It turns out that Apple's Siri voice assistant is enabled by special noise reduction hardware called earSmart, wired right into the Apple A5 processor. The technology is made by Audience, which has an earSmart-less chip in the iPhone 4. The difference is, the iPhone 4 chip was designed to reduce noise when speaking directly into the microphone (like when you're on a phone call), while the chip in the iPhone 4S is designed for a wider field of sound, so you could talk to your phone at arm's-length (as one tends to with Siri) and still benefit from noise cancellation. The bundling of earSmart with the processor explains why the A5 chip was so much bigger than expected, even after taking into account the second core. It also means that, if you're still desperately clinging to the hope that Apple will someday update the iPhone 4 with Siri functionality, now's the time to let go.