"Metal" and "Density" in Phonation according to Complete Vocal Technique: Double-Case Study with Special Reference to...

Introduction: In Complete Vocal Technique (CVT), "metal" and "density" are used as auditory-perceptual concepts to characterize voice quality. Previous studies have explored acoustic, electroglottographic (EGG), and vocal tract-related patterns associated with different metal and density conditions. However, no studies to date have examined how these qualities are reflected in aerodynamic measurements or the glottal flow waveform. This exploratory analysis aimed to provide insights into the concepts of metal and density. The goal was to gain an understanding of how varying degrees of metal and density are reflected in acoustic, EGG, aerodynamic, and glottal inverse-filtering parameters, as well as in vocal efficiency and economy parameters.

Two CVT-trained singers (male and female) were recorded while performing consonant-vowel syllables at three pitches across four CVT vocal modes with varying "metal" and "density" conditions. Subglottal pressure (P_sub), oral airflow (Q), EGG, and acoustic voice signals were recorded. Contact quotient (CQ) from EGG and an estimate of glottal resistance (GR) were calculated. Glottal flow waveform was studied with inverse filtering. Vocal efficiency and EGG-based estimator of vocal economy (quasi-output-cost ratio [QOCR]) were calculated. Samples were rated for auditory perception of strain by two expert groups with different educational backgrounds.

Metallic phonation displayed higher sound pressure level (SPL), P_sub, GR, CQ, vocal efficiency (p < 0.001), and lower Q (p < 0.001) and QOCR (p = 0.009) than nonmetallic phonation. Out of glottal parameters, harmonic richness factor was higher and H1-H2 and NAQ lower in metallic phonation. Compared to fuller density, reduced density exhibited lower SPL, P_sub, Q, vocal efficiency (p < 0.001), CQ (p = 0.044), AC flow, and MFDR, and higher QOCR (p = 0.055) and NAQ. Strain was rated higher in metallic samples than in nonmetallic samples by vocologists and speech-language pathologists (p < 0.001).

The examined parameters distinguished between nonmetallic and metallic phonation as well as between reduced density and fuller density conditions. While metal and density share common measures, they appear to form their own specific profiles. Larger sample sizes are needed to refine these preliminary findings. Indicators of vocal economy warrant further investigation.