Application of transparent microperforated panel to acrylic partitions for desktop use: A case study by prototyping

The authors addressed all the points raised by the two reviewers. Below is the reply to the reviewers in which the summary of the revision is given in detail.

[To the comments from Simone Torresin]

Thank you very much for your positive and constructive comments. Although the reviewer does not require an amendment, the comments encouraging for the future study are suggested. We consider that they are important for this paper to mention the points of future studies. Thus, we added the sentences below:

In the last paragraph of Section 3, we added:

For this purpose, a parametric study with perforation ratio, hole diameter, etc, will be needed both in theoretical and experimental method.

In the last paragraph of Concluding remarks:

For this purpose, a parametric study both in theory and experiment will be necessary. Particularly, a theoretical model analysis will be helpful for a practical design.

[To the comments from Tin Oberman]

Thank you very much for constructive and detailed comments on our paper. Although this review is structured in the style of an article, in the following, the response to the comments is summarised in point by point style.

Q: I found that the application of transparent microperforated panels as acoustically enhanced social distancing tool is a very useful idea in the times of a pandemic and potentially beyond. It would be also very useful to find out how much worse acoustically do the standard acrylic partitions make the spaces they are added to and to what extent can the microperforated ones mitigate the issue without introducing significant reduction in visual transparency and significant increase in costing.

A: Thank you very much for this positive appreciation. Many people have experienced changes in the acoustic environment caused by a desktop partition such as the one discussed here, but little concrete data has been published. There are studies by Sugie et al. [12, 13] on the effect of partitions on sound transmission, but to the authors' knowledge there is no data on the effects on room acoustics. It would be ideal if we could measure these effects and add the results, but this research covers the presentation of the idea and up to the sound absorption measurement test of the prototypes: we are firstly investigating whether it is possible to create a transparent partition with sound absorbing properties. Therefore, it remains to be seen to what extent sound-absorbing partitions using an MPP can actually have an effect on the acoustic environment in a room, however, if the partition is placed over a certain area, it can be easily expected that a sound-absorbing effect will appear. These limitations of the present study are mentioned in the text of Concluding remarks.

Q: Moreover, it would be useful to understand how this prototype visually and acoustically compares with the commercially available solutions such as the Clearsorber (RPG). While some of these answers might be beyond this particular study, I believe they are important to bear in mind while reporting the results and writing concluding remarks so to make them as useful for further research as possible. I found the text to be overall clear and easy to read. However, there is still room to improve the structure and the clarity. As there is no other content than the absorption measurements, I would suggest making the report even more robust to ensure reproducibility. While the main message is easy to understand, English language would need to be improved as well.

A: Clearsorber (RPG) is designed to be large in size and to have a larger air-back layer, therefore, it cannot be applied directly for the present purpose. The sound-absorbing performance of RPG is better than one we proposed here: about 0.3-0.5 in NRC and higher than 0.5 at peak absorption coefficient. It is a pioneer product as MPP with transparency. From the point of view of its intended use, Clearsorb is not designed for the application of our study, but it is an innovative commercial product of transparent MPP. The webpage of this product is cited as a reference [8] of a pioneer product of transparent MPPs in Introduction. A brief comparison was made in Appendix using the published data. We expect that this will give some idea.

Regarding English language, the manuscript has been edited by a professional English editing service, however, this time we have proofread the text carefully to make it readable.

Q: Regarding the structure, the section 3 should be made more efficient. Please reorganise this section and report the experiment in a more technical and less narrative manner. For instance, the Figures 5 and 7 are obsolete, all the data needed is visible in the Figure 9. The distinction between the experiments 1 and 2 is currently not clear. It reads as it is about one experiment with four different levels of intervention into the original panel (no intervention, two microperforated panels added, adhesive tape added, adjustments made to add acrylic seals). If so, please adjust the structure accordingly.

A: As pointed out, it is appropriate and clear to consider this point as one experiment. However, based on the results of the experiment, the final specification of the prototype was decided through a trial and error process in three steps. For this reason, we do not refer to the experiments as Experiment 1 and Experiment 2, but describe each study as a series of experiments, such as Step 1, Step 2 and Step 3. This is because we did not compare the experiments under the three conditions from the beginning, and by describing them step by step, we can make the findings from the series of experiments clearer. In addition, we would like to leave the figures as they are, although there is some duplication of information. This is because it makes clearer the problems and their solutions at each stage.

Q: I believe the middle panel is the key feature in the light of the epidemiological measures which motivated this study so I would suggest moving the explanation behind that specific design trait to the introduction and including it in the abstract (by expanding the existing sentence and stating the reason for having the non-perforated droplet barrier in between).

A: The existing text at the end of the Introduction has been changed to a more detailed description and this has also been added to the Abstract.

Q: For the purpose of the reproducibility, please report more data related to the experiment and in accordance with the ISO 354, i.e. the surface area of the 10 panels, the reverberation time without any panels inside, number of measurement points used in measurements, measuring equipment etc. Perhaps there is a paper published with a more detailed description of that very room and measurement system you could simply refer to?

A: Some more detailed information about the measurement settings, including arrangement of the specimens, surface area of the specimens, etc, are now added.

Q: Moreover, adding a supplementary datasheet with the exact measured absorption values would potentially help further research that could look into modelling the potential effect of such panels on noise levels or indoor soundscape.

A: As suggested, a table of the numeric data for Specimen (c) is given in Appendix, as a supplementary datasheet.

Q: How was the target sound absorption frequency range decided?

A: We have targeted the main frequency bands of speech (mid-frequencies), but due to the nature of sound absorbers, we have not covered all of them completely.

Q: Figure 3 could be horizontally arranged so it takes less space.

A: The figure is now arranged horizontally, as suggested.

Q: Figure 4 is not completely clear. Would it be possible to add better pictures that show the whole of the panel so the background does not get in the way of understanding the details and so it is clear how the panel is standing, i.e. is it touching the floor or not. While the Figure 2 is clear in that regard, the other photographs are ambiguous. The size of the ‘legs’ might not be that important for absorption, but I can see no reason not to report it. The same goes for the details about the joints and if any dampening materials were used anywhere in the design. Also, a figure showing the visual effect of combining small and large holes would be useful to understand the influence of perforations and extra layers on the overall visual transparency.

A: It was difficult to take a clear photograph as the specimens were transparent, however, in this revision, we replaced some of photographs with better ones. The size of the ‘legs’ are now added in the caption of Figure 2, which is 120x80 mm made of 5 mm thick acrylic sheet. Some photographs include the images of the joints. As is observed in figures, we did not use any damping materials. Even when we cannot provide better photo/picture, we added necessary information in the text or the captions.

Q: More references could be added to the introduction or some less important paragraphs featuring very general and not-referenced statements could be omitted.

A: Thank you for your advice. Due to the nature of the journal, we have tried to include some general information to make it more accessible to non-acousticians. We cite a book suitable for general readers in Introduction. However, we did not omit the existing paragraphs, because we believe that a certain amount of general content is better to be included for a general reader, please understand.

Q: Overall, this is brief, clear and concise paper, hopefully leading to further studies and application

A: Thank you very much for your detailed review. We hope that this revision meets your scientific standard.

There are various measures currently in place to prevent the spread of COVID-19; however, in some cases, these can have an adverse effect on the acoustic environment in buildings. For example, transparent acrylic partitions are often used in eating establishments, meeting rooms, offices, etc., to prevent droplet infection. However, acrylic partitions are acoustically reflective; therefore, reflected sounds may cause acoustic problems such as difficulties in conversation or the leakage of conversation. In this study, we performed a prototyping of transparent acrylic partitions to which a microperforated panel (MPP) was applied for sound absorption while maintaining transparency. The proposed partition is a triple-leaf acrylic partition with a single acrylic sheet without holes between two MPP sheets, as including a hole-free panel is important to a possible droplet penetration. The sound absorption characteristics were investigated by measuring the sound absorption in a reverberation room. As the original prototype showed sound absorption characteristics with a gentle peak and low values due to the openings on the periphery, it was modified by closing the openings of the top and sides. The sound absorption performance was improved to some extent when the top and sides were closed, although there remains the possibility of further improvement. This time, only the sound absorption characteristics were examined in the prototype experiments. The effects during actual use will be the subject of future study.