A spectral filtering method for tracking formants in children's speech

Children's speech is typically characterized by high fundamental frequencies (e.g., 250-600 Hz) that create widely spaced harmonic components, producing an apparent undersampling of the vocal tract transfer function. The purpose of this study is to describe a formant measurement technique based on cepstral analysis that does not require modification of the cepstrum itself or transformation back to the spectral domain. Instead, the spectrum is low-pass filtered with a cutoff point (i.e., cutoff "quefrency" in the terminology of cepstral analysis) to preserve only the spectral envelope. To test the method, speech representative of a 2 to 3 year-old child was simulated with an airway modulation model of speech production. The model includes physiologically-scaled vocal folds, vocal tract, and trachea and generates sound output analogous to a microphone signal. The true formant frequencies can be calculated independently of the output signal and thus provide test cases that allow for assessing the accuracy of the formant tracking algorithm. Formant analysis will also be applied to children's natural speech samples to demonstrate the method.