There's a pic from wikipedia. Here's an explanation
The most common method is the overhung design where the height of the voice coil is greater than the magnetic gap's height. The underhung design which is used mostly in high-end speakers has the coil's height smaller than the gap's. The differences, advantages and disadvantages of both methods are given in the table below.
Overhung coil
• Coil height is greater than the gap's height.
• This method keeps the number of windings within the magnetic field (or flux) constant over the coil's normal excursion range.
• Higher coil mass, sensitivity medium to high.‡
• Soft non-linearity as the coil exceeds limits.
Underhung coil
• Gap's height is greater than the coil's height.
• This method keeps the magnetic flux that the coil experiences, constant over the coil's normal excursion range.
• Low coil mass, sensitivity low to medium.‡
• Hard non-linearity as the coil exceeds limits.
Here's the details I was able to find off wikipedia, hope it helps. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif