20. The medium of claim 15 , 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: identifying a maxima in the portion of the audio input signal; generating a representation of a frequency spectrum of the audio input signal at the identified maxima; and determining that the representation matches a representation of a frequency spectrum of the audio trigger signal at a corresponding maxima in the audio trigger signal.
15. A non-transitory computer-readable medium storing software comprising instructions executable by one or more computers which, upon such execution, cause the one or more computers to perform operations comprising: obtaining an audio input signal representing an utterance of a user; 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; 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 utterance as a trigger utterance that corresponds to the keyword.