Mono and Stereo audio signals have been discussed for many years and can confuse many engineers that are new to mixing. While it does seem to be an abstract subject the difference between mono and stereo (monophonic and stereophonic) signals is simple. A mono signal is an audio signal that is routed through a signal channel. While a stereo signal is sound that is reproduced using 2 or more independent audio channels.
Monophonic sounds typically lie in the center of the stereo field while stereo sounds give the impression that they surround the listener. For example, many drum sounds are recorded as a mono channel. The Kick and snare are usually panned dead center and the listener can tell that they are coming from that one specific space in the stereo field. While synths, electric guitars, brass, and strings (while they may be panned to the left or right) give the impression that they are surrounding the listener and are on both sides of the stereo field. This is why when you are mixing a stereo sound in your DAW you will have 2 pan pots, in order to control both of the stereo sounds channels. While a mono track will only have one pan pot because the sound is only coming from one space in the stereo field.
Mono and stereo audio signals are both necessary for a full well-rounded mix. You don’t want to make every instrument stereo because the mix will become cluttered and you don’t want everything mono because your mix will sound small. My approach is usually to create my drums as mono signals and create my synths, guitars, brass, strings and other instruments and stereo channels. I also will create my vocals as mono channels, although I will later add depth and width with reverb and delay effects so that my vocals are not dead center and feel small in the mix. The best way to decide if a sound should be mono or stereo is to listen to the sound and try to determine if the sound is coming from a single audio channel or many audio channels. Using this method you can balance your mono and stereo tracks in order to achieve a perfect balance in the stereo field.
Below you will see 2 mono tracks on the left side and 2 stereo tracks on the right side of the image.