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A little thought on how a speaker operates will tell you that the cone produces a sound wave from both sides, and by the very nature of the speaker and its operation, these two sound waves are 180 degrees out of phase, which means that they will very effectively cancel each other.If the wavelength of the soundwave is short, [less than the radius of the speaker cone] cancellation will be markedly reduced, however in the case of bass tones which have soundwaves many feet long, no bass could be produced at all unless the speaker was enclosed in a suitable cabinet to provide significant separation of these two out-of-phase sound waves.Unfortunately, the converse of this was not always true.
The audio signal is applied to a small coil (called the voice coil) of wire wrapped on an insulated tube which is an extension of the speaker cone at the center as shown. Diagram of electrodynamic speaker as used in many Hammond tone cabinets.
Here is a cross section of an electrodynamic speaker.
To make this simpler, I have left out the supporting frame or "basket" which holds everything in correct alignment and the speaker cone center suspension.
This effectively cancels the action of the slight field current ripple voltage and eliminates the otherwise noticeable resulting AC power line humming from the speaker.
In the newer PM speakers, there is no ripple in the field of a permanent magnet and thus no need for a hum-bucking coil.
When current flows in one direction through the voice coil, it creates a magnetic field which reacts with the field in the gap and makes the cone move slightly to the right.