Up until the 1960’s the big deal with recording and reproducing sound was fidelity. That was the effort to make a speaker reproduce what a microphone heard as accurately as possible. Once fidelity goals started to approach what the human can hear, the fun began.
The first enhancement was to notice that humans have two ears. Two ears means we need two sound sources so stereo was added to FM radio and recordings. The Beatles recordings provide an illustrated history of the transition from monaural to stereo to the realm of multi-channel recording (see Wikipedia). This transition from consumer monaural to stereo took place in the 1960’s for the most part.
Multi channel recording was the transition from using one microphone to using many of them and then mixing the sound from each channel to achieve a one or two channel stereo recording. This was the beginning of the multi-channel audio that did not achieve broad market status until the 1990’s.
It was noticed that, despite the two ear limitation, human hearing could detect more than what two sound sources could deliver. That resulted in the four channel systems of the 1970’s. These were mostly experiments and novelties. Standards did not get well defined or accepted and recording techniques were having enough trouble getting cost effective two channel recording media with good fidelity and low noise. The digital audio developments of the 1980’s were the key to the next phase. That was illustrated by the arrival of Dolby Digital 5.1, also known as AC-3 in the movie Batman Returns in 1992.
The problem with a good sound system is that the ear is a sophisticated hearing device and sound is a complex signal. At the very basic end, this is seen by the problems of wiring polarity in stereo speaker systems. Hook one speaker up backwards and the sound would seem to come from the wrong place. Then there was the problem of microphones that heard the same sound but at different times and with different room ambiance due to their having different locations. Some efforts to solve this problem have involved putting microphones inside the ears of a dummy head but even that doesn’t work because people don’t sit still for a 3 hour concert and move their head to ‘see’ sound.
Where it is now is in the computers. The old style bass and treble controls are still there, sort of, but they have been superseded by digital audio processing that can be tuned for a particular speaker’s characteristics and its position in a particular room and its position relative to the listener. This is, in part, where there is the “.1” in 5.1 channel audio and why modern home theater speaker systems are often much smaller than they used to be.
Back in the heyday of hifi, speakers were combined units designed to produce the entire sound spectrum. They had big ‘woofer’ speakers to handle bass notes, medium sized mid-range speakers for common frequencies, and small tweeters for the high pitched sound. There were internal filters that split up the signal sent to the speaker so each of its components would get the part it was designed to handle. These were all-in-one acoustic transducer designs.
Modern speaker systems take note of the fact that there is little directionality in low frequencies. The job of producing low audio frequencies and even sounds so low you really don’t hear them but only feel them is left to the “.1” of the 5.1. This is called the low frequency effects (LFE) channel and takes care of the massive air pushing so the other speakers don’t need to be as big or as bulky. Splitting the audio signal for the frequencies needed is done in the amplifiers or even farther upstream in the recording studio when you have a multi-channel sound source.
While the 5.1 or six channel audio is becoming rather standard, there are still productions that provide additional channels for better sound environment creation and there are many viewers who have only two channel systems to play them on. This is the backwards compatibility problem. Techniques that grew out of the quadraphonic experiments of the seventies are often used to help handle this problem.
For more, see WikiPedia on loudspeakers and Dolby Stereo and a good rundown on what modern electronics is doing to improve speakers at answers,.com
The key to note is that you can use those old massive 70’s style hifi speaker systems in your modern home theater. The new generation of AV receiver can be programmed so it knows to use their woofers for the LFE channel and how much signal to feed each speaker for your seating arrangement and room acoustics. Those old speakers might not be as efficient or convenient as the new ones but modern electronics is making it much easier to accommodate for any deficiencies of individual components.