7. The apparatus of claim 1 wherein the acoustic models are characterized by a plurality of parameters, each node representing a set containing one or more of the parameters.
7. The apparatus of claim 1 wherein the acoustic models are characterized by a plurality of parameters, each node representing a set containing one or more of the parameters. 8. The apparatus of claim 7 wherein the selected level is arranged lowest of all of the levels, each node on the selected level representing a set containing a different one of the plurality of parameters.