8. A system comprising: one or more computers; and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to 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.
8. A system comprising: one or more computers; and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to 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. 9. The system of claim 8 , wherein classifying the utterance as a trigger utterance that corresponds to the keyword is based at least on: performing endpointing that identifies one or more potential individual words in the portion of the audio input signal; generating a transcription of at least a portion of the utterance of the user from the endpointed portion of the audio input signal based 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; determining the keyword appears in the transcription; and based on determining the keyword appears in the transcription, classifying the utterance as the trigger utterance that corresponds to the keyword.