Vowel nasalization might affect the envelop of the vowel signal by reducing the magnitude of the rising and falling slope amplitude

Electrical analog study of the vocal tract has long time ago shown that nasalization can cause a drop of 5–10 dB in the overall vowel amplitude (House and Stevens, 1956). In this preliminary study, the magnitude of rising and falling slope amplitudes from the vowel signal was introduced as a new index to discriminate oral vowels from nasalized vowels. Two speech samples of /iti/ and /iki/ were produced by two normal children and two age-matched children with cleft lip and palate and hypernasal speech. Speech samples were digitally recorded using a microphone and CSL software. The PRAAT software was used to extract the text file of the vowel segment (i.e., from the initiation of the vowel periodicity to the initiation of the constriction interval of the following plosive). Then, the text files were analyzed by MATLAB to measure amplitude rising and falling slopes of the vowel signal. Results indicated that mean falling and rising slopes of the amplitude in the nasalized vowel are smaller than those of the oral vowel. This technique might be considered as an objective index to measure vowel quality in disordered speech studies as well as speech synthesis.