Deriving individualised /r/ targets from the acoustics of children's non-rhotic vowels

Abstract

In visual-acoustic biofeedback for rhotic errors, learners are guided to match the third formant (F3) location to a visual target on a real-time acoustic spectrum. As the acoustic properties of correct English /r/differ across speakers, this study aimed to improve target selection by investigating the validity of individualised targets derived from children's non-rhotic vowels. A previously proposed prediction formula was adjusted using data from a child normative sample and tested in two groups of children. Study 1 found that predicted values were unexpectedly higher than actual F3 values in children whose /r/ errors had been remediated. To understand this discrepancy, Study 2 applied the formula to typically developing children and found that predicted values were also higher than actual F3 values, suggesting that different normative data might better represent the current samples. An updated formula is proposed, which can be used to generate individualised targets within acoustic biofeedback applications.

Keywords: Articulation disorders; biofeedback; intervention; residual speech errors; speech sound disorders.

Figure 1

A screen shot of a live LPC spectrum with a superimposed target F3 of correct /r/ (line with starfish), adapted from ‘staRt’ the Speech Therapists’ App for /r/ Treatment from McAllister Byun, Campbell, Liang, Park, and Svirsky (in press).

Figure 2

For each age group for females (top) and males (bottom) in Lee et al. (1999) normative data, dark bars represent average of corner vowels /i,æ,ɑ,u/ while light bars represent actual F3 of /ɝ/ values. To the nearest hundredth, predicted F3 of /ɝ/ values (dots) correspond most accurately with actual F3 of /ɝ/ values at 70% of vowel average for females and at 67% of vowel average for males.

Figure 3

Plotted F3 of vocalic /ɝ/ tokens for Autumn and Lilianne across the course of ultrasound biofeedback therapy in McAllister Byun et al. (2014). Vocalic /ɝ/ production in words improved from baseline (BS) to maintenance (MN) sessions, consistent with the decreasing F3 across treatment (TX) sessions. The grey dashed line is the 80% critical frequency. The solid black line is the F3_pr value, which was 70% of the vowel average for females.

Figure 4

The 80% critical frequency (green triangles), predicted F3 of /ɝ/ (blue diamonds), and actual F3 of /ɝ/ values (dots) for each participant and each task.