Improving the Directionality of Low-Frequency Acoustic Radiation by a Finite Array of Quadrupolar Sources with Acoustic Metamaterials

Round 1

Reviewer 1 Report

Review of the manuscript entitled ‘Improving the directionality of low-frequency acoustic radiation by a finite array of quadrupolar sources with acoustic metamaterials’

In the manuscript, authors propose a new structure to obtain a directive radiation pattern by a quadrupole with high directivity without side lobes in a broad band of frequencies. The results are verified by numerical simulations and experiments. A quadrupole array is devised to obtain better results than the dipole array. The results show a new approach of acoustic radiation by quadrupolar metamaterials.

The study is interesting and deserves to be published. However, there are some missing informations and details.

- Authors say that the results show a new approach of acoustic radiation by quadrupolar metamaterials. Explain why it is a metamaterial?

- ‘number of linear array units can’t be very large. When the number of structural units is 176

large, the difference of the radiation angle of each unit should be considered.’ Could you give details? The radiation angle of each unit is increase or decrease?

- Fig. 6: There are some differences between the theoretical calculation results and the simulations although the main lobe is almost the same. Which ones? Give the differences?

- The manuscript must be checked for English spellings and re-write. Some sentences are not clear. For example: - ‘From the Figure 2, quadrupole-like radiation patterns can be obtained from the experiment, which is line with the numerical simulations …’

Author Response

Dear Editors and Reviewers:

Thanks for your letter and for the reviewers' comments concerning our manuscript entitled “Improving the directionality of low-frequency acoustic radiation by a finite array of quadrupolar sources with acoustic metamaterials” (ID: crystals- 2089358). Those comments are valuable for revising and improving our paper with important guiding significance. We have made correction according to the comments. The responds to the reviewer's comments are as follows:

Author Response File: Author Response.docx

Reviewer 2 Report

The authors in the first sentence wrote "An increasing number of researchers are paying attention to improving the directivity of wave propagation", but did not confirm this with a review of the world literature. The introduction and the literature review should be rewritten. The literature is scarce and there are no new articles.

The results section should be clearly presented and the results of the experiment, uncertainty should be stated there. In addition, validation of numerical models must be provided.

Author Response

Dear Editors and Reviewers:

Thanks for your letter and for the reviewers' comments concerning our manuscript entitled “Improving the directionality of low-frequency acoustic radiation by a finite array of quadrupolar sources with acoustic metamaterials” (ID: crystals- 2089358). Those comments are valuable for revising and improving our paper with important guiding significance. We have made correction according to the comments. The responds to the reviewer's comments are as follows:

Author Response File: Author Response.pdf

Reviewer 3 Report

Section 1 must be improved.

-       Authors should emphasize contribution and novelty, the introduction needs to clarify the motivation, challenges, contribution, objectives, and significance/implication. 

-       You should introduce the problem in more detail so that the reader is immediately clear about the purpose of your study. Specify better the essential elements of the problem.

-       You should add more information in the introductory part, you should add other works that have also addressed the problem.

-       The bibliography you have added must be enriched with works that have dealt with the topic recently. In the works you mentioned there is not even one in the dates ranging from 2018 to 2022.

-       Add references to works that have already dealt with the topic, for example:” Membrane-type acoustic metamaterial using cork sheets and attached masses based on reused materials”, “A review of acoustic metamaterials and phononic crystals”, “A review of tunable acoustic metamaterials”.

-       You must properly introduce your work, specify well what were the goals you set yourself and how you approached the problem.

-       At the end of the section, add an outline of the rest of the paper, in this way the reader will be introduced to the content of the following sections.

Section 2 must be improved.

-       In this section introduce your model, do not start by referring to a figure but describe in detail what you want to achieve, describe the structure you are elaborating.

-       Briefly introduce how acoustic metamaterials work.

-       Add references to support the statements that you added in this section.

-       review the format of the equations, in some cases there are problems.

-       You must properly introduce the equation, list in detail the variables contained in it with a concise description of the meaning. To make them more readable show them in a bulleted list. In this way the reader will be able to understand the contribution of each variable.

-       In section 2.2 you present your simulation model. a description of the hardware and software used for data processing is completely missing. Describe in detail the hardware used:  Extract this data from the datasheet of the hardware manufacturer. To make reading the specifications of the hardware more immediate, you can insert them in a table, listing the instruments used and the specific characteristics for each. Also, you should describe in detail the software platform you used.

-       Describe in detail which models you used to model your system and how you entered them into the Comsol software. Add screenshots of the settings you set. Readers should be able to repeat your experiment.

-       In figures 2 and 3 you already present some results, but in this section you should only describe the materials used and the methodologies adopted. Move results to a specific section.

-       In section 2.3 you describe the experimental approach you have adopted for the validation of the simulation model.

-       Describe better the specimen you have made, in particular better present the element in figure 2.d. This is the essential element better describe how it works, compare it with the drawing presented in figure 1.

-       Describe in detail the equipment used to make the measurements. Extract this data from the datasheet of the instrumentation manufacturer. To make reading the specifications of the instruments more immediate, you can insert them in a table, listing the instruments used and the specific characteristics for each.

-       Were the measurements performed in a reverberant room?

-       Did you use a reference standard to take the measurements?

-       Add a table with the characteristics of the material you have elaborated.

-       I could not find a detailed description of the evaluation metrics you have adopted. How will you measure your model's performance? This section is essential in order to demonstrate the effectiveness of your methodology. Furthermore, only by adopting adequate metrics will it be possible to compare your results with those obtained by other researchers.

Section 3 must be improved.

-       In this section you describe another model you have elaborated. I noticed you do the same in section 4. You always use the same format: theoretical model, simulation. Then it would be appropriate for you to add a subsection naming it Results and Discussion. In this way it will be clear to the reader that in each section there is first the presentation of the model and then the description of the results.

-       The previous considerations apply to this section as well.

Section 4 must be improved.

-       The previous considerations apply to this section as well.

-       A detailed discussion of the results obtained is missing. Try to summarize what was obtained and try to extract useful information from the work carried out. Also add bibliographic references to support your conclusions, to give more weight to your statements.

Section 5 must be improved.

-       In this section you should first summarize your goals at the beginning of your job. Then describe in detail the results obtained.

-       Paragraphs are missing where the possible practical applications of the results of this study are reported. What these results can serve the people, it is necessary to insert possible uses of this study that justify their publication.

-       They also lack the possible future goals of this work. Do the authors plan to continue their research on this topic?

30) Replace “Quan and others” with “Quan et al. [9]. I will not repeat this advice again, it also applies to the other occurrences.

80)Figure 1 must be improved. it is not sufficiently clear how the absorptive action of the material and how the HRs act. Try to describe it better.

92) Figure 2: Make Figure and Caption fit on the same page.

134) Rearrange subplot labels, rename them from left to right and top to bottom

Author Response

Dear Editors and Reviewers:

  Thanks for your letter and for the reviewevers' commments concerning our manuscript entitled“Improving the directionality of low-frequency acoustic radiation by a finite array of quadrupolar sources with acoustic metamaterials”.Those comments are valuable for revising and improving our paper with important guiding significance. We have made correction according to the comments. The respondsto the reviewer's comments are in following document.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Authors have correctly respond to the comments and remarks and have change accordingly the manuscript.

Reviewer 2 Report

The authors have corrected the article well

Reviewer 3 Report

The authors addressed the reviewer's comments with attention and modified the paper with the suggestions provided. The new version of the paper has improved both in the presentation and in the contents.

Minor revision

- Check the references for errors on author names and journal names

- Add the doi of the articles