Inverted Apex Surround
New surround design on the 55 and 35 series subwoofers that compresses the excursion capabilities of the surround. This design provides three major benefits.
First, increased surface area; since this surround is accordian style it is not as wide as other surrounds meaning the cone is bigger which results in more air being able to be moved which translates into greater volume displacement and higher SPL (Vd=Sd*xmax).
Second, the progressive surround works with the spider to control the woofer's excursion at high power, lowering 3rd order distortion which results in better sound.
Third, uniform motion is maintained on both the positive and negative cycles of the subwoofer, unlike subwoofers with tall, narrow surrounds that snap through on the negative cycle. Uniform motion results in consistent compliance which results in less distortion and increased sound quality.