Effects of Coating Ageing on the Acoustic Properties of Norway Spruce (Picea abies)

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Review: Manuscript coatings-3922104

Title: Effects of Coating Ageing on the Acoustic Properties of Spruce Wood (Picea Abies)

 

This paper focuses on the influence of varnish on the physical, mechanical and acoustic characteristics of resonant spruce, with particular attention to its application value in the manufacturing of musical instruments. It is an interesting and well-written manuscript expanding the knowledge on the effects of modifying (varnishing) the surface of Norway spruce wood. However, the manuscript should be slightly improved, in my opinion. I present my substantive comments and proposal for editing corrections in accordance with the order of the article's chapters. 

 

Introduction

 

The introduction is well organized and concise.

Title and the whole article

The full correct English name of the wood tested according to the standard EN 13556 “Round and sawn timber - Nomenclature of timbers used in Europe” is Norway spruce. I suggest using this name in the title and throughout the text.

 

Materials and Methods

Line 62 - 68

To the extent possible, more detailed information should be provided about the origin of the spruce wood, its habitat, and the age of the trees. It should be specified from which part of the trunk the samples were obtained (the butt end, the trunk circumference, mature wood?).

From the point of view of the anatomical structure of the wood, what characteristics do the samples taken under study have (width of annual rings, proportion of early wood/late wood, regularity of annual rings).

The authors present the notations L, R, T, h in the text referring to Fig 1a, but these notations are not found in the respective figure. Do T and h designate the tangential direction?

 

Wood Surface Treatment

This section would benefit from expanded documentation of the two varnishes (N and P). Precise chemical compositions, manufacturer details, and solvent ratios are essential for evaluating reproducibility. Without these specifications, comparative analysis becomes challenging as formulation differences significantly impact penetration and curing behaviour.

 

Characterization of the coated surfaces

 

The authors mention in the title "effects of Coating Ageing". I recommend specifying whether they refer to artificial or natural aging.

What moisture content did the samples have before varnishing and then during the 300 days.

Since the authors performed numerous tests, we recommend that they enter the information in a table specifying how many samples were used in each test and the periodicity of test repetition, where applicable.

 

There is no information on whether and what statistical analysis was used to provide the obtained research results. This information should be supplemented.

 

Results

Gloss of finished surfaces

Can this physical parameter influence the acoustic behavior of wood?

Surface hardness

Did the authors also determine the thickness of the coating?

Was the König hardness measured on the wood substrate (control samples), on varnish films applied to glass slides? Can this data be included in the table?

If so, this information should also be specified in section 2.2.2.

 

Analysis of the free-free vibration resonance

In Fig. 2 it is necessary to explain the components of the experimental stand (what each component of the stand represents).

 

 

I hope my comments will be helpful in improving the manuscript.

Yours sincerely
Reviewer

 

Author Response

We sincerely thank the Reviewer for the thoughtful and constructive comments, which have helped us to improve the quality and clarity of our manuscript. Please find the attached word file, where we address all comments point by point. All corresponding modifications have been highlighted in red in the revised version of the manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

See the attachment.

Comments for author File: Comments.pdf

Author Response

We sincerely thank the reviewer for their careful reading of our manuscript and for the constructive and positive comments. We are grateful for the helpful suggestions, which have further improved the quality and clarity of our study. Please find in the attached word file detailed, point-by-point responses. All corresponding modifications have been incorporated into the revised manuscript, with changes highlighted in red.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript addresses a relevant and specialized topic—the long-term effects of nitrocellulose and polyurethane coatings on the acoustic properties of spruce resonance wood. Given the importance of spruce in musical instrument making, this study is of clear interest to wood science, materials engineering, and musical acoustics communities. The paper is generally well-written, logically structured, and supported by experimental data.  The 300-day aging period is beneficial because it gives information about long-term performance that is often not found in other studies. The manuscript contains valuable information, but several issues require attention before it can be considered for publication.  

• The study used a total of 20 lamellae, comprising seven each coating in addition to a control group. The limited sample size raises questions regarding statistical robustness. The authors must clarify whether the replicates were sufficient for making significant inferences and whether statistical analyses (ANOVA, post-hoc comparisons) were conducted.
• The study is based exclusively on mechanical and acoustic evaluations. The inclusion of microscopic or chemical characterization methods, such as SEM cross-sections and FTIR for monitoring curing, would have enhanced the explanation of the observed aging phenomena.
• The observation that nitrocellulose exhibited cracking at the lowest drop height consistently from the outset, without temporal variation, indicates a potential methodological flaw or inadequate sensitivity of the test for detecting weaker coatings. A detailed examination of the reasons for the test's inability to distinguish nitrocellulose performance over time is required.
• The authors linked the decrease in damping and the rise in ACE over time to the process of coating curing. Although plausible, alternative explanations, such as the redistribution of residual stresses and moisture stabilization at the film–wood interface, are inadequately addressed.
• The enhancement in ACE was characterized as a “restoration” of acoustic properties; however, the values did not attain control levels. This distinction requires clearer emphasis.
• The study, while motivated by applications in musical instruments, retains a degree of abstraction in its discussion. A direct interpretation of the implications of these changes on tonal quality—specifically brightness, sustain, and timbre—would enhance the study's impact.
• The use of terms like “hardness” in pendulum testing should be carefully phrased, since this is not absolute hardness but damping-related.
• The literature review is robust; however, it would be enhanced by incorporating more recent studies on long-term varnish aging in instrument acoustics, such as research on polymer cross-linking in conservation science.

Author Response

We sincerely thank the reviewer for the thorough and constructive comments, which have helped us improve the quality and clarity of our manuscript. We addressed each comment point by point. Please see our responses in attached word file. All corresponding changes have been incorporated into the revised manuscript, with modifications highlighted in red.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

This work focused on the long-term ageing effects of nitrocellulose and polyurethane coatings on the acoustic properties of spruce wood. It is of significant interest to the fields of wood acoustic for musical instrument making. The finding that surface hardness strongly correlates with acoustic damping and ACE is a notable and potentially useful contribution. Here are some suggestions for the authors as following.

1. It is recommended to add error bars to Figures 5-7.
2. Are there 7 samples for each of the two types of coatings? From the author's description, it seems that there are a total of 7 samples (L72). If so, how many samples are there in each of the two groups respectively? And from the Title of Table 1 (L90-91), it seems that there are 10 samples in each group.
3. Acoustic impedance is also a very important parameter that reflects the acoustic vibration performance of wood. It is recommended to supplement the analysis of acoustic impedance values in “3. Results”.
4. Some figures and tables in the manuscript are not placed appropriately. For instance, Figure 1 should be placed after L68, Figure 2 after L137, and Table 2 after L264.
5. Is ISO 4211-4 standard (1995) a currently valid standard? Please check whether it has been replaced by a new version.
6. The unit of the dynamic elastic modulus (E_n) value calculated according to Formula 1 should be Pa, not GPa. To make the unit of the value GPa, it should also be multiplied by 10^-9.
7. Was the acoustic vibration performance of the wood tested before coating? The acoustic vibration performance after coating should be compared with that before coating, rather than with other uncoated control.
8. When analyzing the effects of coating on acoustic performance of wood, the analysis of its influence mechanism was not in-depth enough, and the relevant evidence revealing the influence mechanism was insufficient.

Author Response

We sincerely thank the Reviewer for their positive and constructive evaluation of our manuscript. We address each comment point by point. Please see our responses in the attached file. All corresponding revisions have been incorporated into the revised manuscript. The modifications are highlighted in red in the revised version.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have responded point by point to all the recommendations made, and have improved the manuscript with the requested information. As a result, I consider that the manuscript can be published in its current form.

Reviewer 3 Report

Comments and Suggestions for Authors

The revised version of your manuscript demonstrates that you have carefully and thoroughly addressed all reviewer comments. The inclusion of statistical analyses (ANOVA with Tukey’s HSD), clarification of methodological details, and expansion of the discussion—particularly regarding residual stresses, moisture equilibration, and polymer cross-linking—have substantially improved the scientific quality and clarity of the work.

The terminology has been used with greater precision (e.g., clarification of the pendulum “hardness”), and the discussion now provides a meaningful link between the measured acoustic parameters and perceptual aspects such as tonal brightness and sustain. The addition of recent and relevant literature has further strengthened the study’s contextual foundation.

Overall, the revisions have been implemented effectively and comprehensively. The manuscript is now scientifically sound, clearly written, and suitable for publication in Coatings.