Weíll skip the evolution of stereo but stop here to discuss four speaker audio. Simply, this is having two speakers in front and two in the back. This will be referred to as a 4.0 system if the speakers all have discrete sources.
With stereo, identical sounds from both speakers will sound like they are coming from between the speakers if the sounds reach your ears at the same time. This makes sense, but we must consider that the sounds from two speakers can only reach your ears at the same time if you are more or less equidistant from the speakers.
The next evolution came with the addition of the center channel in between the front speakers. This was primarily used for movies to carry to voices of the actors. Center channels were traditionally placed near the TV screen so that actorís voices would appear to be coming from the screen. It turns out that this new channel now carried nearly 70% of all sound during a movie. Many people loved the center channel as it allowed voices to be reproduce without the interference from ambient sounds that were fed through the other speakers, others hated it because it robbed the imaging and soundstage that they had with two speakers before. One of the reasons for this is that many center channels are not as well designed and built as a main speaker. In addition, the aesthetic horizontal placement of the center channel wasnít ideal and the TV screen affected the sound.
Overall, the consensus was that the center channel was a good development. It allowed people across the entire sofa to properly place sounds in space when watching a movie and made it so people didnít have to sit directly in between the main speakers.
Around this same time, people began adding a subwoofer to their sound system. At the time, most subwoofers were actually just woofers that made up for the inadequate bass response of smaller speakers. BOSE is most famous or infamous for its design, but JBL and others had sub-sat systems in the 1970s. The reason that subwoofers have more freedom of placement is because the low frequency sounds are non-directional. This is because the wavelength of the sound is much bigger proportionally to the distance between our ears and the temporal sensitivity of our ears. Itís high school physics all over again. This is on the part of the truth however, as subwoofer placement is just as important as placement of any other speakers. Room resonances begin to play an important factor in frequencies below 80hz. A subwoofer that has a perfectly flat response in an anechoic chamber may not perform as nicely in a room. The shape of the room, the volume, the material and stiffness of the walls can all determine its resonance peaks although predicting it beforehand is difficult. Subwoofers placed in a corner sound louder because they have the reinforcement of two walls and the floor. This is similar to cupping your hands when you yell.
JBL's sub-sat system from the 70s
That said, there is not a perfect location for a subwoofer. If you want the most output, then corner placement would be the first thing to try; the problem is that the bass can become boomy and inaccurate. Placement in the middle of a room avoids wall reinforcement but can result in tepid bass. This all gets twice as complicated if you add a second sub and need to deal with interactions between the subs and the room. Best thing to do with subs is to buy a Sound Pressure Level meter and some test cdís to find places in the room without resonances or to use an equalizer to tame the room resonances.