Conference interpreters work in a soundproof booth. They receive sound from the speaker through headphones and immediately translate the speech into a microphone which is the beginning of a channel taking the sound to the audience.

http://www.youtube.com/watch?v=MA2fWvtMPDU

The interpreter listens to the speakers speech in full or in large blocks. Then, and usually with the aid of notes jotted down, the interpreter translates the whole speech.

The interpreter is close to the participant/s and translates the speakers speech in a whisper.