I got up to look out the window, but the fire engine never went by. It sounded like it was going by, right past my home, but it really wasn't there. The hook & ladder crew was probably somewhere in England, circa 1965, when the BBC sound effects library, that the radio station was using, was recorded. How could it be so convincing that a 40-year old recording of some distant sound be so present, so real, in my own home?
The answer: stereo. That magical imitation of having two ears on opposite sides, a right one and a left one, is what separates the media today from a scratchy AM transistor radio. Stereo is twised and altered to approximate reality as much as posible, though such tricks as Dolby Surround Sound and THX full-immersion digital sound systems. Either way, they all come down to an attempt to record reality & play it back later.
Visual media hasn't progressed any further than the transistor radio, in terms of reality. 3D movies are still pale imitations of what 3D stereo is capable of. The ears must me easier to trick than the eyes, but there are still subtleties that are constantly missed by recording studios.
Sampling is a common practice today, the recording of someone else's sounds and putting it to a beat. Recently, I was listening to some early attempt at techno music, bopping along to the heavily syncopated digital beat, and suddenly a sound hit me - there was a bell, somewhere, inside my car, following along to the beat. That bell wasn't computer generated; it was a stereo sample of a real bell. The rest of the recording had been in stereo, simply by recording drums on the right track, and keyboards on the left track, but this sampled bell was recorded in true stereo, and stuck out like a jeweled tiara.
Simply recording different levels on different tracks (called 'panning') is nowhere near the real process which causes stereo sound. When something occurs off to your left, the sound doesn't just enter your left ear. First, the sound enters the left ear, certainly, but an instant later, the sound wave passes to the other side of your head, registering to the right ear as a 180 degrees out of phase, slightly delayed version of the same sound the left ear just picked up. The sound hasn't finished yet -- the sound waves expanded from the original source in all directions, and echos are created by the walls behind the source, the ceiling, the floor, all of which create new phase permutiations, new delays, new echos to be heard. It's even entirely possible an echo is heard in the right ear before any echos reach the left ear. Echos aren't the end result of a sound; the physical waves created in the air by the sound source also interact with themselves an the echos. Interaction, alteration, reflection, delay -- this complexity of echos, waves, phases, levels, and time is how we percieve the world around us in terms of sound waves.
The amazing thing is that true stereo sound, recordings made by having two side-by-side microphones in the same space, actually does record these nuances. These nuances has be reproduced with most normal stereos, too. The background sound of echos & noise is called 'ambient sound'. The delays & the phase variations picked up by the microphones are recorded, and by adjusting the phase, those sounds can come right out of a speaker again. The trick is this: to bridge the positive connectors for left & right on your stereo with one speaker, and the negative terminals with a second speaker. Note: This may cause damage to some stereos. This results in the main sounds of the left & right channels, presumably the central sounds which were recorded, to be mixed out of phase, cancelling each other. What this leaves behind are entirely audible reflections -- those delayed & phase-altered sound waves which are completely different between the left and the right channels on a recording. The two speakers bridging left & right are now the rear channels in a simple surround-sound setup.
This doesn't work all the time, though. Through a freak of electronics damage, my sister's stereo mixes the two channels, in-phase. Generally, this just sounds like monophonic sound, but when the left & right channels are distinctly different (due to panning), the difference is magnified. On a CD of mine, there is some noise in the background that sounds like digitally scrambled voice. However, when the two channels are mixed, the result is uncanny: the voice is clear as day, every word understandable. Play the CD on a normal stereo, you can't understand it. Dolby surround sound is designed to enhance that trick: since movies are not filmed in real-time, in true stereo, ambient sounds have to be added in later. It is impossible to artificially create the ambient noise of the real-world, Dolby surround mimics reality, allowing for audio producers to pan in 3 dimensions, rather than just left and right. When a rock band goes into a recording studio to cut a record, the record may be labeled 'stereo', but it was not recorded in true stereo. Each musician is sequestered in their own soundproof room, walls padded to defend against unwanted reflections, and the impersonation of stereo sound is added later. None of this creates true stereo sound; it is all designed to fool the ear into hearing something that never happenned.
When I was in high school, I read a science experiment: The task was to take a styrafoam wig-head, and install microphones where the ears should be. Wires were to be run to another room, where a blindfolded lister was to sit, headphones on, listening to the true stereo picture of the other room. I could only imagine, because we never implemented this, but the book described the results as startling. The blindfolded subject would turn their head, while listening, as though they were actually in the room. Disorientation would result, because the styrafoam head wouldn't move in concert with the listener's head - things on the left would always be on the left, regardless of the listener's movement. Sound itself isn't composed of two unrelated halves - it is all one system, interrelated & interacting with itself. There is a recording studio which specializes in jazz & folk groups. Unlike their high-budget brethren, they don't eliminate the stereo sound with the intention of fabricating it later. Albums recorded in this studio are recorded on two microphones, a left and a right. Balance & mixing is accomplished by physically moving the musicians in relation to the microphones. The recordings which result from these sessions have an unprecedented feel to them; the 'live' sound persists, giving the recording more life than a standard recording.
I've put my knowledge of this to good use: My living room stereo is an excellent example. I have two main speakers, set left & right of my TV set, for the central stereo effect. My second set of speakers are very non-traditional. A speaker style from the 60s & 70s (and is resurfacing today) is a very futuristic layout -- the speaker lays on it's back, direcing the sound upward towards a swooping cone, which acts as a reflector, directing the sound waves outward at 90 degree angles from the direction of the speaker, and spreading the sound in a 180 degree circle. This layout removes my need to cross wires on the back of my stereo - these two 'fill' speakers create an overabundance of ambient sound waves in my living room. The two main speakers give a general direction for the source of the audio, but the center is very indistinct due to the fill speakers. I generally don't notice, because artificial stereo diminishes any reality in the audio, except when a fire engine is neccessary in the background of a news story.