Review Reports

Round 1

Reviewer 1 Report

Microfabricated electroacoustic transducers with perforated moving plates was investigated. Therefore, the analytical model of aminiature transducer with moving electrode was shown.  Numerical analysis and analytical models for acoustic pressure field and air gap are well described. However, there is no simulated results for pulse-echo response because it is one of the critical simulated results to show the proposed model verifcation. Therefore, authors need to provide mores simulated results.

 

1. I am wondering why authors focused moving electrodes. Is there any motivation ?

2. What are differences between moving and fixed electrodes ?

3. Please use Figure instead of Fig. Please check others in the manuscript according to MDPI author guidelines.

4. In Figure 6, why numerical values in higher frequency looks a little bit unmatched ?

5. Authors need to provide in detail analysis for previous transducer model research in Introduction section.

6. Authors need to emphaize the movitation in Abstract section.

7. Please use Equation instead of equation.

8. Authors need to provide the pulse-echo response in FEM analysis or numerical analysis.

None

Author Response

See our report in attached file

Author Response File: Author Response.pdf

Reviewer 2 Report

The present work describes a new fabrication process and characterization of high acoustic impedance and high attenuation multiphasic metallic backings for high-frequency P(VDF-TrFE)-based transducers.

 

 

 

In the abstract it is written : “Theoretical simulations showed that increasing the backing's acoustic impedance from 4.5 to 25 MRayl, increases transducer sensitivity by 5 dB, …………”. This sentence, written in this way and out of context, is false concerning  piezoelectric resonators electroacoustic transduction.

 

The abstract states : “ Microstructural characterizations of these new multiphasic composites showed that impregnation was incomplete and that a third air phase was present”. This is evident in these materials and the text does not quantify the final percentage of air in the samples taken.

 

The relative novelty is the use of bronze-specific metal sinters with optimized grain sizes to increase attenuation. The work finally compares two different composites, one with a tin infiltrate and one with an epoxy infiltrate. They have no control over the degree of infiltration and therefore the percentage of air inside. Some of the authors have worked on other multiphase ceramic-based materials as backing materials.

 

In the introduction, a well-known one-dimensional model of piezoelectric transduction - the KLM model - is used to conclude something very well known, namely the variation of the response of a piezoelectric resonator as the impedance of the backing is varied. It really does not belong in an article like this.

 

Section 3.1.3 is redundant. Figure 6 is sufficient.

 

When moving on to experimental characterization (3.2), it is concluded that the objective is best met with bronze- epoxy-air. It is the most mitigating. Good conclusion.

 

When transducers are manufactured and acoustic responses are measured (3.3), two types are made – bronze-tin-air and bronze- epoxy-air. There is a mistake with the maximum frequency of bronze- epoxy-air. It is not 21.5 MHz but 24.5 MHz.  Curiously, the solution with the best band is disregarded because it is less sensitive. Are there no simple solutions to amplify the signal by 12 dBs and therefore use this type of backing?

 

The reasoning between line 476 and line 483 is not acceptable. Even less so by making comparisons with a one-dimensional model with perfect solids.

Difficult to read. A rewrite by a scientist with English as a mother tongue is recommended.

Author Response

See comments in attached file

Author Response File: Author Response.pdf

Reviewer 3 Report

Overall, the contents of the manuscript are interesting, and the presentation quality is excellent. I am somewhat concerned about the narrow scope of the article, and thus the overall novelty, but I believe that the presented methods are useful enough to the reader to warrant publication.

Specific comments:

Minor edits/comments:

A note on why specifically 7.5 mm diameter width was chosen would be useful. This was mentioned briefly but I would like a bit more discussion.

Why was an excitation signal of a sinusoid with two cycles chosen? Why not more cycles?

p. 14 mentions "computer-controlled robot" - I assume you meant motorized stage. If so, I would change the terminology.

Major edits/comments:

In breaking with the quality of the rest of the paper, section 3.2 was very poor in comparison. The entire setup description was very "hand-waving" and a much more detailed description of the data acquisition and processing is necessary. Citing a previous paper is insufficient (especially since I could not find an English language version of the reference in several databases). My suggestion is to add an image of your experimental setup, as well as a schematic for your acquisition and processing technique. Additionally, it was stated that "The longitudinal wave velocities measured for dense bronze and brass agreed with the literature and thus validated the measurement setup". Having similar wave velocities as a single source (and a very poor one at that, as the front page of a website was linked as the source and not the direct source of the literature values) does not validate the experiments. Literature values for wave velocity on different materials can vary wildly and can't be used as validation for experimental technique.

I would like to see experimental validation of the choice to use a 15 MHz receiver. Yes, the spectral characteristics of the transmitted signal can be altered by the medium, but such a drastic difference in center frequency of the transmitter and receiver will require justification.

 

Author Response

Se comments in attached file.

Author Response File: Author Response.pdf

Reviewer 4 Report

This work develops composite materials as the backing for high frequency transducers. The materials have high acoustic impedance and are conformable to some extent, these are two advantages. However, several issues need to be addressed before further consideration:

1. The acoustic impedance of the backing material is important. The acoustic impedance of the active material is around 4.5 Mrayl, and the backing in this work is above 25 Mrayl. This impedance mismatch causes the acoustic energy reflects back, therefore sensitivity is increased and bandwidth is decreased. The selection of the backing should balance the sensitivity and bandwidth, please add more details about why this high impedance backing is needed for your applications.

2. The SEM images and simulation results for longitudinal velocities show that the composite is not homogeneous. Do the authors think it will affect the performance of the transducer? Since the reflected acoustic energy could not be homogeneous and make the acoustic field of the transducer to be non-homogeneous. It is recommended to provide hydrophone measurements.

3. Figure 8(b) also shows the effect of inhomogeneity. The red line will be more linear if the backing material is homogeneous. Is there any reason to make it inhomogeneous? Maybe in the manufacturing process, please add it in the manuscript. 

4. Figure 7, why the attenuation in 40 MHz is higher than 50 MHz?

5. Figure 11, the dynamic range is 40dB, I am assuming that the sensitivity of the transducer is not satisfied, maybe it is because the performance of the active material is not good. it is recommended that the authors add some descriptions about why this active material is selected and what is the specific application to selecting this backing for this active material.

Good

Author Response

See comments in attached file.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Authors defended my questions very well so I think the paper can be published with some minor comments. Thus, there are some remaining questions before publications.

1. Why high frequency PVDF-TrFE transducer instead of low frequency one ? Is the movitation for imaging on small animal, skin, or eye imaging applications ?

2. In Line 37, please correct >20 MHz to > 20 MHz. Please check others in entire manuscript.

3. In Line 122, please correct Fc=27 MHz to Fc = 27 MHz. Between the equal indicator, there are supposed to be space.

4. Please add ref. (Special attention is paid to the perpendicular~) with ref. (https://www.sciencedirect.com/science/article/abs/pii/S0263224116306157)

5. In Figure 7, why two different frquencies (50 and 15 MHz) transducer were used for measurement ?

 

None

Author Response

Thank you for the comments and pleas find in attached file our answers.

Author Response File: Author Response.pdf

Reviewer 2 Report

Now it's Ok

Author Response

Thank you very much

Reviewer 3 Report

The changes made to the manuscript were sufficient to advance it to publication. I am still not convinced that utilizing a 15 MHz receiver was the correct choice. I also do not believe that the experimental methods were the best choice for this work, but the results are good enough to warrant publication.

The quality of English language in the manuscript is mostly fine. It might benefit from having a native speaker review the manuscript.

Author Response

Thank you very much for your comment and please find our answer (in italic) to your following question

The changes made to the manuscript were sufficient to advance it to publication. I am still not convinced that utilizing a 15 MHz receiver was the correct choice. I also do not believe that the experimental methods were the best choice for this work, but the results are good enough to warrant publication.

Authors are partly in agreement with this remark because the choice of a transducer in reception with a center frequency at 15 MHz is not optimized and we could surely increase the frequency range for the calculation of the acoustic attenuation coefficient tion with another transducer in reception (for example with a 20 MHz transducer?). But here the main interest is to be able to measure this coefficient in the frequency range that interests us and typically up to about 25 MHz. It is for this reason that we subsequently did not optimize the experimental conditions.

 

Reviewer 4 Report

Accept as it is

Author Response

Thank you very much