The clinical utility of the soft phonation index

This paper investigates the applicability of the soft phonation index (SPI) as an indicator of vocal fold adduction. The SPI is one of 33 acoustic parameters calculated and displayed from a single vocalization by the multi-dimensional voice program (MDVP). A review of pertinent literature revealed that while increased values of SPI are presumed to be associated with incomplete adduction of the vocal folds, results should be interpreted cautiously as the index has not been sufficiently studied. The purpose of this pilot study was to determine how consistently SPI tracks systematic changes in vocal fold adduction. Samples of normal, breathy and pressed phonation produced by female speakers were analysed. As predicted, a significant increase in SPI for breathy productions was demonstrated. However, increases in SPI were also noted for pressed phonation. Possible explanations and implications for clinical application are discussed.