Siri is powered by the powerhouse Apple A5 processor which is a SOC made up of dual ARM Cortex A9 processors and PowerVR SGX543MP2 GPU. Apart from that iFixit noted that it found considerable amount of vacant space, Apple being a design and form factor intensive company it would seem strange to find vacant space on a processor. Audience, a digital signal processing startup revealed details about that mysterious vacant space recently.
Audience is famous for its earSmart technology which basically helps in filtering noise from the actual audio being transmitted into the iPhone 4S. This helps with better audio clarity. Apple A5 has earSmart implement on chip level which explains the mysterious vacant space. The implementation of earSmart helps SIRI to get more clear commands which can be processed with more accuracy when compared to its predecessors.
Audience claims that Apple already licensed their next generation undisclosed technology as well which might be implemented in Apple A6 which is supposed to be powering the next generation iPad and iPhone. But we have to consider the fact that licensing a product doesn’t really necessarily mean that the technology will be implemented in a product.