A feature vector may be extracted from each frame of input digitized microphone audio data. The feature vector may include a power value for each frequency band of a plurality of frequency bands. A feature history data structure, including a plurality of feature vectors, may be formed. A normalized feature set that includes a normalized feature data structure may be produced by determining normalized power values for a plurality of frequency bands of each feature vector of the feature history data structure. A signal recognition or modification process may be based, at least in part, on the normalized feature data structure.