obtaining sets of audio data from different sources, wherein each of the respective sets comprises audio values with respect to time values over a respective time period;

calculating a respective normalization factor for at least four sets of the sets of audio data, wherein the audio values of the at least four sets are for a same type of audio value;

calculating a mixed normalization factor by using the at least four calculated normalization factors, wherein the mixed normalization factor is located at any point within a quadrilateral that includes the at least four calculated normalization factors;

normalizing at least two sets of the at least four sets by using the mixed normalization factor, wherein the normalized at least two sets together constitute training data; and

training an acoustic model by using the training data.