14. The system of claim 11 , wherein generating models for the distribution of values within the quantiles comprises generating, for each of the quantiles, a univariate Gaussian distribution based on values in the speech data that correspond to the quantile.
11. 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: receiving, from a client device over a computer network, audio data that describes an utterance of a user; accessing stored data of an acoustic model that was generated by: accessing speech data that represents utterances of a particular phonetic unit occurring in a particular phonetic context, the speech data comprising values for multiple dimensions; determining boundaries for a set of quantiles for each of the multiple dimensions; generating, for each of the quantiles, a model that models the distribution of values within the quantile; generating a multidimensional probability function that indicates, for input speech data representing speech occurring in the particular phonetic context, a probability that the input speech data will have values that correspond to a given set of the quantiles for the multiple dimensions, wherein generating the multidimensional probability function comprises generating an n-gram model wherein the n-grams are sequences of quantile identifiers and the sequences include quantile identifiers for quantiles in at least two dimensions; storing data indicating the boundaries of the quantiles, the models for the distribution of values in the quantiles, and the multidimensional probability function; using the stored data indicating the boundaries of the quantiles, the models for the distribution of values in the quantiles, and the multidimensional probability function to determine a transcription for the utterance; and providing, to the client device and over the computer network, the transcription for the utterance.