Exploratory Acoustic Investigation of Customizable 3D-Printed Hybrid Acoustic Materials (HAMs) through Interlaboratory Impedance Tube Measurements
Round 1
Reviewer 1 Report
The research article titled on “Challenges in the acoustic characterization of innovative 3D printed materials for architectural applications.” This article examines the sound absorption efficacy of 3D-printed hybrid acoustic materials intended for sound absorption and diffusion. The proposed hybrid materials were designed and manufactured parametrically using Computational Design (CD) techniques, Additive Manufacturing (AM) technology, and feedstock material with adjustable porosity, allowing for advanced control of acoustic performance via geometry-related sound absorbing/diffusing strategies. The novelty of the article seems valid and significant contribution can be obtained regarding acoustic architectural perspective. However, prior to the paper is accepted, minor changes are needed including:
1. Title: Perhaps the title needs revision. The term - “challenge” is not termed representing the whole content.
2. Abstract: Line 14 change Additive Manufacturing (AM) to 3D printing
3. Introduction: Line 15-50 The architectural design of 3D printing concrete building should be also addressed. References suggested include: Thermal and Sound Insulation of Large-scale 3D Extrusion Printing Wall Panel, 3D Printing Technologies in Architectural Design and Construction: A Systematic Literature Review
4. Material and Methods:
Line 123 revised “Materials and Methods” into “Methodologies”
Line 179-180: Elaborate properties of LW PLA
5. The discussion on larger-scale sample or real application for acoustic architecture should be addressed
6. Conclusion: The emphasis the best sample topology should be explained
Author Response
Dear Editor and Reviewers,
Thank you very much for the thorough evaluation of the manuscript and for your suggestions for modifications, which we gladly took into account in the revised version. The answers are given in blue color.
We hope that the manuscript could be accepted for publication after these improvements.
Best regards,
The authors
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This manuscript presents design, fabrication method, and measurement results of normal incidence sound energy absorption coefficient for 3D-printed samples. The manuscript showcases some possibilities of 3D-printed acoustic treatments that provide some absorption, and possibly also some scattering (which is outside the paper’s scope). Design and fabrication methods are explained. Measurements are made in two impedance tubes. I think that the manuscript would benefit from further work, deeper analysis and better presentation.
Overall, the paper raises some issues in acoustic characterization, and presents results by example. The issues raised are not systematically investigated in depth – instead I’d characterise the investigation as exploratory. For example, one circular sample each for the small and medium impedance tubes is chosen from each larger surface pattern – so the representativeness of the sampling choice is assumed to be adequate without systematic investigation. Another example: some discussion of apparent Helmholtz resonance is included, but there is no analysis of how/whether this effect would relate to a larger sample unconstrained by the tube walls. In other words, possibly the tube introduces a measurement artefact, a possibility that is central to the purpose of the manuscript but not considered in analysis. The analysis lacks a theoretical basis, and is essentially a broad discussion/description. Fundamentally, we do not know whether measurement artefacts affect results because the measurements are all that is presented, and this is a substantial epistemological vulnerability. Hence in its present form the manuscript has limited generalisability. To me it is like preliminary results from an ongoing project that might traditionally be presented at a conference, rather than in a journal.
While the study claims to be a 'round-robin' test, this is the smallest round-robin test that I have ever seen (n=2). As I understand it, the testers are the authors, and so are not actually independent. Readers are likely to be disappointed if that term is used, so I discourage its use for in the manuscript.
The manuscript’s introduction is very broad in scope. In my opinion the manuscript would be better served by a more focused introduction, so that the main point of the manuscript is better introduced. The broader context is not so important because the experimental work in the manuscript is limited in scope. This is just a suggestion.
If the purpose of the manuscript is to investigate the effect of measurement method, then it would have made sense to include a simple sample, preferably with a known normal incidence absorption coefficient spectrum, for baseline validation of the two tubes as a function of frequency. At frequencies where the tubes agree, then differences between the 3D-printed samples are more meaningful. This is a reasonable step to take in revising the manuscript.
In discussing the properties of the two types of ‘foam’ it could be helpful to include flow resistivity measurement results of fabricated samples (without the front surface). This could support interpretation of the data by others (and hence encourage citation).
Strangely, the definition of sound absorption coefficient is not correct (lines 253 and following). Firstly sound absorption coefficient is not a ratio of levels (because 'level' refers to a value in decibels). Secondly undue confusion is introduced by referring to normal incidence and perpendicular incidence - this makes no sense. If you mean normal incidence, just write normal incidence. Thirdly the meaning of '(without reversal)' is only somewhat clear when I think about why the definition doesn't make sense (the sound field in the tube is not just active, but also reactive). Please write a totally accurate definition that is perfectly clear!
The sentence starting line 258 does not make much sense (grammatically nor logically). The 'advantage' of the method is actually a potential advantage, not a proven advantage – and the investigation is needed to prove that the impedance tube can be used accurately to characterise these types of samples.
Repeated measurands are mentioned in the paragraph 295, but no results are presented. Furthermore, the manuscript does not state whether the samples were re-mounted into the tubes for the repeated measurements, or whether it was just another repetition of the test signal without touching the sample.
Section 3.5 is not well-structured. The section can be better written, with a more careful and deeper consideration of relevant theoretical expectations.
Line 335 claims that the reflections from rough surfaces will lead to non-plane-wave reflections at low frequencies. This seems highly unlikely considering the scale of the samples within the tube, which is a small fraction of a wavelength at low frequencies. If this is occurring in the impedance tube, then it should be demonstrated (because it seems to be a controversial claim).
The 60 mm impedance tube's upper cutoff frequency (for torsional modes) is about 2.8 kHz. Hence the claim that the method allows measurements up to 5 kHz (line 268) is incorrect. Also it does not make sense to present results outside of the valid range. Results are presented without acknowledging this obvious limitation. In line 349 and following it is suggested that the divergence at high frequencies is due to diffusive elements, but in fact a wide divergence is already expected above the tube 60 mm tube cutoff frequency - where plane wave propagation breaks down anyway.
The effect of sample orientation is only reported in the 35 mm tube, not the 60 mm tube. The effect is shown to be negligible. How then can the authors justify the claim that “there is potential for non-plane wave propagation” (line 351) when they present no evidence? If this is an issue, then the effect of sample orientation must be checked in the 60 mm tube – where non-plane propagation is expected above its cutoff frequency anyway. Please ensure that the discussion (and conclusion!) is linked to evidence rather than speculation.
Line 365: Reference 27 does not exist. I do not know on what basis the claim about damping efficiency is made because no other substantiation is provided. I encourage the authors to explicitly use acoustics theory to interpret their results, rather than merely mentioning references in discussion.
Minor issues:
The manuscript mainly uses centimetres, which is unusual in science. This might be an editorial style issue.
43 ‘one’ is not the right word
75 – ‘Fiber’ presumably should be singular (the plural of the compound noun is ‘composites’)
99 - imperfect
106 – syntax is not correct
113 – shall is not the correct meaning
115 – performance (singular) makes better sense
240 – there is no correlation analysis in the manuscript, so the term ‘correlations’ should not be used (also line 246).
319 – ‘frequency’
328 – ‘two times lower’ is poor expression (almost a paradox).
390 – ‘coherent’ is not the correct word
Author Response
Dear Editor and Reviewers,
Thank you very much for the thorough evaluation of the manuscript and for your suggestions for modifications, which we gladly took into account in the revised version. The answers are given in blue color.
We hope that the manuscript could be accepted for publication after these improvements.
Best regards,
The authors
Author Response File: Author Response.pdf
Reviewer 3 Report
The entire abstract section must be revised to give a brief explanation of the importance, investigations and outcomes with the advantages/significance of this research study. Also, the novelty of the study should be reflected in the abstract.
Present another detailed graphical abstract for this work, which could be more interesting for the reader community.
The authors should highlight the novelty of the work presented. Show your innovation in this work. Would you mind identifying blatant discrimination between your work and others?
Please add other more recent references in the Introduction section (more recent references not cited). I also suggest improving the literature review by discussing those mentioned above and other journal papers facing similar issues, and it is necessary to clearly mention the originality and novelty of this study in the end of this section.
The introduction section is not up to the mark. In the Introduction section, you need to connect the state-of-the-art to your paper goals only. Hence modify the entire section accordingly and present the specific goals/research objectives in the last part of the introduction section.
Revision of the conclusion section is much required. It is not showcasing the entire essence of the detailed work presented in the paper.
A complete revision of the document is necessary. Improved bibliography, as there are more recent references not cited.
In my opinion, the paper can be accepted after major corrections.
Author Response
Dear Editor and Reviewers,
Thank you very much for the thorough evaluation of the manuscript and for your suggestions for modifications, which we gladly took into account in the revised version. The answers are given in blue color.
We hope that the manuscript could be accepted for publication after these improvements.
Best regards,
The authors
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Thank you for carefully revising the manuscript. One comment I should make is that while reviewers do their best to provide constructive advice, the are not always right. For example, a reviewer asked to change 'Materials and Methods' to 'Methodologies' - but the journal style guide indicates 'Materials and Methods' (https://www.mdpi.com/journal/acoustics/instructions). Similarly, the idea that every reviewer's comment needs material added to the manuscript can result in an unnecessarily long paper (but sometimes a shorter paper is better). Anyway that's just a comment. Overall the revisions are reasonable, and I have confidence that the authors understand the limitations of their work. Some minor issues will be corrected by copy-editors. The addition of appendices helps substantiate the paper.
Reviewer 3 Report
Accept in present form.
