Numerical analysis on the improvement of sound insulation performance of ple-num doors : Effect of sound absorption and MPPs

Review
The authors have addressed the reviewer´s comments satisfactorily.

Note:
This review refers to round 2 of peer review.

Review
The study reports a series of Finite Element Method (FEM) analyses to evaluate the sound transmission loss of plenum doors following the application of sound-absorbing materials in different positions and with different properties (absorption coefficients). The study is interesting, and the following minor comments are provided:

1) The abstract could perhaps include a brief numerical summary of the performance achieved and the frequency range in which the solution is effective across the different investigated scenarios.

2) “First, the sound insulation performance of plenum doors treated with sound-absorbing materials while maintaining transparency was evaluated.” This sentence—although clarified later in the manuscript—is not immediately clear when transparency is first mentioned in the introduction. Adding further context would improve comprehension.

3) Figure 3: it is unclear what the colors in the caption text refer to.

4) “To maintain the transparency of the door, absorption is applied to only half of the panels.” As an additional suggestion, also relevant for expanding the conclusion section, the authors could mention the possibility of integrating translucent metamaterials to achieve both acoustic attenuation and visibility. See, for example, the work of G. Fusaro and colleagues.

5) Part of Figure 5 is not clearly legible.

6) It would be useful to clarify how the door moves and how the lateral anchoring is handled, possibly with a sketch, when the door is opened and closed.

7) A paragraph discussing the limitations of the study is recommended. For example, the use of global absorption coefficients rather than real materials with frequency-dependent absorption curves is a minor limitation.

8) The conclusion section is currently too concise; expanding it with future research perspectives is recommended.

Note:
This review refers to round 1 of peer review and may pertain to an earlier version of the document.

Review
The paper provides a systematic FEM-based investigation of plenum doors treated with sound-absorbing materials and micro-perforated panels (MPPs). Its main contribution lies in extending earlier work on plenum doors by examining both partial-surface absorption strategies and multiple MPP insertion configurations. This represents a meaningful incremental advancement, though the underlying physical principles and modelling frameworks largely follow established approaches in plenum window/door research. The study is scientifically sound in its general methodology, with clear parameter definitions and a structured progression from baseline cases to increasingly complex MPP configurations. However, several modelling assumptions, such as the uniformity of absorption coefficients and the limited justification of target frequencies, would benefit from deeper physical explanation or sensitivity analysis.
The scientific English is understandable overall, though at times repetitive, and occasionally vague or missing essential qualifiers (e.g., regarding boundary conditions, material models, or uncertainty). Figures and tables are clear but would benefit from improved captions and greater integration with the discussion.
Overall, the paper is relevant to UCL Open Environment because it addresses ventilation-acoustics trade-offs, a topic situated at the intersection of building physics and environmental comfort. With revisions to strengthen contextualisation, methodological transparency, and clarity of argumentation, the work could become suitable for publication. I would recommend its publication after minor revisions are addressed. Hopefully, the following comments will help the authors to improve the paper:
1. Page 1, lines 14-20: The abstract uses general descriptions (“especially significant”, “relatively small”, “indicating the need for further evaluation”) without quantifying effects. Perhaps the authors could add numerical ranges (e.g., TL improvements in dB) to strengthen scientific precision.
2. Page 1, lines 1-13 (Abstract title block): The abstract does not clearly state the novelty compared to Ref. [8]. I would recommend to explicitly highlighting what aspects of absorption/MPP insertion configurations were never evaluated previously.
3. Page 1, lines 21-30 (Introduction): The introduction emphasises natural ventilation but does not connect sufficiently to environmental outcomes or the journal’s scope. Situating plenum doors within broader environmental building strategies would benefit the readership of the paper (IAQ, energy reduction, façade design).
4. Page 1-2: Discussion of previous plenum window research is descriptive but not analytical. Maybe the authors could critically discuss the limitations of existing window-based studies and why analogous door structures may behave differently. Moreover, the bibliography should be expanded a bit to show robustness in the literature review, which is at the base of this study.
5. Page 2, lines 48-58: The transition from Ref. [8] to the new study is slightly abrupt and does not fully motivate why partial-absorption cases or MPP integration are practically important. Perhaps the authors could add justification related to manufacturability, cost constraints, or architectural integration.
6. Page 2-3, (model description): FEM model description lacks numerical mesh details and convergence criteria. I would recommend that the authors include mesh density, element types, and sensitivity checks to ensure reproducibility.
7. Page 2-3, (diffuse incidence method): A method using 300 plane waves is mentioned but not described in detail. Moreover, justification for this number or convergence testing is not provided. Maybe the authors could add a brief validation demonstrating that increasing to 400 or 500 waves does not alter TL results significantly.
8. Page 2 (peripheral absorption coefficients): The assumed absorption coefficients (0.1-0.7) do not correspond to any specific material model. Can the authors clarify whether these are idealised coefficients or derived from real porous materials? Can they provide references related to such materials?
9. Page 3, lines 2-8 and Fig. 3: The interpretation (“likely due to the smaller area”) lacks quantitative surface area comparison. I would recommend including a paragraph or a table computing treated surface-area percentages.
10. Page 4, lines 1-20 (panel absorption cases A-D): The discussion suggests small differences but does not show statistical or numerical measures. Perhaps the authors could add ΔTL values between cases at specific frequencies or frequency bands.
11. Page 5, lines 1-16 (MPP parameters): MPP tuning to 300 Hz is mentioned but not explained physically. Please provide the MPP resonance formula and show how parameters were selected.
12. Table 2 caption is hidden.
13. Page 5-6 (MPP1-4 results): The statement “This trend is particularly pronounced…” lacks quantification. Maybe the authors could provide TL gains (e.g., +3-6 dB near 300 Hz).
14. Page 6-7 (MPP5 & MPP6): The visibility argument is slightly qualitative and not supported by any diagram showing optical obstruction. Maybe a schematic or discussion could be added to quantify the occluded area for MPP5 and MPP6.
15. Page 7-8 (MPP7-9 mid-cavity): The explanation “MPP works primarily near openings” is plausible but not supported by acoustic field plots. Since the software you are using allows to have such visualisation, the authors could add sound-pressure or particle-velocity contour maps.
16. Page 9, Concluding remarks: Conclusions are slightly descriptive and do not emphasise design implications or limitations. I would recommend the authors to add explicit guidance for practitioners (e.g., minimum back-space thickness, preferred MPP location). Moreover, a subsection on limitations and future work could be added too for the sake of the readership.

Note:
This review refers to round 1 of peer review and may pertain to an earlier version of the document.