Space sound absorbers with next-generation materials: additional sound absorption for post-pandemic challenges in indoor acoustic environments

Acoustical Materials and the related technologies are nowadays essential for the always-changing indoor and outdoor environments. This article reports solid strategies to mediate the post-pandemic “new style” conditions with people’s comfort and optimal indoor functionality. The article lists several limits that could derive from safety measures conditions and effectively gives solutions for each of them. Not only the materials technologies previously designed by the authors are interesting, but the way they address them to the current indoor safety situation makes them valuably inspiring.

I think it is especially appreciable the fact that the authors constantly include design quality value to their overall evaluation, which from an ergonomic point of view is crucial for the user’s comfort. I particularly appreciated such discussion on the analysed systems’ aesthetic designability, which sometimes tends to be disconnected from the physical impact of it.

Overall the manuscript fits the primary purpose of the journal. There is a valid correlation between the reported acoustic systems for indoor and best practice strategies discussed. Overall, the contents of sections and subsections are very clear, and the path that connects the report section with the discussion and conclusions is very precise.

I think that the paper is ready for publication, however, I add only a few comments that might help to improve the article at this stage:

1.  Page 5, raw 8, It has been reported that masks affect the acoustic characteristics of voices rather drastically [3].

From the Literature Review reference, it is clear how the safety mask use affects the speech intelligibility; however, it seems that also the SPL of the voices is quite lowered down. Perhaps I would include some discussion or considerations on how the designed acoustic systems for the post-pandemic “new style” indoor conditions would answer to this issue.

2. Page 9, raw 22, obserbed should be spelt as observed

3. Page 13, raw 23, In such cases, a 3D-PMA can be more efficient, as the equivalent sound absorption areas are higher in the case of the 3D-MPA .

I think it would sound better written as something like “In such cases, a 3D-PMA can be more efficient, as its equivalent sound absorption areas are higher than standard systems with comparable volume.”

4. Page 14, raw 12, However, by introducing this additional sound absorption, the acoustics of the room can be somewhat improved, even though some elaboration will be needed.

I think that adding some words about which sort of elaboration the authors mean would give to the text more completeness

5. Page 14, raws 28-33, In this study, we introduced and summarised the nature of three-dimensional microperforated and permeable membrane space sound absorbers of various types. We also demonstrated their typical absorptive characteristics. As these absorbers have displayed not only practical absorption performance, but also a wide applicability and the additional value of aesthetic designability , they may pose a more efficient solution to the acoustical challenges of ‘new style’ built environments.

I think you should specify that the data analysis work was done previously as it could be mistaken from the audience as your key research question. Whereas I think the key point of this paper is the “ the additional value of aesthetic designability” which, in my opinion, could be further highlighted and discussed (perhaps adding some more applicability examples or including ergonomics consideration).

This is a commentary on potential usage scenarios of the self-standing microperforated acoustic absorbers developed by the authors and more thoroughly described in their previously published works. The potential novelty lies in an original point of view of this commentary. Therefore, I believe the text should be improved so the overall narrative becomes its strongest point. The main improvements could be achieved by briefly describing a wider context. There is a strong intention to advocate for usage of the self-standing absorbers at events held during times when certain epidemiological measures are in place to compensate for the acoustical effect caused by lower numbers of audience allowed due to social distancing. However, I would suggest the C-19 aspect should be either omitted or made stronger. While appreciating the argument that the 3D-PMAs described are hygienic and economically viable as very flexible to install and remove, one of the reasons for suggesting to reconsider the focus on the C-19 pandemic is the temporary nature of all the epidemiological measures we are witnessing all over the world.

The use of self-standing objects intended to acoustically enhance rooms is not new but they are not very common in performance spaces, especially from the viewpoint of a listener / audience, so I wholeheartedly welcome this commentary as a contribution to reflect on the potential reasons behind that and how and why this could be changed. The mention of the three-dimensional absorption units that are usually hung from the ceiling weakens the overall argument as they are common as they allow a more flexible plan interieur. A clear distinction should be made between those and the units this commentary focuses on.

For instance, there is a record of self-standing items for enhancing acoustics used in ancient theatres and churches in Europe. On the other hand, in more recent times transparent microperforated absorbers became commercially available so perhaps the ongoing development of acoustic metamaterials should be briefly mentioned. Also, I would suggest that the introduction incorporates a clearer connection between the two concepts (self-standing acoustic units and (transparent) microperforated absorbers) with a very brief history overview of both concepts and how the authors came to combine them. This could be reflected in the introduction and discussion. I believe there is a strong argument for such an approach from the design point of view, even for residential uses and perhaps combining similar design with furnishing such as lighting. It should be made clearer in the introduction if the focus of the commentary is on multipurpose halls or residential use and why.

It is not clear from the text how much is known about the interaction between coronaviruses and the usual porous absorber materials. It is sensible to suggest that the harder, self-standing panels are easier to clean and disinfect but it might be too hasty to suggest all the other absorbers would now be in need of replacing and/or cannot be used anymore. More references on this point are needed.

Another important point related to the context is the acoustic effect of human bodies. It might be self evident, but considering the wide range of backgrounds the readers of this journal come from, please stress already in the introduction that multipurpose public spaces usually host many people where human bodies then significantly contribute to the overall sound absorption in the room. A reader should perhaps get more knowledge about the significance of the effect in rooms of different sizes than mentioned in the ‘Some simple examples’ section. Further, information on the effect of human bodies on room modes and speech intelligibility would be beneficial so readers could fully understand the context. This is crucial as this is a strong part of the conclusions but not elaborated in sufficient depth throughout the text.

A minor point I came across is the mention of the effect of wearing face coverings which is not clear. While it is clearly related to speech intelligibility per se , it is not clear in relation to 3D-PMAs so it could be omitted or explained in more depth, i.e. source vs receiver relation.

In the face of venues all over the world struggling to survive the financial effects of the pandemic and the subsequent lockdowns, it is hard to imagine they could afford additional costs to compensate for the acoustic effect of missing human bodies. I believe this aspect should be considered to strengthen the narrative. Perhaps suggesting how the 3D-PMAs could be used in the pre/post-pandemic life could be beneficial for the narrative.

Moreover, the narrative is mostly built based on the epidemiological measures implemented in Japan so far. This should be acknowledged in the title and discussed as a limitation.

Some minor typos are present, but the overall presentation and language are very good and very clear. I enjoyed reading it.

To conclude, I believe this commentary on potential usage scenarios of self-standing microperforated absorbers is a valuable contribution. Therefore, I would suggest to slightly revise the paper by giving more attention to the context and the narrative, mainly: different usage scenarous, the absorptive effect of human bodies compared to 3D-PMAs across the scenarios, the acoustic requirements of multipurpose halls, and the infection risks related to coronaviruses and porous materials, with clearly laid out limitations of the commentary, such as policy (focus on Japan and the temporary nature epidemiological measures) and economics.