2. A prefiltering method as described in claim 1 wherein: said acoustic description of the utterance to be recognized includes, in addition to said succession of acoustic descriptions which are compared against the succession of acoustic models of said wordstart cluster models, one or more additional acoustic descriptions which represent a portion of the utterance occurring after that represented by said succession of acoustic descriptions; said method includes storing additional acoustic description cluster models, which represent sounds occurring in vocabulary words after the sounds represented by wordstart cluster models; said using of the cluster likelihood scores to produce prefilter scores includes using the cluster likelihood score produced for each wordstart cluster model to produce an intial prefilter score for words represented by that wordstart cluster model; said method further includes the following extra prefilter scoring steps for each of a plurality of vocabulary words: comparing one or more of said additional acoustic descriptions from the utterance to be recognized against one or more additional acoustic description cluster models representing sounds of that word, and producing an additional score as a result of that comparison; combining said additional score for a word with its initial prefilter score to produce a combined prefilter score for that word; and said selecting of said subset of vocabulary words to undergo more lengthy comparison is based on the combined prefilter scores associated with the vocabulary words.
2. A prefiltering method as described in claim 1 wherein: said acoustic description of the utterance to be recognized includes, in addition to said succession of acoustic descriptions which are compared against the succession of acoustic models of said wordstart cluster models, one or more additional acoustic descriptions which represent a portion of the utterance occurring after that represented by said succession of acoustic descriptions; said method includes storing additional acoustic description cluster models, which represent sounds occurring in vocabulary words after the sounds represented by wordstart cluster models; said using of the cluster likelihood scores to produce prefilter scores includes using the cluster likelihood score produced for each wordstart cluster model to produce an intial prefilter score for words represented by that wordstart cluster model; said method further includes the following extra prefilter scoring steps for each of a plurality of vocabulary words: comparing one or more of said additional acoustic descriptions from the utterance to be recognized against one or more additional acoustic description cluster models representing sounds of that word, and producing an additional score as a result of that comparison; combining said additional score for a word with its initial prefilter score to produce a combined prefilter score for that word; and said selecting of said subset of vocabulary words to undergo more lengthy comparison is based on the combined prefilter scores associated with the vocabulary words. 3. A prefilter method as described in claim 2 further including pruning those vocabulary words represented by the wordstart cluster models having the worst cluster likelihood scores from further consideration, including preventing them from having said exta prefilter scoring steps performed for them.