They don't, it's just a stupid advertising gimmick.
No, it's a real thing. Actually, it's a mind game that mimics the real thing. And, as above, it has nothing to do with the speakers. The physics and psychology are very well known, and documented in both peer-reviewed scientific journals and patents.
The round hole in a guitar is a version of a "Helmholtz resonator", named after a *really* smart guy in the 1850's. His protoge was the father of experimental psychology, and performed the first laboratory-controlled investigations into acoustics and hearing perception. Modern psycho-acoustic experiments date back to the 40's, but Bob Carver brought things out of the lab in the 70's with some of his high-end stereo gear. He had a demo box at the Consumer Electronics Show with a switch, and you could select the acoustics of 4 or 5 famous opera houses around the world when playing back a record. Of course no one knew if the replicated acoustics were accurate to those houses, but there definitely were differences among the options. Bob called this "Sonic Holography".
A sound reproducing system having right and left speakers which transmit right and left stereo sounds, respectively. The right signal, in addition to driving the right speaker, is inverted and delayed to produce a compensating signal which is transmitted to the left speaker to produce a delayed...
patents.google.com
The overall effect is built on a combination of delays, frequency-shaping filters, and selective combining of the two channels. I think National Semiconductor had a chip for this, and Sanyo or Panasonic almost certainly did. I have an old, small Panasonic boom box, medium-priced at the time, that has a switch marked "Mono", Stereo", and "Stereo Wide", and the little puppy really does expand the perceived stereo image.
Separate from this is "matrix audio", where multiple physical audio channels are encoded at the source into two signals that are then decoded back into the original channels in the receiver. This is a much more complex process and is not always fully backward-compatible with standard two-channel analog stereo that Blumlein invented in 1931.
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