[ricard]

When erasing with an AC bulk eraser, there's a certain time factor involved, not for the actual erasure, but in order to achieve a reasonably slowly decaying magnetic field when removing the tape from the actual erase magnet, to avoid leaving the tape magnetized. It may not make a difference in a MiniDV application, but for ordinary analog tape, if the tape is magnetized, then unused sections of the tape will exhibit noise, and possibly leak in from surrounding tracks when playing back new recordings, and might also make erasure difficult, if the tape is magnetized all the way to saturation.

When using a powerful enough permanent magnet, I would think that a second or two would be enough to erase the tape, it's not like it takes any time for the field to build up.

While a bulk eraser is in principle a secondary-less transformer there is one huge difference: The laminations are opened on one side to force the magnetic field out into the air (and through the tape to be demagnetized). It's really the same design as for a magnetic head, but on a larger scale. A properly designed transformer on the other hand has as small a leakage field as possible; it is usually still detectable and can affect nearby magnetically sensitive components, but it is way to weak to erase tape.

Similarly with speakers. Although I would certainly avoid leaving my tapes near speaker cabinets, remember that the magnetic field decays rapidly with distance, so in practice I've never noted any ill effects from tapes lying around speakers. If placed on the actual permanent magnet of a speaker element I would be worried. Also, think of how many tape recorders have built-in speakers, with the tape in close proximity to the speaker elements, and no special magnetic shielding.