# Effect of Subglottic Stenosis on Expiratory Sound Using Direct Noise Calculation
## 4 Summary and Discussion
### Limitations  
To properly interpret the results of the current simulation, it is crucial to acknowledge the simplifications made in the setup and how they diverge from real-world scenarios. The most significant simplification is the assumption of axisymmetric geometry, which eliminates turbulence from the flow and promotes more ordered flow patterns. In reality, the glottis takes on a triangular shape and generates a two-dimensional jet, where the bimodal lateral jet instability could play a more significant role in the sound production [57]. This may lead to different tonal mechanisms or even completely turbulent sounds. Under such conditions, the pipe tone from the glottal jet may be weak or entirely absent. It is also more difficult to form a Helmholtz resonator by constricting the glottis, because the glottal opening becomes a slit instead of a hole. In addition to the pipe tone, the hole tone is another important mechanism contributing to the tonal sound of the SGS. Understanding the effects of axisymmetry on the hole tone is also crucial. The work of [PERSON] and [PERSON] [14] revealed that the jet disturbance remained symmetric even when the downstream obstacle was a long rectangular slit rather than a hole. Moreover, jet flows were found to be predominantly pressure-sensitive,regardless of the direction of the external source [14; 27; 29]. It is therefore reasonable to expect the hole tone to be present in a realistic, non-axisymmetric glottal configuration. However, the tones are expected to be less pure compared to the spectra obtained using the axisymmetric configuration, due to the complex flow patterns associated with non-axisymmetric geometries. Although the SGS is commonly modeled using simplified geometries such as axisymmetric [17; 18]) or two-dimensional [19] shapes, irregular shapes of the SGS are commonly observed in clinical examination. Compared to the axisymmetric SGS, the irregularly shaped SGS is less likely to produce whistling tones, likely due to the irregularities in the shape that disrupt the orderly flow structures necessary for whistling to occur. Furthermore, all tissues are modeled as rigid walls without considering possible vibratory and damping effects that could potentially absorb energy from the airflow and acoustic feedback and weaken the tonal sound. This current study focuses solely on expiration, while reports suggest that SGS stridor is often inspiratory or biphasic [11]. Based on these current findings, it is expected that the inspiratory tonal sound in the SGS will also be characterized by pipe tones that are independent of the flow direction [39]. However, the tonal quality during inspiration may differ as the hole tone mechanism is absent. Future studies are needed to confirm this. Additionally, investigating how glottal configuration affects inspiratory sound could aid in distinguishing SGS from other respiratory disorders that produce a similar sound. Conceptualization, X.Z. ; Formal analysis, B.G., Q.X., S.T. and X.Z. ; Investigation, B.G. ; Writing--original draft, B.G. ; Writing--review & editing, B.G., Q.X., S.T. and X.Z. ; Supervision, X.Z. ; Funding acquisition, S.T. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Institute on Deafness and Other Communication Disorders (NIDCD), Grant No. 5R01 DC009616. Not applicable. The data that support the findings of this study are available from the corresponding author upon reasonable request. The data are not publicly available due to data volume and complexity. The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIDCD or the NIH. The numerical simulations were supported by the Extreme Science and Engineering Discovery Environment (XSEDE) (allocation award number TG-CTS180004) and The University of Maine System Advanced Computing Group. The authors declare no conflict of interest.