The surround is a component on a conventional cone driver. Cone drivers are widely used particularly for the low (20-500 Hz) and midrange (500-3000 Hz) parts of the audio spectrum. The surround provides a flexible air seal between the cone and chassis.
Clearly, the surround must be designed so that it does not impede the motion of the cone—even under large excursions. A common design of surround is the half-roll layout, as depicted in FIGS. 1 and 2. This consists of a annular flange 10, which fits around the (circular) cone and forms a bridge to the (substantially circular) aperture in the chassis into which the cone fits. A flat circular flange 12 extends around the outer circumference of the surround, and allows it to be fixed to the chassis. An inner circumferential flange 14 defines a truncated cone and substantially matches the outer rim of the cone (not shown), allowing the surround to be attached to the cone.
A “half-roll” 16 is provided between (and bridging) the inner flange 14 and the outer flange 12. This is an approximately semi-circular (in section) length of material which initially extends from the inner flange 14 away from the cone and forward of the driver before curving back towards a junction 18 with the outer flange 12. The two flange areas 12, 14 are located at approximately the same axial position. The length of rubber material around the roll shape 16 is greater than the gap 20 between the chassis edge and the cone edge; thus, as the cone moves, the increase in the gap between the cone edge and the chassis edge is accommodated by the extra material around the roll shape 16. Hence, the half roll design impedes the cone very little at low frequencies, when the cone and surround are moving in a simple manner.
The surround is commonly manufactured in a flexible material such as rubber. It is necessary for the material to have a low elasticity, so that the surround does not impede the motion of the cone. However, because of this low elasticity, the bending wavespeed in the material is typically very low. This can cause problems at mid frequencies, where the surround can resonate quite severely. As the surround is quite large in surface area—typically a significant proportion of the cone area—this surround resonance will normally radiate quite effectively. Additionally with a soft cone, such as those made from paper, polypropylene or Kevlar, where the cone is used partly in “breakup” mode, i.e. where the cone is bending in its bandwidth of usage, the surround behaviour has a great effect on the cone motion. In addition, in these modes the surround resonances commonly coincide with bending of the cone edge, which further degrades the radiated frequency response. This is partly because the mechanical impedance presented to the cone edge by the surround typically varies widely with frequency when the surround is close to resonance.
There are a number of techniques which are conventionally employed to try and avoid these issues:                careful material selection for the surround        small flat areas on the inner edge of the surround        changing the thickness of the surround material        adjusting the roll height and width on the surround        
However, none of these techniques are guaranteed to be successful in every case. None of these techniques completely eliminate the surround resonance; in the majority of cases, they operate by modifying the behaviour so as to alleviate the problem in that the resonance is not evident in the radiated sound. This approach commonly results in designs which are finely balanced, meaning that if it proves necessary to make a small change of geometry or material for other reasons, the surround resonance problems can re-emerge.