What is the working principle of the Loudspeaker?

    An audio signal source such as a microphone or recording produces an electrical "image" of the sound. That is, it produces an electrical signal that has the same frequency and harmonic content, and a size that reflects the relative intensity of the sound as it changes. The job of the amplifier is to take that electrical image and make it larger -- large enough in power to drive the coils of a loudspeaker. Having a "high fidelity" amplifier means that you make it larger without changing any of its properties. Any changes would be perceived as distortions of the sound since the human ear is amazingly sensitive to such changes. Once the amplifier has made the electrical image large enough, it applies it to the voice coils of the loudspeaker, making them vibrate with a pattern that follows the variations of the original signal. The voice coil is attached to and drives the cone of the loudspeaker, which in turn drives the air. This action on the air produces sound that more-or-less reproduces the sound pressure variations of the original signal.
Dynamic loudspeakers are just linear motors sized and shaped to produce certain frequencies.
Planar magnetics have the voice coil in the diaphragm.
Ribbon speakers, the ribbon itself is the voice coil.
Electrostatic speakers have no voice coil or magnets. A charged diaphragm is driven between two high voltage grids.
Then there were Hill Plasmatronics........
They are all transducers, devices that translate one form of energy into another. The operating principle is transduction.

The OP's title question is answered by millercarbon and mijostyn, and blindjim is not wrong either.

I would add:
Different speaker designs exist because their designers saw different goals, and had different budgets and timelines, educations and hearing abilities.
It is easy to be impressed with the technical details in a design and the dedication of its designer. Of course, this does not validate the science behind the design. Those with the requisite education know quite well the simple math ignored in so many designs, math also not known by reviewers or editors. Those designs turn out to be ones difficult to set up, and those that cannot play certain types of music.

To keep research grounded in reality, one must state the desired goal and then always ask the right questions (and many wrong questions and figuring that out!).
When designing a new speaker, we can agree the main goal is to get that complex waveform seen on a computer to be how our eardrums move in and out. Of course, that motion will be overlaid (not obscured) with room reflections and echoes, so we pull our speakers out and make our rooms 'not lively'

To reach this goal, a good designer must describe his average listener--
  • How far away?
  • Across what range of angles?
  • In what size of room?
  • Can the speakers be pulled out and positioned?
  • What music is played, and how loud (soft)?
  • How much power in the amp?
  • How conventional must be the speaker's cabinet?
  • Budget
Then it's off to the races! The good designer would then determine which types of designs best achieve that one goal above, for that average listener. The good designer would also learn the limitations of each design, since no designs are perfect.
Of course, consumers must rely on listening, which means also training their ears to listen for details important to the music. Go to live, intimate musical events of top-notch performers. Go often as possible to help train your ears. While we all have different abilities to hear into the music, we all benefit from conscious efforts to actively listen.

Best regards,
Green Mountain Audio
Dear @ztzaudio : "  They are all transducers..."""

@millercarbon is just rigth. Speakers are like phono cartridges: both are transducers.

Regards and enjoy the MUSIC NOT DISTORTIONS,
The OP made a statement. He didn't pose a question except in the title. Seems like everyone gave good answers though.