[Electronic M]

There are also many different forms of stereo mic placement, mixing, and listening that achieve different effects of spatial imagery to the listener.

We only have 2 ears but we can detect if a noise in the room with us comes from infront of us or behind us. We perceive this through the differential timing/phase at which the sound reaches each of our ears and the differential timing of echoes off objects around us. In the 50's and 60's there were 'Dynaural' stereo records recorded with 2 mikes in a dummy head designed to pickup the spatiality as a human would hear it, and to only accurately convey that spatial image to someone listening with headphones.

Some stereo is captured with only 2 mikes on several feet apart on stage and designed to give a console listener the effect of sitting in the audience. Some stereo is created by recording each instrument to its own mono tape track then manually mixed to each channel so the engineer/producer can paint their own soundscape by controlling the volume and L/R channel distribution of each instrument on a mixing console....Many pieces of popular music that predated stereo recording were later "rechanneled for stereo" by isolating each instrument (or family of instruments) by frequency using equalizers, then remixing the individual 'isolated' instruments to each channel to paint a soundscape electronically...This process was less accurate than creating the stereo mix live from many individual mikes or tape tracks.

In the 2 mike approach in classical music there are often back echoes off the concert hall that give a sense of spatial depth. Some consoles had built-in reverbs that could add a simulated echo to add synthetic depth. There were also boxes that effectively connected a pair of backchannel speakers in series across only the hot leads of the front channels to place the differential echoes of a concert hall behind the listener.

Another thing to consider is the ability to localize sound and the ways it echoes vary with the frequency of the sound. High frequencies are easy to localize but bass especially DEEP bass is more difficult for human hearing to localize thus modern surround systems usually have 5 tiny speakers that are only good for mid-range and treble, and direct all the bass to a single large subwoofer.

Quadraphonic sound (which died in music but survived in movies thanks to Dolby 5.1) is another fascinating topic to dig into...I could go on quite a bit on that topic as I have collected every record and decoder format and tape format sold in the US.