John's Corner

Add stereo to a mono audio file

This tip covers how to add stereo effects to a mono audio file.

Listening to single channel audio sounds flat, especially in headphones. There are some simple enhancements you can apply to give this audio some life. These enhancements trick the ears into hearing a wider spatial sound and can provide a convincing stereo effect. They can be applied separately or in combination, depending on the type of audio. It is worthwhile spending some time experiementing with different combinations to get the best results.

• Add a small delay to one channel only
  This will usually provide a good stereo effect on its own with delay values in the range 5 milliseconds (msec) to 30 msec. Due to the range of frequencies in the audio in relation to the delay, this tricks the ears to spread the apparent position of voice and music across the left and right channels. A side effect is that the delay tends to make the non-delayed track sound louder, which may also require some adjustment to audio levels between the tracks. Note: 1 msec = 0.001 sec
• Add echo or reverb to both channels
  This works best with a short echo less than 50 msec. It is not strictly a stereo effect on its own but does provide ambience and doesn't change apparent loudness between channels. Can also be used in conjunction with other effects.
• Invert the phase of one channel
  This effect is best used with the other effects. With single channel delay, it can help to restore the volume balance. With echo, it can help to give a wider stereo spread.

It is also possible to apply numerous special effects which can be useful but should be used with caution as these effects can markedly change the overall sound. The recommended tool for this tip is Audacity. Note that you can undo a change using Ctrl-Z and redo it using Ctrl-Y. This can be very helpful when trying different values.

1. Start by loading your audio file into Audacity. This will allow you determine whether the audio is in a single channel or whether there are two channels with identical audio in each channel.
   
   • If the audio is in two channels, press Shift-M to open the Channel menu and select Split Stereo Track. You can now work on each channel separately.
   • Otherwise, go to the Edit menu and click Duplicate to add a second track with the same audio content. These tracks are split.
2. Add delay to one channel
   
   • Click at the start of the second track then go to the Generate menu and select Silence.
   • Start with 00h00m00.010s duration and use Preview to test.
   • Try increasing the duration in small amounts and/or inverting the phase in one channel. Click Ok to save.
3. Add echo to both channels
   
   • Press Ctrl-A to select all audio in both channels
   • Audacity allow you to add echo or reverb as separate effects. The echo settings in Audacity are very simple but are still suitable for this purpose. Go to the Effect menu and select Echo.
   • Start with a delay time of 0.030 seconds and a decay factor of 0.5. This means that the echo will follow every 30 msec after the original audio, each time reducing in amplitude by half.
   • Try increasing the duration in small amounts and/or inverting the phase in one channel. Click Ok to save.
4. Invert the phase of one channel
   
   • Select the audio in one channel only - click and hold in the space after the end of the audio, drag back to the start then release.
   • Go to the Effect menu and select Invert.
     Please note that due to a bug in Audacity, you will need to join the tracks then split them before you can hear the result.
5. Other effects
   
   • The Reverb effect allows more extensive control of echo. Go to the Effect menu and select Reverb. Refer to the Audacity manual for details on the various settings available.
   • Click Manage then Factory Presets to view the range of preset. The Default setting is a fairly neutral and is a good starting point. The Vocal I produces a softer, less noticeable reverb while Vocal II is brighter and more live. The other presets provide much more noticeable reverb and are likely to be less suitable. Of course, you can tweak any of the components independently. Click here for a broader discussion on echo and reverb.
6. Press Shift-M to open the Channel menu and select Make Stereo Trackto recombine the channels. Export to mp3 to create the final file and check in your favourite player or drop onto a web page.