Table 2.

Descriptions of the acoustic and perceptual metrics.

Metrics type	Description
Perceptual measures
Intelligibility	Percentage of words correct from a transcriptional task. Data were originally reported in Liss et al. (2013).
Severity	Perceptual rating of global, integrated impression of dysarthria severity obtained from five SLPs.
Vocal quality	Perceptual rating of global, integrated impression of overall vocal quality obtained from five SLPs.
Nasality	Perceptual rating of global, integrated impression of nasal resonance obtained from five SLPs.
Articulatory imprecision	Perceptual rating of global, integrated impression of precision of articulatory gestures obtained from five SLPs.
Prosody	Perceptual rating of global, integrated impression of speaker's rhythm, stress, and intonation obtained from five SLPs.
Rhythm metrics	Acoustic measures of vocalic and consonantal segment durations (Liss et al., 2009).
ΔV	Standard deviation of vocalic intervals.
ΔC	Standard deviation of consonantal intervals.
%V	Percentage of utterance duration composed of vocalic intervals.
VarcoV	Standard deviation of vocalic intervals divided by mean vocalic duration (× 100).
VarcoC	Standard deviation of consonantal intervals divided by mean consonantal duration (× 100).
VarcoVC	Standard deviation of vocalic + consonantal intervals divided by mean vocalic + consonantal duration (× 100).
nPVI-V	Normalized pairwise variability index for vocalic intervals. Mean of the differences between successive vocalic intervals divided by their sum.
rPVI-C	Pairwise variability index for consonantal intervals. Mean of the differences between successive consonantal intervals.
rPVI-VC	Pairwise variability index for vocalic and consonantal intervals. Mean of the differences between successive vocalic and consonantal intervals.
nPVI-VC	Normalized pairwise variability index for vocalic + consonantal intervals. Mean of differences between successive vocalic + consonantal intervals divided by their sum.
Articulation rate	Number of (orthographic) syllables produced per second, excluding pauses.
EMS metrics	The EMS variables were obtained for the full signal and for each of the octave bands (Liss et al., 2010).
Peak frequency	The frequency of the peak in the spectrum with the greatest amplitude. The period of this frequency is the duration of the predominant repeating amplitude pattern.
Peak amplitude	The amplitude of the peak described above (divided by overall amplitude of the spectrum).
E3–E6	Energy in the region of 3–6 Hz (divided by overall amplitude of the spectrum). This is roughly the region of the spectrum (around 4 Hz) that has been correlated with intelligibility (Houtgast & Steeneken, 1985) and has been inversely correlated with segmental deletions (Tilsen & Johnson, 2008).
Below 4	Energy in the spectrum from 0 to 4 Hz (divided by overall amplitude of the spectrum).
Above 4	Energy in the spectrum from 4 to 10 Hz (divided by overall amplitude of the spectrum).
Ratio 4	Below 4/above 4.
LTAS metrics	The measures of LTAS were normalized to RMS energy of entire signal and derived for 7 octave bands with center frequencies ranging from 125 to 8000 Hz (Utianski et al., 2012).
N1RMS	RMS energy of entire signal and each octave band.
nsd	Standard deviation of RMS energy (for 20-ms windows) of entire signal and each octave band.
nRng	Range RMS energy (for 20-ms windows) of entire signal and each octave band.
PV	Pairwise variability of RMS energy: mean difference between successive 20 ms windows of nRng. Computed for entire signal and for each octave band.
Vowel metrics	Vowel measures were derived from formant frequencies of vowel tokens embedded in six-syllable phrases produced by each speaker (Lansford & Liss, 2014a, 2014b).
Quadrilateral VSA	Vowel space area. Heron's formula was used to calculate the area of the irregular quadrilateral formed by the corner vowels (i, æ, a, u) in F1 × F2 space. Toward this end, the area (as calculated by Heron's formula) of the two triangles formed by the sets of vowels, /i/, /æ/, /u/, and /u/, /æ/, /a/, is summed. Heron's formula is as follows: $\sqrt{\mathrm{s}\left(\mathrm{s}-\mathrm{a}\right)\left(\mathrm{s}-\mathrm{b}\right)\left(\mathrm{s}-\mathrm{c}\right)}$, where s is the semiperimeter of each triangle, expressed as s = ½ (a + b + c), and a, b, and c each represent the Euclidean distance in F1 × F2 space between each vowel pair (e.g., /i/ to /æ/).
FCR	Formant centralization ratio. This ratio, expressed as (F2u + F2a + F1i + F1u)/(F2i + F1a), is thought to capture centralization when the numerator increases and the denominator decreases. Ratios greater than 1 are interpreted to indicate vowel centralization.
Mean dispersion	This metric captures the overall dispersion (or distance) of each pair of the 10 vowels, as indexed by the Euclidean distance between each pair in the F1 × F2 space.
Front dispersion	This metric captures the overall dispersion of each pair of the front vowels (i, ɪ, e, ɛ, æ). Indexed by the average Euclidean distance between each pair of front vowels in F1 × F2 space.
Back dispersion	This metric captures the overall dispersion of each pair of the back vowels (u, ʊ, o, a). Indexed by the average Euclidean distance between each pair of back vowels in F1 × F2 space.
Corner dispersion	This metric is expressed by the average Euclidean distance of each of the corner vowels, (i, æ, a, u), to the center vowel /^/.
Global dispersion	Mean dispersion of all vowels to the global formant means (Euclidian distance in F1 × F2 space).

Note. SLPs = speech-language pathologists; EMS = envelope modulation spectra; LTAS = long-term average spectra; RMS: root-mean-square.