1. A method performed by data processing apparatus, the method comprising: accessing, by the data processing apparatus, a aratus speech data that represents utterances of a particular phonetic unit occurring in a particular phonetic context that comprises one or more additional phonetic units, the speech data comprising values for multiple dimensions for each of the utterances; determining, by the data processing apparatus, boundaries for a set of quantiles for each of the multiple dimensions based on the speech data that represents utterances of the particular phonetic unit occurring in the particular phonetic context; generating, by the data processing apparatus and for each of the quantiles, a model that models the distribution of values within the quantile; generating, by the data processing apparatus, 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; storing, by the data processing apparatus, data indicating the boundaries of the quantiles, the models for the distribution of values in the quantiles, and the multidimensional probability function as a portion of an acoustic model corresponding to the particular phonetic unit occurring in the particular phonetic context; and using, by the data processing apparatus, 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 perform speech recognition for an utterance.