The overall slope of long-term-average spectrum (LTAS) decreases if vocal loudness increases. Therefore, changes of vocal loudness also affects the a measure, defined as the ratio of spectrum intensity above and below 1000 Hz. The effect on a of loudness variation was analyzed in 15 male and 16 female voices reading a text at different degrees of vocal loudness. The mean range of equivalent sound level (Le) amounted to about 28 dB and the mean range of a to 19.0 and 11.7 dB for the female and male subjects. The L-eq vs. alpha relationship could be approximated with a quadratic function, or by a linear equation, if softest phonation was excluded. Using such equations a was computed for all values of L-eq observed for each subject and compared with observed values. The maximum and the mean absolute errors were 2.4 dB and between 0.1 and 0.6 dB. When softest phonation was disregarded and linear equations were used, the maximum error was less than 2 dB and the mean absolute errors were between 0.2 and 0.7 dB. The strong correlation between L-eq and a indicates that for a voice L-eq can be used for predicting alpha. (c) 2006 Acoustical Society of America.
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