1. An acoustic processing, device comprising: a processing unit configured to calculate one or more Hidden Markov Model (HMM) pruning thresholds; a histogram pruning unit configured to prune one or more HMM states based on the one or more pruning thresholds to generate one or more active HMM states; and a pre-pruning unit configured to prune the one or more active HMM states based on an adjustable pre-pruning threshold, wherein the adjustable pre-pruning threshold is based on the one or more pruning thresholds.