Voice Therapy With a Semi-occluded Vocal Tract: From Current Challenges to Next Level Efficacy Studies
Phase: NA
Conditions: Voice Disorders
Sponsor: University Ghent

 
 
 
 
 
 
 
VOICE THERAPY WITH A SEMI-OCCLUDED VOCAL TRACT: 
FROM CURRENT CHALLENGES TO NEXT LEVEL EFFICACY STUDIES 
Protocol and statistical analysis plan 
Document date: March 3, 2023 
 
 
 
 
 
 
 
 
 

PROTOCOL 
This study has been approved by the Ethics Committee of Ghent University Hospital (BC-
09807). 
Participants 
Female students in the first year of study toward a bachelor degree in Logopaedic and 
Audiological Sciences at Ghent University (academic year 2021-2022), will be recruited for 
this study by convenience sampling. They will all provide written informed consent. Exclusion 
criteria are: an organic vocal fold pathology diagnosed by a specialized otorhinolaryngologist, 
smoking, pregnancy and hearing problems.  
Design  
A randomized controlled trial will be used. Participants will be assigned randomly to one of 
three experimental groups or a control group: (1) straw phonation (SP) in air, (2) SP in 2cm 
water, (3) SP in 5cm water or (4) /u/ phonation with similar soft onset and slightly pursed lips 
as in SP but without a straw (control group). An online random number generator will be used 
for this procedure.  
Material and methods 
Straw phonation material 
Ecologic compostable wheat stirring straws with a diameter of 3mm and a length of 20cm will 
be selected for all experimental groups. SP in water will be performed in reusable cups, and the 
water depth (2cm or 5cm) will be set by drawing a line on the straw.  
 
 

Preparatory phase 
Before the experiment, in the second week of the academic year, all students will receive one 
group SP workshop of 20 min guided by an experienced voice therapist (I.M.). The aim of this 
session is to strive for a correct and comfortable SOVT production prior to the actual 
experiment. First, focus will be on an eutonic posture in sitting position and costo-abdominal 
breathing. Participants will be instructed to breath in through the nose and blow out through the 
mouth without phonation. Second, they will be asked to repeat this but now adding phonation 
on a [ɔ] vowel with soft onset [hhhɔɔɔ] during exhalation, at habitual comfortable pitch and 
loudness. Afterwards, both step one (without phonation) and step two (with phonation) will be 
repeated with the straw. Attention will be drawn to sensory feedback, forward focus, and 
avoidance of hyperfunction. At the preparatory phase, students will be unaware of their group 
assignment and will be specifically instructed to not practice the exercises at home.  
Flexible strobovideolaryngoscopy and phonatory tasks 
In the third and fourth week of the academic year, all students will undergo flexible SVL by a 
specialized otorhinolaryngologist (P.T. or F.D.) using an EndoFLEX Spectar laryngoscope 
(Xion Medical). The participants will be examined in seated position with the head upright and 
without administration of topical anesthesia. During the examination, they will be asked to 
phonate an [u] vowel at habitual pitch and loudness (baseline), followed by the specific SOVT 
exercise of their group assignment. For the SOVT phonation, they will be instructed to produce 
an [u] vowel at habitual pitch and loudness with soft onset and slightly pursed lips through the 
stirring straw (either in air, 2cm water or 5cm water) or without a straw (control group). The 
[u] vowel will be selected for all phonatory tasks (baseline and SOVT, both in the experimental 
group and the control group) so that potential differences are more certainly attributable to SP 
and not to differences in vowel production. The same voice therapist (I.M.) will guide them 
through these phonatory tasks, together with a master’s student (K.P.) for practical support.  

Visual-perceptual ratings 
After data collection, all video samples will be evaluated randomly and blindly by an 
otorhinolaryngology resident (C.D.V.) and a speech-language pathologist specialized in voice 
(I.K.). Evaluations will be standardized by use of the Voice-Vibratory Assessment with 
Laryngeal Imaging (VALI) rating form for stroboscopy (Poburka et al., 2017). In advance, a 
half hour training session will be provided in which each parameter will be clarified with the 
definition, a high-quality graphic, and two video examples.  
After the training, the assessors will first independently evaluate each video sample on a self-
paced basis, after which a consensus evaluation will be reached. Ten percent of the samples 
will be randomly repeated to assess intrarater reliability. The video samples will be presented 
without audio to prevent bias of the participant’s voice quality on the judges’ ratings.  
The parameters to be evaluated are glottal closure (complete, anterior gap, posterior gap, 
hourglass, spindle gap, irregular, or incomplete), amplitude (magnitude of lateral movement of 
the vocal folds, in %), mucosal wave (magnitude of lateral movement of the mucous membrane 
in %), vertical level (on-plane, off-plane left lower, or off-plane right lower), nonvibratory 
portion (adynamic segments of tissue that appear stiff, in %), anteroposterior (AP) and 
mediolateral (ML) supraglottic activity (constriction of the supraglottic structures, rated 0 − 5 
with the aid of concentric circles), free edge contour (normal, convex, concave, irregular, or 
rough), phase closure (open phase predominates, nearly equal, or closed phase predominates), 
phase symmetry (the degree of symmetry between the left and the right vocal folds in terms of 
opening and closing, in %), and regularity (consistency of averaged stroboscopic cycles, in %) 
(Poburka et al., 2017). 
 
 

STATISTICAL ANALYSIS PLAN 
The data will be analyzed statistically by SPSS version 28 (SPSS Corporation, Chicago, IL, 
USA) at α = 0.05. 
To determine the interrater reliability of the visual-perceptual ratings, a two-way mixed, 
consistency, average-measures intraclass correlation coefficient (ICC) will be calculated for the 
continuous variables, and a Cohen's kappa (K) will be used for the nominal variables (Hallgren, 
2012; Koo & Li, 2016). Intrarater reliability will be determined using a two-way mixed, 
absolute agreement, single-measures ICC for the continuous variables, and a Cohen's K will be 
used for the nominal variables (Hallgren, 2012; Koo & Li, 2016). 
Linear mixed model analyses will be used to compare the groups over phonatory condition 
(habitual [u] phonation vs SOVT phonation) on each continuous variable using the restricted 
maximum likelihood estimation and scaled identity covariance structure. Group, Phonatory 
Condition and Phonatory Condition * Group interaction will be determined as fixed factors. A 
random intercept for subjects will be included. Model assumptions will be checked by 
inspecting whether residuals were distributed normally. Within-group effects of Phonatory 
Condition will be determined by posthoc pairwise comparisons. Marginal homogeneity tests 
will be used to compare nominal variables between habitual /u/ phonation and SOVT phonation 
within groups.