Your brain determines the direction that a sound is coming from by comparing the how far out of synch the waves reaching the left and right ears are. Since the wavelength depends on frequency this means that the lower the frequency the harder it is for the brain to determine the direction and for a human with an average width head you can't tell where anything lower than about 100 Hz is coming from.
"Wait a minute" you are saying "when I listen to my stereo I can clearly tell where the kick drum is and it's frequency is lower than that!"
You're right but that drum (or bass or any other low sound) doesn't produce a pure sine wave and it is the harmonics (multiples of the fundamental frequency that are cause the non sinusoidal wave form) in the sound that your brain determines the direction from.
Back in the 1970s a fellow named Floyd Toole (head of the physics department at the National Research Council's lab in Ottawa) was thinking about that and came up with the concept that if you played the frequencies below 100 Hz through one speaker (which could be anywhere in the room) and everything above 100 through smaller speakers your brain wouldn't notice that the fundamentals weren't coming from the same place as the harmonics. He called this a "subwoofer" and his research was the basis of the field of psychoacoustics.
Many years ago while I was working as a speaker designer I had the opportunity to spend a few hours with Floyd and shortly after I designed a subwoofer system (unfortunately it never made it to production).
In the ensuing years subwoofers have become fairly common, especially in car audio, home theatre sound systems and computer speakers.
Our late son was a bit of a hoarder when it came to electronics, often hauling home things he thought someone might need one day (and usually giving it away when he found someone who could use it). When we cleaned out our late son's apartment a few years ago there were 3 subwoofers, a Coby system (complete & with original packaging), an Altec Lansing VS4121 and a little gray one marked only "CA" (neither of which had the satellite speakers).
Because the Altec system's controls are in one of the satellite speakers I couldn't even test it without them; Maybe if I shopped long enough I might have found a set of the satellites but that would hardly be worth it when I can buy a complete system for about $50.
All I could find out about the little gray one is that CA means Cyber Acoustics; I couldn't find...
