Phase Synchronisation for Tonal Noise Reduction in a Multi-Rotor UAV^†

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors,

Your paper is concise, well structured, very interesting and relevant to UAVs active noise control. As an aircraft emissions and UAV teacher myself, I appreciate your focus on this topic. I believe that some modifications to your manuscript will be needed to make your paper suitable for publication. For your review and consideration, I have the following recommendations:

1. The paper title is very good. It highlights all important things and tells reader what to expect.
2. Maybe you can add one-sentence statement of novelty versus prior work in Abstract. Although the full paper includes psychoacoustic annoyance analysis, the abstract does not mention human-perception metrics. The abstract is succinct but packs in many numeric details. Consider dropping one or two of the less critical figures (e.g. “third harmonic…5.3 dB”), so the abstract can focus on the top‐line finding and its significance.
3. Introduction: In Equations (1) and (3), you use two different multiplication symbols.
4. What about practical implementation discussion? Real UAVs experience greater speed fluctuations and nonlinear motor dynamics than in the lab.
5. In conclusion you did not suggest anything for the future work or possibilities of next research according to your results.
6. The Journal of Sound and Vibration paper by Shao & Lu (2022), experimentally investigates phase synchronization in multi-rotor drones and reports tonal noise reductions across all azimuths (DOI:10.1016/j.jsv.2022.117199). I recommend adding this reference and compare those results with yours.

I hope that these comments will be helpful.

Reviewer

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Congratulations on this clearly presented experimental study. The results are particularly good and provide evidence on the use of phase synchronisation as an active noise reduction strategy for UAVs. The work addresses a timely and relevant subject with practical implications for UAV noise control.

 

In the study, experiments using an electronic phase locking system evaluate how relative phase angle, propeller spacing, and blade geometry can impact tonal noise levels. The results show that increasing phase Delta Phi to 90 degrees leads to significant SPL reductions due to destructive interference from coherence effects. Two and five bladed propellers were used in the experiments showing similar trends on reported SPL behavior and displayed directivity patterns.

 

I provide below a series of comments and suggestions aimed at further strengthening the manuscript. I believe the work deserves publication in MDPI after some minor corrections and revisions. In particular, the study could benefit from supporting theoretical modeling to help generalize the results or predict performance beyond the given configurations.

 

Results are presented from page 7 in the submitted version of the manuscript.

 

• The authors emphasize that the acoustic characteristics of the propellers are analyzed from a time-frequency perspective using the continuous wavelet transform (CWT), and later refer to the mother wavelet function used. For the sake of clarity, could the authors briefly explain how the CWT was applied in this study, possibly by including a simplified model? A short discussion on the choice and role of the mother wavelet function would be welcome to understand its role in aeroacoustic signal processing.

 

• Figures 4 and 5, which illustrate the acoustic radiation field with striking constructive and destructive interference patterns, are particularly nice and add robustness to the study. As a conceptual insight for readers, could the authors briefly comment on whether these results, arising from phase synchronization that ensures coherence between sources, can be analogously related to Young’s double-slit experiments?

   

• In figure 6, panels (a), (b), and (c) compare the first (f1), second (f2), and third (f3) blade-passing frequencies (BPF) as a function of the relative phase angle between 2-bladed propellers. The results are summarized in Table 2; however, the actual frequency values for these BPFs are cannot found in the text unless I missed them somewhere. It may be useful to include a formula relating these frequencies to the rotational speed (rpm) and the number of blades to help readers better understand and reproduce the results. Additionally, there is no mention of whether any A, B, or C weighting procedures were applied to account for human auditory perception, which would be important for estimating the practical annoyance of the noise suppression.

   

• If any weighting procedure (A, B, or C weighting) was implicitly applied during the experiments or data processing, it would be helpful for the authors to explicitly mention this in the manuscript for the sake of completeness and to better relate this to human sound perception.

 

• It is clear that the highest sound pressure level corresponds to the first blade-passing frequency (f1), making it the ground component. However, for the sake of completeness, it would be nice if the authors could clarify whether the observed noise reduction with increasing relative phase angle Delta phi also persists when considering the combined superposition of the first three BPFs.

   

• To improve clarity and emphasize the total effect of phase synchronization, the authors might consider plotting the total SPL, adding SPLs from f1, f2, and f3, on the same plots as Figures 6(a) (b) (c). This would provide a more comprehensive view of the overall noise reduction achieved through phase synchronization.

   

• Figure 7 presents the directivity patterns. For the sake of completeness, it may be useful to increase the spacing above the legend. Additionally, while the legend appears to indicate more entries, only five directivity curves in both, the cases a) and b), are visible in the plot. Could the authors make clear whether some curves are overlapping, or I maybe just misunderstood anything?

   

• The directivity patterns in figure 7 appear to follow trends similar to those predicted by simplified acoustic source models such as dipoles and quadrupoles. The latter are often illustrated through analogies to tuning forks or coupled tuning forks (even more than 2 in some models). It may be worth briefly mentioning that rotating blades can, in some cases, be modelled by dipole sources, interacting dipoles, or even coupled quadrupoles, depending on the flow and configurations. Including such a theoretical reference could shed light on the shapes of the radiation patterns from a theoretical acoustics point of view.

   

• After figure 8, the manuscript starts with paragraph: “Figure 10 illustrates...” in reference to results for the 5-bladed propellers configuration. To improve clarity for the reader, it would be helpful to introduce this part as a new subsection ?, clearly marking the transition from the 2-bladed to the 5-bladed case...

   

• Could the experimental set-up for the 5-bladed propeller configuration be presented in a similar format to that in table 1, as was done for the 2-bladed case ? This would help reader clearly identify the parameters used and facilitate comparison between the two configurations.

   

• The directivity patterns shown in Figures 9 and 10 could potentially be interpreted using simplified acoustic source models such as dipoles or quadrupoles, as often done in theoretical aeroacoustics. While such analogies are more evident in simpler configurations, like the 2-bladed case, they become less trivial as the number of blades increases. Nevertheless, I suggest the authors to briefly comment about those aspects in the conclusions, as it may help connect the experimental observations with classical acoustic source theory and deepen the physical interpretation of the radiation patterns.

   

• In Figure 11, the psychoacoustic annoyance is shown as a function of the directivity angle. The trends are clear, however it would be helpful to comment further on the annoyance scale. Is there a threshold beyond which the perceived sound by human beings becomes intolerable or painful? Within Zwicker’s psychoacoustic annoyance formulation, could the authors clarify whether any scaling to physical levels, e.g., in phons, was applied?

• The conclusions could be revised or extended to incorporate some of the remarks and suggestions addressed above, particularly those related to the interpretation of the results through simplified acoustic models and the relevance of the psychoacoustic results.

I thank the authors for their careful experimental work and results. In my opinion, with the minor revisions suggested above, this paper will provide a robust contribution to the field and therefore, I strongly recommend it for publication in MDPI.

 

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The article is very good. The methodology followed is correct. The experimental part is well defined and implemented. However, I have a question: why did the authors choose the corotating system? The way the propellers are arranged in the wing system doesn't make sense.
Another problem with the article is its graphics. They are small, and the legends, as well as the curves for each parameter, are not clearly visible.

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

This article analyzes the hindrance of drone noise to its wide application. The article's investigation identified the main noise sources of unmanned aerial vehicles (UAVs), thereby determining the research objective: to study the impact of different parameters in phase synchronization methods on noise reduction. A large number of experiments were conducted. The effect of noise reduction was evaluated by using the psychoacoustic annoyance model and some classic indicators (SPL, Loudness). By analyzing the experimental results, the influence laws of various additional parameter changes on noise reduction in the phase synchronization method and the optimal noise reduction conditions were obtained.

However, there are still some problems with the article :

1. In Figure 11, the legend shows that the three lines are 0°, 24° and 36° respectively. However, in line 363 and the description in Figure 11, it is "0°, 18°, and 36°"
2. In the first and second experiments (the corresponding experiments of Figure 4 and Figure 5), Figure 5 has four relative phase angles (0°, 60°,75°, 90°) to present the variation trend of SPL, but only two phase angles were adopted in Experiment 4. The variation trend of Wx with the relative phase Angle was not fully demonstrated.
3. Figures 8 and 11 use the rectangular coordinate system, but the other graphs of the change in the directivity Angle θ in the article are all drawn in the polar coordinate system (Figures 7,9,10). It is suggested that the form of the drawing of the graph be unified.
4. In the Discussion section, the conclusion was put forward that the maximum noise reduction occurs at Δψ=180°/Nb. However, the article only contains experiments for Nb=2 and Nb=5. It is suggested to add experiments and charts for Nb=3 and 4. Otherwise, this can only serve as an inference rather than a conclusion.
5. The title of this article is "Phase Synchronization for Tonal Noise Reduction in Multi-Rotor UAV", but the article does not elaborate on how to achieveand keepthe phase synchronization of the propeller. The current article content merely discusses the influence of propeller phase difference on noise based on the premise of propeller phase synchronization. A flowchart of the phase synchronization method, model establishment, or an appropriate title is needed.
6. This experiment was conducted in the aeroacoustic facility, which can be regarded as a highly idealized environment. Whether the conclusion has limitations should be discussed in the Discussion section. For instance, consider whether the wind in the real environment has an impact on the phase synchronization system.
7. The conclusion needs to be more precise: The conclusion section should include (1) the discovery of practical problems, (2) the purpose of the research, (3) the significance of the research, (4) the summary of the experimental results, and (5) the indication of possible future work directions. The current conclusion section only contains (4) and is not concise enough.

In conclusion, this article has certain practical application guiding significance. It is recommended that it be reviewed again after the author has completed the revision.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The article has improved substantially. The methodology is correct, and the authors have explained the propeller assembly, its location, and rotation. Therefore, the work is well-constructed, and the conclusions are interesting for further studies of various kinds, such as propulsion systems in unmanned aircraft design and multirotor systems.