In REAPER, send the trigger track’s audio from channels 1/2 to channels 3/4 of the track being processed, then select ReaComp’s auxiliary detector. This keeps the trigger separate from the main stereo input. ReaEffects Guide

This walkthrough creates a steady synthetic tone that dips while a second tone plays. It is for a Mac desktop user who wants to understand and verify the routing without recording an instrument or microphone. Audacity supplies the tone files; REAPER performs the sidechain processing.

What should travel through each channel pair?

Give the destination track four internal audio channels. Its own tone occupies channels 1/2, while the incoming trigger occupies 3/4. REAPER’s current guide documents this multichannel sidechain arrangement and explains that track channels and hardware outputs are separately routed. You can use an ordinary stereo output for this exercise. REAPER User Guide, sections 17.1 and 17.5

Signal Destination Purpose
Steady tone Processed track, channels 1/2 Audio whose level will change
Trigger tone Processed track, channels 3/4 Control signal for compression
ReaComp stereo output Processed track, channels 1/2 Processed audio sent to the master

The distinction matters when editing the send: the source pair stays 1/2; the receiving pair becomes 3/4. Changing both ends to 3/4 would request a different source signal from the one this example creates.

Create two synthetic tone files in Audacity

Prepare these files in separate empty Audacity projects so each export contains only one tone. Open Generate → Tone, choose Sine, and enter the following illustrative settings. Audacity documents frequency, amplitude and duration as the tone generator’s controls. Tone — Audacity Manual

File name Frequency Amplitude Duration
steady-tone.wav 220 Hz 0.02 10 seconds
trigger-tone.wav 880 Hz 0.25 2 seconds

These values are chosen for diagnosis, rather than as a production preset. The steady tone has a much lower amplitude than the trigger. That lets you set a compressor threshold between their levels and later check which signal controls the detector.

Generation starts at the selection’s beginning, or at the cursor when nothing is selected. Start each tone at time zero in its empty project. Confirm the duration before applying the generator; otherwise an existing selection can put the generated sound somewhere unintended. Tone — Audacity Manual

For each project, open File → Export audio. Select WAV, choose stereo and use the same sample rate for both files, such as 44,100 Hz. Name each file as shown above. The export dialog provides format, channel and sample-rate controls; exporting the full project mixes its contents into the resulting file. Export your audio — Audacity Manual

Stereo exports make this exercise’s paired routing explicit. They do not create a stereo image from a single generated tone. Keep the two exports separate: combining the steady and trigger tones into one file would defeat the independent routing test.

Put the tones on separate REAPER tracks

Create a new REAPER project with two ordinary tracks named Steady and Trigger. Drag each WAV onto its corresponding track. REAPER’s official Quick Start documents importing by drag and drop and editing item position through Item properties. REAPER Quick Start

Place steady-tone.wav at zero seconds. Place trigger-tone.wav at two seconds, so it ends at four seconds. For exact placement, right-click each item, open Item properties, and set its position using a seconds-based time display. Keep the tracks outside folders for this example.

The timeline now provides three useful checkpoints: an initial steady-only interval, a two-second trigger interval, and a longer interval after the trigger ends. This arrangement makes a persistent reduction easier to distinguish from a response confined to the trigger.

Route the trigger to channels 3/4

Open the Steady track’s routing window using its ROUTE button. Set Track channels to 4. Keep its master send enabled, with Master send channels from/to restricted to track channels 1-2 feeding master channels 1-2. The guide explicitly documents this arrangement for a four-channel track reserving 3-4 for sidechain detection. REAPER User Guide, sections 17.13 and 17.17

Open the Trigger track’s routing window and add a send to Steady. In that send’s audio mapping, leave the source at 1/2 and change the destination to 3/4. This is the channel mapping prescribed in REAPER’s sidechaining documentation. ReaEffects Guide, Sidechaining

Use a send level of 0.00 dB. Select Pre-Fader (Post-FX) for this exercise so later changes to the trigger track’s fader do not change the detector feed. Disable the Trigger track’s Master send, leaving its send to Steady active. REAPER documents the independent send paths and the pre-fader send mode. REAPER User Guide, sections 2.21 and 17.3

This setup gives the trigger one intended destination: the auxiliary channel pair. Leave both track faders at 0.00 dB while checking the example, and leave both tracks unmuted.

Tell ReaComp to use the auxiliary detector

On Steady, open FX, add ReaComp (Cockos), and choose Auxiliary Input L + R in Detector input. The official guide identifies this selection as the detector for the signal arriving on channels 3/4. REAPER User Guide, section 17.5

Use these illustrative starting settings: Threshold at -24 dB, Ratio at 4:1, Attack at 10 ms, Release at 200 ms, and Knee size at 0 dB. Turn Auto make up off, set Wet to 0 dB, and lower Dry fully. These controls govern the compression response and output mixture. ReaEffects Guide, ReaComp

If the plug-in’s routing has previously been customized, open its pin connector using the input/output button. Check that track channels 1/2 feed the main inputs, 3/4 feed the auxiliary inputs, and the stereo outputs return to 1/2. Pin connectors determine the track channels used by a plug-in. REAPER User Guide, section 17.4

Verify the route with three controlled checks

The following is a diagnostic sequence derived from the documented signal flow. Its observations are expected behavior, not results from a performed listening test.

  1. Play across the trigger interval. Watch ReaComp’s gain-reduction display. Reduction should develop while the trigger plays between two and four seconds, then recover according to the release setting.
  2. Temporarily disable only the send. Replay the same interval. With no auxiliary feed, the trigger-related reduction should disappear. Restore the send afterward.
  3. Temporarily select the main detector. With the stated levels, the quiet steady tone should remain below the chosen threshold. Restore Auxiliary Input L + R when finished.

The amplitude choices make the third check meaningful. Calculating peak level as twenty times the base-10 logarithm of amplitude gives approximately −34 dBFS for the steady tone and −12 dBFS for the trigger. The illustrative threshold of −24 dB lies between those peaks. These are calculated signal levels, not measured REAPER meter readings; detector averaging and timing settings affect the displayed reduction.

If reduction never appears, inspect the path in order: trigger item placement, active send, source pair 1/2, receiving pair 3/4, four destination channels, auxiliary pin mapping, then detector selection. Once those agree, inspect threshold and ratio. ReaComp documents that a 1:1 ratio produces no reduction. ReaEffects Guide, ReaComp

If the trigger reaches the audible stereo output, inspect its master send and any other output routes, then check that the receive lands on 3/4. If reduction continues throughout the steady tone, repeat the detector-selection check and confirm the stated source levels.

The completed arrangement preserves the steady tone on the main stereo pair and uses the second tone solely to control its compression. When substituting another effect, confirm that it supports an auxiliary sidechain and map its inputs accordingly; extra track channels alone do not supply that plug-in feature. REAPER User Guide, sections 17.4–17.5

Sources