## 1 Introduction  
The voice, the carrier signal of speech, is modulated by pharynx constrictions, tongue motion, and oral-nasal area. The process of voice production can be described by the interaction between the tracheal airflow and the two elastic VFs inside the larynx that oscillate periodically. Thus, the two oscillating vocal folds (usually between 100 Hz and 300 Hz) interrupt the expiration air stream periodically forming the primary acoustic voice signal [1; 2]. It is the prevailing assumption that for a healthy primary voice signal the VF vibrations need to fulfill three conditions: (1) The VFs have full contact during the oscillations, i.e., the gap between the VFs is temporary fully closed during an oscillation cycle. (2) The VFs oscillate symmetrically. (3) The VFs oscillate periodically. The primary generated voice signal is further modulated within the vocal tract and emitted from the mouth resulting in the audible voice. We use our voice permanently and take it for granted. However, the exact causalities between airflow, vocal fold dynamics, and resulting acoustic signal are still not fully understood [3]. Besides computational advantages of the hybrid approach in aeroacoustic simulation of human phonation, it enables the systematic analysis of the sources of sound according to the used aeroacoustic formulation. Highly resolved, three-dimensional visualization of aeroacoustic sources is applied to technical applications to get insight on the sound generation process, such as fan noise [4; 5]. However, few authors visualized source terms for phonation. [PERSON] et al. investigated aeroacoustic sources of a synthetic larynx model based on Lighthill source terms computed from PIV measurements [6]. Therefore, the two-dimensional distribution of acoustic sources and their harmonic content was assessed. In [7], the ground-work for understanding and predicting the characteristics of aerodynamically generated speech was developed. [PERSON] et al. characterized the aeroacoustic source strength by measuring the trans-glottal pressure and exploiting the equivalence between vocal fold drag and trans-glottal pressure force [8]. A purely numerical investigation of aeroacoustic sources based on the Fowcs Williams-Hawkings analogy is provided in [9; 10]. However, a well-resolved visualization of the three-dimensional distribution of the aeroacoustic sources cannot be found in the literature. Thus, we provide a visualization of the aeroacoustic sources based on the perturbed convective wave equation (PCWE), which describes the flow-induced sound generation and propagation based on incompressible flow. The analysis is performed in the time domain at characteristic instances of the oscillation cycle of the vocal fold vibration by plotting iso-contours of the source term. Additionally, a transformation into the frequency domain provides information on the relation of acoustic sources to the vocal fold oscillation. Moreover, applying such an analysis to abnormal vocal fold motion can contribute to a deeper understanding of certain phonation disorders and might suggest treatment approaches. The derivation of the PCWE starts with the acoustic/viscous splitting technique of [PERSON] [11; 12; 13; 14; 15]. This method splits the flow field quantities into compressible and incompressible parts  
\[p = \bar{p}+p^{\rm ic}+p^{\rm c}=\bar{p}+p^{\rm ic}+p^{\rm a}\,, \tag{1}\] \[\mathbf{v} = \bar{\mathbf{v}}+p^{\rm ic}+\mathbf{v}^{\rm c}=\bar{\mathbf{v}}+p^{\rm ic}+p^ {\rm a}\,,\] (2) \[\rho = \bar{\rho}+\rho_{1}+\rho^{\rm a}\,. \tag{3}\]  
Thus, the flow field variables are separated into mean (\(\bar{p}\), \(\bar{\mathbf{v}}\), \(\bar{p}\)) and fluctuating parts. Additionally, the fluctuating parts are assumed to be a sum of acoustic (\(p^{\rm a}\), \(\mathbf{v}^{\rm a}\), \(\rho^{\rm a}\)) and incompressible flow components (\(p^{\rm ic}\), \(p^{\rm ic}\)).