B&M rocks: Try this: Cup your left hand and then press it firmly against your left ear; now slowly stop pressing in. Feel that slight pressure? There is a very weak vacuum caused by displacing the air in your ear canal. Now if more air was displaced, you'd create a stronger vacuum, one *possibly* able to rupture a weak ear drum.
The ripples or ridges allow for a constant flow of air (basically an air 'leak') that prevents the formation of a vacuum thus eliminating that possibility.
lata,
jeremy
--who actually figured that out the first time riding OL:FOF