1. A non-transitory computer storage medium encoded with a computer program, the program comprising instructions that are operable, when executed by data processing apparatus, to cause the data processing apparatus to perform operations comprising: receiving a voice search query from a user device; determining a query context associated with the voice search query; determining, based on the query context, to apply one or more language model rules; dynamically adjusting one or more probabilities in a base language model according to the applied language model rules to generate a customized language model; using the customized language model to generate a text search query from the voice search query; and providing for display on the user device, one or more search results responsive to the text search query.