Apple’s A5 processor includes noise-reduction circuitry licensed from a start-up called Audience, and a chip analyst believes that’s why the iPhone 4 lacks the Siri voice-control system, reports “CNET.”

According to Apple, Siri — which is currently in beta testing — isn’t like traditional voice recognition software that requires you to remember keywords and speak specific commands. Siri understands your natural speech, and it asks you questions if it needs more information to complete a task.

Audience revealed details of its Apple partnership in January, when it filed paperwork ( for an initial public offering (IPO) of stock. Teardowns by iFixit ( and Chipworks ( revealed a dedicated Audience chip in the iPhone 4. However, the iPhone 4S integrates Audience’s “EarSmart” technology directly into the A5 processor, the company’s S-1 filing said.

Linley Group analyst Linley Gwennap says this explains why the A5 chip that powers the iPhone 4S is so big, notes “CNET.” Read more at .