In a Uni-Q driver array the midrange cone must be free to move around the centrally mounted tweeter – typically there is a 1mm cylindrical air gap between them. Because of the pressure changes inside the speaker enclosure as the cone moves inwards and outwards, air is pushed and pulled through this air gap, and at high volume levels some noise can be generated due to turbulence. For normal sized enclosures the internal acoustic pressure is not sufficient to cause any turbulence – it only noticeable when the enclosure is very small and the required maximum output level is high – such as with the premium KHT3005 system. SST involves placing a flexible membrane between the cone and the tweeter housing, which allows freedom of movement for the cone but prevents the air-flow through the gap – thereby eliminating the source of the noise.