To make one generated tone follow another’s amplitude in REAPER, send the controlling tone to the other track’s channels 3/4, then use Audio control signal (sidechain) to modulate the receiving synth’s volume with a positive direction.

This walkthrough is for a Mac user who wants one synthetic tone to grow louder and quieter with another. You will build a two-track example, give the controlling tone a repeatable level pattern, and calibrate the response. REAPER’s current User Guide documents both audio-driven parameter modulation and the routing needed to control a parameter from another track. Up and Running: A REAPER User Guide, chapter 19

What will the second sound actually follow?

The controlling sound supplies a changing audio level. REAPER uses that level to move a parameter on the receiving track—in this example, the synth’s output volume. The receiving tone keeps its own note and sound settings. The controls for input range, modulation strength, shaping, attack, and release determine its response. REAPER User Guide, sections 19.3–19.5

Treat this as following the source’s loudness contour, rather than copying its waveform or guaranteeing identical amplitude values. A positive direction makes the controlled parameter increase as the controlling level increases. The parameter’s range and the modulation settings still affect the result. The useful target here is a repeatable rise and fall, with an adjustable quieter level and louder level.

The example uses two instances of the included ReaSynth. It accepts notes from MIDI items and provides a Volume parameter suitable for this task. You do not need a microphone, an external keyboard, or an additional instrument plug-in. The REAPER Cockos Effects Summary Guide, section 25

Create two synthetic tones with different jobs

Start with a new REAPER project and a working Mac audio output. Create two ordinary tracks outside any folder, and name them Control tone and Following tone. Open each track’s FX window, search for ReaSynth, and add one instance.

On each track, select the same short time range in the arrangement, then choose Insert → New MIDI item. Double-click each item to open the MIDI editor. Draw one note near the beginning and extend its right edge across most of the item. Use different pitches so the tracks are easy to distinguish when checking them individually. REAPER documents this MIDI-item creation and editing workflow in chapter 13. REAPER User Guide, chapter 13

Keep the receiving note sustained throughout the part you want to control. Parameter modulation changes a synth parameter; it does not supply the MIDI note needed to generate the tone. If the receiving note ends halfway through the example, increasing its volume afterward will not create a new note.

For both synths, start with Volume around -12 dB. Use a short Attack, a Sustain setting that keeps the note present, and a short Release. These controls determine how the synthetic tone starts, holds, and ends. The values are example settings, not a measured result. The REAPER Cockos Effects Summary Guide, section 25

Play each track separately to confirm that its MIDI item produces a sustained tone. Then clear any solo or mute settings before building the routing. Save the project.

Give the control tone an obvious rise and fall

Open the Control tone track’s envelope window and enable its ordinary Volume envelope. Use Trim/Read while editing the envelope manually. Add points to create a quiet section, a rising section, a louder section, and a falling section. You can use the envelope’s context menu to add points and edit their values. REAPER’s ordinary volume envelope acts after the track FX; this placement matters because the synth generates its audio in that FX chain. REAPER User Guide, sections 18.3–18.5

For an illustrative eight-second pattern, use the following envelope points. The decibel values describe changes applied by the source track’s envelope, not guaranteed readings at the modulation detector.

Time Source volume-envelope value Purpose
0 seconds -24 dB Quiet starting section
2 seconds -24 dB End of quiet section
4 seconds 0 dB End of rising section
5 seconds 0 dB End of louder section
7 seconds -24 dB End of falling section
8 seconds -24 dB Quiet ending section

This pattern gives you four useful checkpoints: the low plateau, rising slope, high plateau, and falling slope. It makes routing and response problems easier to identify than an unchanging source tone.

Route the control tone to separate channels

Open the Following tone track’s routing window and set its track channel count to 4. Add a receive from Control tone, with source audio channels 1/2 feeding destination channels 3/4. Equivalently, create that send from the source track’s routing window. This is the channel arrangement used in REAPER’s documented sidechain parameter-modulation example. REAPER User Guide, section 19.11

Set the send to Post-Fader (Post-Pan) so the source track’s changing volume reaches the destination. A Pre-Fader (Post-FX) send would bypass the source fader’s level changes, undermining this particular demonstration. Leave the send level at 0 dB initially. REAPER User Guide, section 17.3

Disable Master/parent send on Control tone if you want only the following tone in the final output. Keep its explicit send to Following tone enabled. This removes the source’s direct master contribution while retaining the route you created. Leave Master/parent send enabled on Following tone. REAPER User Guide, section 5.12

Your routing should now have these roles:

Signal Destination Job
Control tone audio, channels 1/2 Following track, channels 3/4 Supplies the changing control level
Following synth output Following track, channels 1/2 Supplies the audible generated tone
Following track’s main output Master Carries the finished result

Keep the receiving synth’s output assigned to channels 1/2. You do not need to mix channels 3/4 into the audible stereo pair. REAPER’s plug-in pin connectors determine these channel assignments; unmapped channels can pass through an FX when that option is enabled. REAPER User Guide, section 17.4

Make the receiving synth’s volume follow the source

Open ReaSynth on Following tone and click its Volume control. Then click Param and choose Parameter modulation/MIDI link. Check that the last-touched parameter shown in the menu is the volume control you intended to change. REAPER also allows selecting a parameter through the envelope window’s Mod column. REAPER User Guide, section 19.4

In the modulation window, enable parameter modulation and Audio control signal (sidechain). Select track audio channels 3/4, and choose Positive for the direction. Leave LFO and parameter linking disabled for this example so the incoming audio level supplies the movement. REAPER User Guide, sections 19.2–19.3 and 19.11

Use the following settings as an initial calibration setup. They are illustrative starting values; the next steps establish the actual working range.

Control Starting choice What to adjust afterward
Baseline A quiet receiving-synth volume Sets the lower working level
Strength 25% Adjusts the amount of volume movement
Attack 20 ms Adjusts how quickly the rise is followed
Release 150 ms Adjusts how quickly the fall is followed
Min volume -60 dB initially Refine using the incoming signal
Max volume 0 dB initially Refine using the incoming signal
Shaping graph Straight diagonal Leave unchanged during initial calibration

REAPER documents the baseline as the starting parameter value, strength as the intensity of modulation, and attack and release as response controls. Higher timing values generally produce more gradual movement. The minimum and maximum volume settings define the input-level range used for modulation. REAPER User Guide, section 19.3

Calibrate the input range before increasing the effect

Play the complete pattern and watch the modulation window’s input indication. Confirm that it rises and falls with Control tone. Adjust Min volume and Max volume to encompass the source’s useful quiet-to-loud range, allowing some margin at each end. Do not assume the source envelope’s -24 dB and 0 dB values are the detector levels: the generated signal and send level also contribute.

Next, lower the baseline until the following tone’s quiet section is suitably quiet. Raise Strength gradually until the louder section reaches the receiving volume you want. Watch the actual Volume control while doing this. If it reaches its endpoint early, reduce strength or revise the input range.

Finally, adjust timing. Reduce Attack if the receiving volume rises too late; increase it if the rise needs smoothing. Reduce Release if the following tone stays elevated too long after the source falls; increase it for a gentler descent. These adjustments follow REAPER’s documented response controls. REAPER User Guide, sections 19.3–19.5

A quiet baseline can leave some tone present during the source’s quiet sections. If your task requires complete silence between events, check the receiving parameter’s available minimum and verify that behavior separately. The setup here establishes contour following; it does not assume that every parameter mapping produces exact silence.

Diagnose the setup in signal-flow order

Use the same four-part pattern to locate problems. These checks are practical deductions from the documented routing and modulation controls, rather than reported listening-test results.

Symptom Check first Next action
No receiving tone at all MIDI note and synth output Temporarily disable modulation and confirm the receiving note plays
Tone plays, but modulation input does not move Source send and selected channels Confirm source 1/2 feeds destination 3/4, with 3/4 selected in modulation
Source fader changes do not affect the detector Send position Select Post-Fader (Post-Pan)
Input moves, but receiving volume barely changes Modulation enable, baseline, strength, and input range Adjust one control at a time while watching the target parameter
Receiving volume falls as the source rises Direction Choose Positive
Baseline changes unexpectedly during playback Automation on the receiving volume parameter Inspect that parameter’s automation envelope
Control tone is audible in the master Source master routing and destination channel mapping Disable its direct master send and check that control channels stay separate

An automation envelope on the receiving parameter overrides the modulation window’s baseline setting. Keep the source track’s volume envelope, which creates this example’s changing signal, distinct from an envelope on the receiving synth’s volume parameter. REAPER User Guide, section 19.10

If channels 3/4 disappear somewhere in a longer receiving FX chain, inspect the intervening plug-ins’ pin connections and whether unmapped channels pass through. Start with the single-synth arrangement above, then add other processing only after the basic response works. REAPER User Guide, section 17.4

Save the project once the low plateau, rising slope, high plateau, and falling slope produce the intended receiving-volume movement. You can then replace the illustrative source envelope with another synthetic level pattern while retaining the same channel routing and calibration method.

Sources