To give a generated mono sound stereo width in Audacity, combine two identical copies into a stereo track, then add stereo reverb. Duplication supplies the channels; the reverb supplies differences between them. This walkthrough uses a synthetic tone and includes a mono playback check.

Why copying mono into both channels does not create width

An identical signal in the left and right channels is often called dual mono. It has two channels, but no difference between them. Audacity’s documentation confirms that a mono track already plays through both speakers; converting its channel format alone offers no general audio benefit. FAQ:Editing

For this task, the useful distinction is between the original tone and its added ambience. Keep the original equally represented on both sides, then let the reflections differ. Audacity explicitly documents this approach: duplicate a mono track, join the copies into stereo, and apply reverb with stereo width enabled. Its width control operates on stereo tracks only. Reverb

The result is a stereo treatment of your original generated sound. Its spatial character comes from processing that sound, with no additional recording required.

Set up the Mac project and generate the original tone

Audacity’s official Mac download page currently lists version 4.0.1. The steps below use its newer menu terminology. The download page lists testing on macOS 14 and 15; compatibility with other releases is qualified rather than guaranteed. Download for macOS

Generate one mono source

Start an empty project and choose Tracks → New mono track. This creates an empty single-channel track. Tracks

Select that track, place the cursor at the beginning, and open Generate → Tone. Enter these illustrative values:

Control Value
Waveform Triangle
Frequency 440 Hz
Amplitude (0-1) 0.2
Duration 3 seconds

Generate the tone. These values are a repeatable starting point, not an Audacity preset or a tested optimum. The generator supports the selected waveform, frequency, amplitude and duration controls; generation begins at the selection start or cursor. An amplitude of 1 represents full scale, so 0.2 leaves room for subsequent processing. Tone

Leave space for the reverb tail

Place the cursor exactly at the tone’s end, with no time range selected. Choose Generate → Silence and enter 3 seconds. The intended timeline now contains the original tone from 0–3 seconds and silence from 3–6 seconds. Silence generation starts at the cursor when nothing is selected. Silence

This padding gives the reflections somewhere to decay after the tone stops. Audacity’s reverb instructions specifically recommend adding silence at the end before processing. Three seconds is an example allowance; if your chosen settings still produce an audible tail at the endpoint, extend it before rendering again. Audacity’s reverb walkthrough

Turn the mono source into a stereo processing track

Select the source track and choose Tracks → Duplicate. Copies are independent, so later processing can change one without changing the other. Tracks

Select only the upper mono track. Open its track menu beside the name and choose Make stereo track. This command requires exactly one selected track and another mono track directly underneath. You should now have one stereo track displaying two stacked waveforms. Audio Track Item

At this point, the two channels still contain identical audio. Treat this as the preparation stage. To preserve a comparison version, duplicate the new stereo track and mute one copy. Keep only one version audible at a time during comparisons.

Add stereo reflections while retaining the original tone

For an adjustable workflow, select the working stereo track, open Effects, choose Add effect, and add Reverb. Audacity 4 supports reverb as a realtime effect: it processes playback without rewriting the source, provides bypass controls, and includes the processing in an export. Realtime Effects

Use the following illustrative settings. They define a starting experiment, not a promised listening result:

Control Starting value
Room size 30%
Stereo width 80%
Pre-delay 10 ms
Damping 50%
Reverberance 30%
Low tone 100%
High tone 100%
Wet gain −12 dB
Dry gain −3 dB
Wet only Off

These values fall within the documented ranges. Stereo width controls the spread of reflections, Wet gain controls their level, and Dry gain controls the original. Leaving Wet only off retains both components. Reverb controls

Play the full six-second timeline, including the padded ending. Compare with the effect bypassed at roughly similar playback levels. Make one adjustment at a time: first decide how much ambience belongs around the tone, then adjust its spread. A louder processed version is not sufficient evidence that the spatial treatment is useful.

Use this decision aid to keep those choices separate:

What you want to change First adjustment
Less prominent ambience Lower Wet gain
A narrower spread of reflections Lower Stereo width
A shorter decay Lower Reverberance
More separation before reflections begin Increase Pre-delay slightly
The original tone restored after disappearing Turn Wet only off

This sequence follows the documented function of each control. Width and wet level answer different questions: one concerns the reflections’ stereo spread, while the other concerns their contribution to the result. Reverb controls

Why a fixed delay can behave strangely on synthetic tones

Delaying one channel is another way to introduce a difference, but a steady periodic tone makes the result highly dependent on frequency. A delay measured in milliseconds can equal a whole number of waveform cycles—or a half-cycle offset.

Consider an ideal sine wave with equal channel amplitudes and a 10 ms delay on the right. At 500 Hz, the delay equals five complete cycles, so the steady portions line up again. At 450 Hz, it equals 4.5 cycles, putting the steady portions in opposite polarity. Averaging those channels into mono cancels that ideal 450 Hz steady component.

For this illustrative calculation, the mono amplitude relative to the original is abs(cos(π × frequency × delay)), with frequency in hertz and delay in seconds. Substituting 500 and 450 Hz gives 1 and 0 respectively. These arithmetic results were checked with Python; they are not listening-test results. They assume ideal sine waves and exclude the onset and ending.

Audacity describes a sine as containing energy at one frequency. Its other generated waveforms have different spectral content, so the exact single-frequency cancellation example should not be applied wholesale to the triangle tone above. Tone

The practical inference is that a fixed delay needs checking against the actual generated sound. Stereo reverb provides a documented alternative when the desired result is ambience around a retained original tone.

Check mono playback and export the complete result

Save the project, then make a separate copy for the mono check using File → Save as. Exporting and saving serve different purposes: saving preserves the editable project, while exporting renders an ordinary audio file. File

In the check copy, remove the unused comparison track, select the processed track, and use Tracks → Mix-down to… to create a mono result. Mixing down incorporates the current volume, pan and effect settings and replaces the selected tracks. Keep the stereo master separately so this check does not replace your working version. Tracks

Compare the mono result with the stereo version. The spatial spread will change; the useful question is whether the tone’s level, character and ending remain suitable for your task. If the change is excessive, return to the stereo master and reduce the wet contribution before checking again.

Also watch the level meters. Audacity’s track meters retain a clipping indication until cleared. Reverb can increase level, so the example’s starting amplitude and gain settings do not replace a check of the actual output. Audio Track Item, Reverb

Return to the stereo master, leave only the intended version audible, and choose File → Export audio. Use Export full project audio to include the padded ending; an export limited to a shorter selection can omit it. Exporting creates a rendered copy while retaining the editable project. File

Your finished treatment should have a clear purpose: the generated tone remains recognizable, the added reflections supply stereo differences, and the ending has room to decay. Keep the project alongside the export so you can revise the balance when the sound is used in a larger mix.

Sources