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The gap is the space between the coil on the cone and the magnet around it. Not the center of it.

Thanks for all the diagrams Glass! I just ordered the 7th edition of the Loudspeaker Design Cookbook. Any other reference material is helpful.

GlassWolf wrote:

Here, look at this picture to better understand where "the gap" is:

The pole piece in this diagram has a big 'ol pole vent drilled through it... so air can escape from behind the dustcap, the dustcap working double duty as an air pump, pumping air in and out of the vent, expelling hot air away from the voice coil, keeping things cool.

I mention this, only because they drew the vent SO large, it's almost hard to picture that as a "pole piece".

Anyway:

You see the "Magnet" is in a sandwich between the "front plate" and the "Back plate".

Draw an "N" on the plane of the magnet that is flat against the "front plate", and draw a "S" on the plane of the magnet that is touching the "Back plate".
That represents the magnetic polarization of the ring magnet (or magnets, for a sub with a stack of magnets).

In this way, the "front plate" becomes charged magnetically 'north'.
And, the "back plate" becomes charged magnetically 'south'.
...and, since the pole piece is physically conjoined to the back plate, it is also charged magnetically 'south'.
(the back plate / pole piece assembly is referred to as the "T-yoke", because it resembles a T - at least when it doesn't have a big pole vent drilled through it! )

You see the 'magnetically south' pole piece extends right up through the hole in the 'magnetically north' top plate.
...that creates a strong magnetic field, jumping the gap, as the pole piece and top plate try to pull themselves towards each other.

And of course, you see the "voice coil" windings are suspended right in the middle of this magnetic field - held there by the suspension, of course!
"spider" and "surround".

Hopefully that clarifies the magnetic "circuit".

This is a good one too, for a more 3-D view of a good cutaway - definitely helps visualize it better:

GlassWolf wrote:

And please just disregard this mutant... I couldn't even get a Memphis dealer to say kind words about it... I didn't even try to ask if he could possibly justify such a goofy design!

GlassWolf wrote:

solacedagony wrote:Another weird thing I was thinkign about was:
To play at 50hz, the speaker needs to be going in and out 50 times a second. Now, speakers play ranges of frequencies, can't they only be doing one thing at a time? The speaker can physically only move one speed at a time, correct?

Don't think of it this way - picture what the speaker is doing, instead, when you feed it ANY signal.

It's easiest to visualize a sine wave - so start there.
Picture a sine wave drawn on paper... usually it's drawn centered about a horizontal line, starting with a hump rising above the line to some peak, then returning back down to the center line.. passing through it, turning into an equal sized negative hump dipping down to some minimum value, rising back up to the center line (and repeating, over, and over...)

The center line is labeled 0v, the sine wave rises to some maximum voltage, and dips to some equal negative voltage.

What is the cone doing?
It's as simple to picture as this:
Erase the "maximum voltage", "0v", and "minimum voltage" marks - and replace them with "maximum positive excursion", "0mm", and "maximum negative excursion".

When the voltage rises in a positive direction, the cone moves out - breifly reaching a peak excursion level (when the input signal reaches maximum voltage, on our sine wave signal ) before starting to head back in - passing that 0mm 'at rest' position on its' way down to the maximum negative excursion level (when the input signal - our sine wave signal - reaches maximum negative voltage).

You can demonstrate this yourself - grab any subwoofer and a 9v battery from your smoke detector in your house.

When you hold the battery to the subwoofer terminals one way - with the positive battery terminal touching the positive subwoofer terminal - you are sending a positive voltage through the subwoofer, and as a result, it moves outward, to some maximum excursion level - and stays there, until you remove the voltage, at which point the cone returns to it's 'at rest' position (0v = 0mm ).
...If you turn the battery around the other way, you are sending a negative voltage through the voice coil (relatively speaking of course), and the subwoofer moves IN instead - to some maximum excursion level, until voltage is removed... and the cone moves back to 'at rest'.

When you play multiple frequencies at the same time, the result is a complex waveform - it's not a bunch of sine waves somehow all playing individually - rather, they are additive - if you stacked them all on top of each other, and added the voltage levels up at every point - you'd have a complex looking waveform, that wouldn't resemble a sine wave at all - but would still oscillate equally above and below that 0v level, it's just that it would have a complicted shape.

And, the cone motion would replicate that - when the voltage level moved in a positive direction, the cone moves that way... stays at a voltage level for a moment... cone stays put. Voltage level rises suddenly, or drops suddenly - cone motion replicates that.

Does that help?