, whereas the Decay and Size knobs don’t take effect until you release the knob.
One interesting thing about convolution is that it is a symmetrical process: with a 100 percent Dry/Wet setting and no EQ or other effects processing, swapping the IR and source files produces the same result, although possibly at a different level. You can verify this by duplicating the track, swapping the IR and source samples on the new track, matching the two tracks’ playback levels, adding Live’s Utility effect to one of the tracks and turning on Utility’s Left and Right Phase Invert buttons. Silence ensues. For reverb, swapping the IR and source files is of little use because the IR represents a fixed space through which you want to process many different sources. But for sound design it is a very handy trick. If you want to convolve a lot of piano chords with a drum loop, use the drum loop as the IR. If you want to convolve a lot of drum loops with a piano chord, use the piano chord as the IR. For one‑off applications, keep in mind that the Dry/Wet balance and EQ process the source file, whereas Convolution Reverb Pro’s four additional effects process the IR file.
Convolution Reverb Pro adds several handy features for sound design. You can load two IR files for A/B comparison or for separate early and late reflections. You’ll also find four additional built‑in effects: Shape, Damp, Mod(ulation) and Pos(ition). The same effects settings apply to both IRs when two are loaded. Shape lets you fade the IR in and out (Fade sliders), trim the early reflections portion of the file (Early slider), reduce the length of the entire IR file (Length slider) and mix some early reflections into the rest of the IR (Cascade slider). At the bottom of the Shape window you’ll find a button to reverse the IR file and a ‘Direct Mode’ menu to trim onset transients. Reverse is always worth a listen. Of the five sliders, the Fade In, Fade Out and Length are the most useful.
For sound design with long files inserted in the IR slot, you’ll often want to radically reduce the length and set the Direct Mode to ‘No Auto Trim.’ Use fades to eliminate rough edges and unwanted transients at each end. Damp is a three‑band EQ for reshaping of the IR file. Its effect is reflected in the IR graphic as well as in the Damp window; the gray background shows the changing frequency spectrum over time after the damping EQ is applied. Both graphics update as you move the EQ handles or change the numerical settings.
The examples in Screen 2 show the setup for processing a drum loop with a short pitched‑noise sample from Live’s Core Pack (top) and for processing an orchestral strings loop with a female (A) and a male (B) voice sample. The IR graphics reflect the changes using the settings shown. For comparison I’ve displayed the original shapes at the top right of each IR graphic in Screen 2.
Adding Other Effects
Screen 3 shows two examples of combining other Live audio effects with Convolution Reverb Pro (‘CRP’ hereafter). In the first example, the processed file is a drum loop and the IR file is a short snare roll. CRP’s EQ (not shown) carves out a narrow band of the drum loop around 1kHz for processing. A Predelay setting of 40ms delays the convolution processing to add a light swing feel. CRP’s Pos(ition) effect’s Pan and Depth controls are automated using two LFOs from the Max Audio Effects folder in the Core Pack. The LFO settings cause the X‑Y controller for the Pan/Depth settings to travel counterclockwise in an oval, which pans the IR processed output while modulating its depth. The LFOs’ Phase settings control the path. Changing the Pan LFO’s Phase to 50 percent, for example, changes the rotation to clockwise. For more flexible curves, replace either or both LFOs with the Shaper Max audio effect.
In the second example an electric piano loop is processed with a two‑chain Audio Effect Rack in which the first chain is empty to provide the dry piano portion of the mix. The second Rack chain holds Live’s Ping Pong Delay set to 100 percent wet output followed by CRP. The Rack’s Chain selector is mapped to the Mod Wheel and set up to fade in the CRP chain. The CRP’s EQ carves out a narrow band of the Ping Pong Delay’s output for processing. Ping Pong Delay’s time is set to 40ms with no feedback. Increasing the feedback enhances the effect. The IR sample is a short guitar note near the middle of the electric piano clip’s pitch range, which is sculpted using CRP’s Damp section.
Mid‑sides processing is another way to use convolution for either reverb or sound design. Try CRP with or without other effects in either the mid or sides processing chain. For details on how to set up and use mid‑sides processing in Live, check out the SOS November 2015 Live column ‘Too Good To M‑S.’