An Objective Critical Distance Measure Based on the Relative Level of Spectral Valley

Spectral integration is a subjective phenomenon in which a vowel with two formants, spaced below a critical distance, is perceived to be of the same phonetic quality as that of a vowel with a single formant. It is tedious to conduct perceptual tests to determine the critical distance for various experimental conditions. To alleviate this difficulty, we propose an objective critical distance (OCD) that can be determined from the spectral envelope of a speech signal. OCD is defined as that spacing between the adjacent formants when the level of the spectral valley between them reaches the mean spectral value. The measured OCD lies in the same range of 3 to 3.5 Bark as the subjective critical distance for similar experimental conditions giving credibility to the definition. However, it is noted that OCD for front vowels is significantly different from that for the back vowels.