## 2 Related Works
### Evaluation of Speech Changes
#### 2.2.1. Phonation  
Studies about the influence of PD in the phonatory system mainly analyze the impairments in phonatory-related structures and muscles like the diaphragm, the muscles connected to the larynx, the vocal folds, or the supraglottal resonant cavities [45]. In the case of phonation change analysis, recording of the \"a\" vowel with prolonged phonation is most often used as research material, due to how easy it is for a patient to utter and the relatively simple-to-observe phonation stability. Due to the fact that such signals consist of many vocal fold operating cycles, parameters are adopted to relate to each of them. Most often, in the case of this type of examination, disorders related to impairments in phonatory-related structures and muscles are studied. This includes the works of the diaphragm, the muscles connected to the larynx, the vocal folds, or the supraglottal resonant cavities. The results of these studies generally show that patients with PD show a general decrease in phonatory abilities, which is manifested primarily by airflow insufficiency, irregular pitch fluctuations,microperturbations in frequency, microperturbations in amplitude, tremor of jaw, and aperiodicity [9,46].  
Basic studies in this field indicate abnormal laryngeal function in 89% of patients [47]. This is mainly manifested by shortened phonation, because the expiratory phase is very short. This contributes to the occurrence of difficulties in speaking longer phrases, as a result of which there are frequent pauses in speaking to take air into the lungs. On the other hand, examination of the larynx showed that there was tremor in the range of 2 to 7 Hz. [PERSON] et al. [48] observed that patients had an irregular frequency of vibrations of the vocal folds. Additionally, men have a higher base frequency (\(F_{0}\)), while women have a lower \(F_{0}\) variation. Further studies by [PERSON] and [PERSON] [49] on \(F_{0}\) in PD patients showed instability of \(F_{0}\) and the \(F_{1}\) formant during vowel pronunciation [9]. According to [PERSON] et al. [50], changes in \(F_{0}\) can be observed as early as 5 years before the diagnosis is established.  
The most studied parameters of acoustic analysis in the current literature are fundamental frequency \(F_{0}\) and parameters describing its variability in time (jitter). Jitter can be determined based on its several measures, including parameters like Jitta, PPQ5, and RAP. Jitter is mainly affected by the lack of control over the vibration of the cords. The next group of parameters describes the signal energy in general. Shimmer, which is the variation in amplitude of the sound wave, is determined by shimmer parameters such as ShdB, APQ3, and APQ5 [51, 52, 53]. These various classification schemes usually provide accuracy over 75% [39,54,55]. In a different approach, the authors of [56] used perceptual linear prediction (PLP) feature vectors to characterize sustained vowels of 34 subjects (17 patients). In contrast, other related studies used several corpora and a broad set of state-of-the-art acoustic features such as jitter, shimmer, noise, complexity, PLP, linear predictive coding (LPC) or MFCC [57], and reached accuracy below 75% [39].