## 1 Introduction  
In the last few decades, it has been a challenge to standardize the evaluation of overall voice quality in practice. Until now, auditory-perceptual evaluation of voice quality has been considered to be the gold standard due to its accessibility and the perceptual nature of voice quality. Subsequently, it is often used to verify the validity of other evaluation methods [1]. Several assessment tools have been developed to standardize this perceptual rating of voice quality in practice, of whom the GRBAS (grade, roughness, breathiness, asthenia, and strain) scale [2], proposed by the Japan Society of Logopedics and Phoniatrics, and the CAPE-V (consensus auditory-perceptual evaluation of voice) [3], proposed by the American Speech-Language and Hearing Association, are two well-known and widely-used scales. Nevertheless, the subjective nature and the wide range of factors related to listener, stimulus, and scale, influencing intra- and inter-judge reliability, still remains a major drawback [1,4]. Consequently, our interest in objective assessment of voice quality has grown and has been the subject of many studies. Acoustic assessment is a much-discussed and appealing method of objective voice evaluation and is becoming increasingly used in clinical voice practice and research due to its non-invasive and accessible character and its relatively low cost [5]. To overcome the limited validity of single acoustic parameters and also recognize the multidimensionality of voice, [PERSON] et al. [5] developed the acoustic voice quality index (AVQI), a multiparametric tool based on six acoustic measurements, to quantify overall voice quality (an elaborate description of the acoustic measurements is represented in the section 'Materials and Methods'). This tool (Phonanium, Lokeren, Belgium) is the result of stepwise multiple linear regression analysis of thirteen acoustic measures to sort out the most robust combination yielding a single number predicting dysphonia severity, ranging from 0 to 10. The lower this number, the less dysphonia and the better the voice quality. These measures are all obtained within the software Praat ([PERSON] and [PERSON], Institute of Phonetic Sciences, University of Amsterdam, The Netherlands). Analysis of both sustained vowels and continuous speech samples is implemented, since both sample types offer valuable information in voice quality assessment. Sustained vowels are relatively unaffected by individual speech characteristics such as speech rate, dialect, intonation, and articulation, and other factors like phonetic context and stress. On the other hand, continuous speech is more representative of daily speech and, therefore, can be considered to be more 'ecologically valid' [6]. Several studies in the last decade already demonstrated the robust diagnostic accuracy of AVQI [5,7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24], its consistent and high concurrent validity [5, 7, 8, 9, 10, 11, 12, 21, 22, 23, 24, 25] and its high sensitivity to voice changes across voice therapy [8, 12, 16, 18, 21]. Furthermore, studies validating the use of AVQI in different languages showed diagnostic accuracy according to inter-language phonetic differences [9, 10, 11, 12, 13, 14, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25], although AVQI was originally developed for Dutch speakers. The main contributor of AVQI is the smoothed version of the cepstral peak prominence (CPPS), which represents the distance between the first rahmonic's peak and the point with equal quefrency on the regression line through the smoothed cepstrum. The more periodic a voice signal, the more harmonic the spectrum and the higher the CPPS value. It was introduced in the field of voice assessment by [PERSON] et al. [26] and [PERSON] [27], and, meanwhile, many studies have proven it to be a reliable and valid measure of overall voice quality, especially of breathiness [5, 8, 28, 29, 30, 31, 32, 33, 34, 35, 36].