## 2 Related Works
### Evaluation of Speech Changes  
Speech sounds are formed through the cooperation of three organ systems: respiratory, phonatory, and articulatory. The respiratory organs generate a flux of air of sufficient pressure required by phonation, which creates sounds. The phonatory organs are made up of the voice-generating part of the vocal organ (generates sounds of appropriate frequency). It is composed of the larynx with vocal folds and innervated nerves in the larynx. The articulatory organs comprise the pharyngeal cavity, the resonant space complex formed by the paranasal sinuses, nose, oral cavity, and mouth [29]. A dysphonic voice is formed when any element of the phonatory system is disturbed. The way of breathing, phonation-respiratory coordination, phonation time, activation of resonators, volume and pitch of the voice, range and average position of the voice, and its timbre and sonority may be incorrect. In the case of people with [PERSON]'s disease, the muscles of the larynx, throat, soft palate, tongue, and mouth are weakened. The patient's speech is characterized by respiratory-articulatory-phonatory disorders resulting from damage to the subcortical nuclei (extrapyramidal system), primarily the striated body and the globus pallidus [30; 31].  
Characteristic voice changes experienced by ca. 70 to 90% of patients already at early stages of the disease are usually of variable nature [32]. Parts of the vocal cords affected by PD include phonation, prosody, and articulation. Numerous tests and experiments have been conducted over the years that allow a synthetic analysis of voice change in a person with PD [9]. For the first time, the pathomechanism of speech disorders in PD was described by [PERSON] et al. [33]. They identified characteristic features of speech in PD, such as decreased speech volume, monotony, disturbances in the pitch and volume of speech, reductions in stress and breathing, hoarseness, inaccurate articulation, and acceleration of the pace of speech. The first articulation and breathing studies were performed in 1970. As a result, the presence of hoarseness, voice roughness, and breathing disorders was observed in 89 patients. Moreover, 25% of the 200 patients experienced problems with producing the sounds \"p\", \"b\", \"s\", \"f\", \"g\", and \"z\" [34]. In the case of patients with PD, speech disorders include dysarthria, aprosody and dysprosody (monotonous speech, no modulation, rhythm, and stress disturbance) [35], parahasia (using words, syllables or sounds of similar sounding instead of proper ones) [36], breathy and harsh sonority, differences in contrastive stress, shorter phrases, increased length of pauses and hesitations (so-called reflection moans), and a variable speed of speech, including bradyalia (slowing down speech) [37]. Stiffness in the mandible and lip area is also observed. Furthermore, pallialia, which is an involuntary repetition of the same word, may also appear [38; 39]. In this context, the literature shows a very large number of studies and approaches aimed at assessing the impact of PD on the voice of patients. Particular intensity in this aspect has been noticed since 2010. In general, these studies can be divided into three main groups depending on the aspect of speech being analyzed, i.e., phonational, articulatory, and prosodic [39]. Since then, a plethora of studies have presented evidence that the neurodegenerative processes associated with PD cause dysphonia and dysarthria, particularly hypokinetic dysarthria [40; 41; 42; 43], in different stages of the disease [44]. Dysphonia can be defined as the speaker's incapacity to produce a normal phonation due to the phonatory system's impaired functioning, while dysarthria is more related to problems with articulation when pronouncing words.