When you speak to someone who is using a speaker phone, or when you chat with some one else who is not using headphones, you can often get an annoying echo of your own voice. This occurs because whatever you say gets broadcast into the other person's room, bounces around, gets picked up by their mike, and then gets played back to you along with whatever the other person is saying. This feedback can get extremely annoying. Acoustic echo cancelers are clever devices that try and estimate the 'room transfer function' of someone else's room. They try to estimate, based on whatever you just said, what the echo of that will sound like as it gets fed back to you. This estimate is then subtracted from the sound being fed to your speakers/headphones so that you will only hear the other person's voice, and not the echo of your own voice.
[with VU Reddy]
Kaushik Ghose and V. Umapathi Reddy (2000) A double-talk detector for acoustic echo cancellation applications. Signal Processing, 80 (8), 1459-1467. (pdf)
Kaushik Ghose (1999) A double-talk detector for acoustic echo cancellation applications. Masters Thesis. (pdf)