1. A method for recognizing a user input gesture, the method comprising: receiving, at a sound detector, ultrasound signals that are encoded with temporal information, wherein the ultrasound signals are frequency modulated with a pseudorandom noise signal; calculating a channel impulse response based on the ultrasound signals; processing the channel impulse response to extract at least one feature; comparing the extracted at least one feature to a database of channel impulse response features to identify a particular matched feature; and selecting a command to be executed by a processor based on the particular matched feature.