1. A computer-implemented method comprising: obtaining, using one or more computing devices, an audio input signal representing an utterance of a user; determining, using the one or more computing devices, that a shape of at least a portion of the audio input signal matches a shape of at least a portion of an audio trigger signal corresponding to a keyword; and based at least on determining that the shape of at least the portion of the audio input signal matches the shape of at least the portion of an audio trigger signal corresponding to the keyword, classifying, the one or more computing devices, the utterance as a trigger utterance that corresponds to the keyword.
3. The method of claim 1 , wherein determining that a shape of at least a portion of the audio input signal matches a shape of at least a portion of an audio trigger signal corresponding to a keyword comprises: determining a relationship, between one or more maxima in the portion of the audio input signal and one or more minima in the portion of the audio input signal, satisfy more than one condition.