The Haas Effect is a binaural psychoacoustical effect that causes humans to hear two separate sounds as a single one when they’re separated by a very short delay, and are of similar volumes and waveforms. Many engineers have been using this effect to widen mono sounds, or provide the illusion of wide stereo width to a mono (or a too-cantered stereo) sound. The simple trick is to delay either the left or the right channel by enough milliseconds so that your ears will start perceiving it as a stereo signal.
It’s a quick and instant satisfaction tool that works nicely on many sounds. In the example given below, we’ll take the technique a bit further, specifically for synth sounds, adding some modulation of filters and pitch so that your mono signal will sound wide and interesting.