# Prosody-Based Measures for Automatic Severity Assessment of Dysarthrie Speech
## 2 Acoustic Measures
### Articulation and Phonation Based Measures  
MFCCs are commonly used in automatic speech recognition (ASR) as they represent the relevant frequencies shaped by the vocal tract while removing irrelevant F0 information. A baseline set of acoustic features representing articulation (MFCCs) and phonation (voice quality) are extracted for comparison of prosodic features with non-prosodic features. We extract 12-dim MFCCs and log energy along with delta and delta-delta features from all utterances. This process leads to a 39-dim feature set representing articulation.  
Voice quality refers to the properties of speech related to the vocal folds within the larynx. Our study includes the following voice quality measures, which are used in acoustic studies of dysarthric speech [52; 53]: relative jitter, relative shimmer, harmonics to noise ratio (HNR), # of voice breaks, and degree of voice breaks. Some studies, such as References [29; 36], include jitter and shimmer in prosody-based feature sets; however, these features are related to phonation, and thus, are better categorized as voice quality features. Therefore, we consider jitter and shimmer as voice quality features similar to the studies in Reference [23; 28]. All voice quality measures are extracted with the Praat software using the voice analysis function [54].  
Jitter represents the variations of F0 within a time period. We calculate relative local jitter by the average absolute difference between consecutive periods, divided by the average period. The calculation for jitter can be examined in Equations (5)-(7), where \(T_{i}\) is the duration of the \(i\)th interval and \(N\) is the number of intervals.  
\[\text{Absolute jitter}\ (\text{sec})\ =\ \sum_{i=1}^{N}\abs{T_{i}-T_{i+1}} (N-1) \tag{5}\]  
\[\text{Mean Period}\ (\text{sec})\ =\ \sum_{i=1}^{N}T_{i}/N \tag{6}\]  
\[\text{Relative Jitter}\ =\text{Absolute Jitter}/\text{Mean Period} \tag{7}\]Shimmer, as shown in Equation (8), where \(A_{i}\) is the amplitude of the \(i\)th interval, is very similar to jitter, except that F0 perturbations fall in the amplitude domain; therefore, the average absolute difference between the amplitudes of consecutive periods are divided by the average amplitude.  
\[\text{Relative Shimmer}\ =\ \frac{\frac{1}{N-1}\sum_{i=1}^{N-1}\lvert A_{i}-A_{i=1} \rvert}{\frac{1}{N}\sum_{i=1}^{N}A_{i}} \tag{8}\]  
HNR refers to the periodicity of a speech signal over noise. Harmonicity is measured in decibels (dB) by the ratio of the energy of the periodic part (\(E_{p}\)) related to the noise energy (\(E_{n}\)) as shown in Equation (9).  
\[\text{HNR (dB)}\ =\ 10\log\ (\frac{E_{p}}{E_{n}}) \tag{9}\]  
Lastly, we include two measures related to breaks in voicing. The first is the number of voice breaks which [PERSON] calculates by taking the number of distances between consecutive glottal pulses that are longer than 1.25 divided by the pitch floor which is set at 50 Hz. Any interval longer than 25 ms between pulses is considered a voice break. Second, we measure the degree of voice breaks, which is the total duration of the breaks in the signal, divided by the total duration, excluding silence at the beginning and the end of the sentence.