Review Reports - Methodology for Testing Acoustic Absorption of Lightweight Fabrics with 3D Microstructures Using Impedance Tube

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

There should be a more complete characterization of the tested fabric.
What is the weave of the fabric? What is the linear density of the yarns? How many warp and weft threads are there in each fabric?
Please explain what is visible in Figure 1b.

I suggest that the Results in Table 5 be shown on a graph – this will better illustrate the differences between the samples.

Standard deviation from the measurements should be added to Tables 4, 5, 6, and 7. Standard deviation bars should also be added to the graphs showing these results.

Why are there results for 8 flat samples in Figure 6? As I understand it, there are 4 samples but measured with 2 methods.
Figure 6 is not legible.

There is no explanation why the absorption magnitude decreases at a frequency of around 1000 Hz.

Why do the MP.R measurements in Figure 7 not show a reduced acoustic absorption magnitude? Explain this relationship and phenomenon.

The differences are statistically significant.

The conclusions are general. There is a lack of explanation for why such dependencies occur.

Author Response

Comments 1: There should be a more complete characterization of the tested fabric.

What is the weave of the fabric? What is the linear density of the yarns? How many warp and weft threads are there in each fabric?

Response 1: Agree. We appreciate the proposal to include more information about the fabrics very interesting; this way they are better characterized. The following data related to the 4 types of fabric has been completed in Table 1 (Characteristics of tested textile sample): Composition, structure, number of threads-warp, number of threads-weft, yarn count warp and weft, weight per unit area and contraction.

Comments 2:Please explain what is visible in Figure 1b

Response 2: Agree. Effectively, the microscope photo is not clear and requires a brief explanation. In the most illuminated area of the picture, series of 3 threads intertwined perpendicularly can be seen. Over them, a series of thinner threads forms a dark mass, which appears in the shape of an ellipse. This darker “ellipse” coincides with the part of the textile that is white, as seen in image 1a. In the upper right of the image, it can be seen how this mass passes over the intertwined part. The white area appears as something opaque to the light received from below the sample, while the darker threads allow light pass though. A shorter explanation has been introduced in the text for better reader comprehension. Starting from line 101

Comments 3: I suggest that the Results in Table 5 be shown on a graph – this will better illustrate the differences between the samples.

Response 3: Agree. It is very accurate to suggest making a graph with the results from Table 5 to better see the differences. We have taken advantage of the idea to organize two graphs, reverberant chamber and impedance tube showing together the practical absorption coefficients from flat samples and pleated samples. A graph with those results has been introduced in the text. Figures 7 and 8

Comments 4: Standard deviation from the measurements should be added to Tables 4, 5, 6, and 7. Standard deviation bars should also be added to the graphs showing these results.

Response 4: Agree. We appreciate the comment about Tables 4, 5, 6, and 7; this completes the information properly. The values have been included in the corresponding tables

Comments 5: Why are there results for 8 flat samples in Figure 6? As I understand it, there are 4 samples but measured with 2 methods. Figure 6 is not legible.

Response 5: Agree. We apologize for this part of the text where the number of samples and tests in each one is not clear.

There are four flat samples that have been tested with two different methods: reverberation chamber (MF1R, MF2R, MF3R, and MF4R) and impedance tube (MF1I, MF2I, MF3I, and MF4I). The same has been done with the pleated samples: MP1R, MP2R, MP3R, MP4R, MP1I, MP2I, MP3I and MP4I. This figure has been changed and also figure 7. Colored lines have been added to make it more legible. Now they correspond to figures 9 and 10.

Comments 6: There is no explanation why the absorption magnitude decreases at a frequency of around 1000 Hz.

Response 6: Agree. Thank you for your excellent contribution. We appreciate your contribution and, in the future, we will expand our research to better understand these issues and try to resolve or limit them as much as possible. Indeed, it is not explained in the article and it is necessary to clarify it. It is known that vibrations of the solid phase of poro-elastic material samples in impedance tubes, like the ones we have tested, can significantly influence their acoustic absorption. This usually happens in partial frequency bands, so this is probably the phenomenon we are observing in the decreases around 1000 Hz. This behavior is related to the size of the sample and its boundary conditions within the tube, and is therefore not intrinsic characteristic of the material. This would explain why it does not appear in the tests managed in the reverberation chamber. A small paragraph has been introduced in Discussion section starting from line 385 to explain this phenomenon.

Comments 7: Why do the MP R measurements in Figure 7 not show a reduced acoustic absorption magnitude? Explain this relationship and phenomenon.

Response 7: Agree. The question is very accurate as it relates the two types of samples, flat and folded, with their respective acoustic absorption coefficients. We hope that this brief explanation clarifies the matter. It is a known fact that pleated textile materials absorb more than flat ones, as the pleats substantially increase the amount of material and therefore the surface exposed to the acoustic wave. This fact minimizes, in this case, the drop in acoustic absorption observed around 1250 Hz and 2500 Hz that the flat samples exhibit (fig 6). The MPR results should be considered real and reliable for this case. The MFI results are from the new method (not sufficiently tested) that attempts to obtain comparable results with the standardized test.

Comments 8:The differences are statistically significant

Response 8: Agree. We appreciate the recommendation to specify that the differences are statistically significant. Indeed, there were some sentences where this fact was not pointed out, and differences were discussed in a general manner. Throughout the text, it has been specified that the differences are statistically significant

Comments 9:The conclusions are general. There is a lack of explanation for why such dependencies occur.

Response 9: We appreciate this warning, which is why we have introduced the clarification to the question being asked and conclusions have been completed. This result has occurred because the fabrics are placed at a distance from the wall, and the impedance tube is not a tested system for measuring in this configuration

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript presents the comparison of two method for determination of sound absorption coefficient. By the way, some improvements could be done to clarify the description:

the bottom frequency range in impedance tube should be explained, typically small tubes are used for higher frequencies, above 1600Hz.

Figure1 - images should be improved, the structure and general view of textile fabric should be seen clearly,
Table 1 characteristic of fabrics should be completed- at least fabric construction
Table 5 Are the pleated samples tested with moulds? The scheme of fabric placement within the tube will be better,
Figure 6 - please rearrange the graphs - the lines/dotted lines are not easy to follow, the colours line could be more visibly and easy to diverse,
the statistic results should be elaborated not for all frequencies together, but divided into the groups (as described by analysis of flat fabrics results)

Author Response

Comments 1: The bottom frequency range in impedance tube should be explained, typically small tubes are used for higher frequencies, above 1600Hz.

Response 1: Agree. It is an interesting and controversial issue, so we appreciate the reviewer bringing it up. The question raised by the reviewer is very valid and understandable, since there are impedance tube models that perform measurements using two different diameters where the working frequency ranges overlap by at least one octave. However, the use of two different diameters is recommended but not essential. According to ISO 10534-2 standard, section 5.2, the lower working frequency of the impedance tube (fi) is limited by the uncertainty of the signal processing equipment and the separation between the two microphone positions.

We do not want to leave this situation unexplained, so we are providing you with an example given by the impedance tube equipment manufacturer and distributor.

Low Frequency Uncertainty

The uncertainty in Region 1 of Figure 1 manifests itself as a random error superimposed on the mean SAC result, as seen in the figure below for a sample of 25 mm foam. The results in this figure were obtained using microphone spacings of 29.2 and 76.2 mm. Except for a small amount of random error in the 29.2 mm data, the results are identical for both spacings. If the random error is undesirable, frequency smoothing of the SAC data can be performed.

Our tube may be used to obtain data from 50 Hz to 5650 Hz.

(there is a figure, you can see in the attach file)

However, to be on the safe side, we used a slightly lower frequency range (100-5000 Hz).

Comments 2: Figure1 - images should be improved, the structure and general view of textile fabric should be seen clearly.

Response 2: Agree. Certainly, the photos need to be improved so they can be understood correctly. So, the definition of the photos has been improved and a brief explanatory note has been included in the text. Starting from line 101

Comments 3:Table 1 characteristic of fabrics should be completed- at least fabric construction

Response 3: Agree. We consider the advice to include more information about the fabrics very interesting; this way they are better characterized. The following data related to the 4 types of fabric has been completed in Table 1 (Characteristics of tested textile sample): Composition, structure, number of threads-warp, number of threads-weft, yarn count warp and weft, weight per unit area and contraction.

Comments 4: Table 5 Are the pleated samples tested with molds?

Response 4: Agree. We regret that the sample placement process was not clearly understood and appreciate the question. The question is efficient since the text does not specify that the molds are removed from the back of the tube and are used only to give shape. The explanation of the placement of the pleated samples in the tube has been expanded in the text. Section 2.2.3. starting from line 223.

Comments 5: The scheme of fabric placement within the tube will be better,

Response 5: Agree. Thank you very much for the suggestion, as a sketch is the most appropriate way to show it. A schema representing the placement of the samples in the tube has been included in the text. Figure 6

Comments 6: Figure 6 - please rearrange the graphs - the lines/dotted lines are not easy to follow, the colors line could be more visibly and easy to diverse.

Response 6: Agree. We apologize for this figure where the lines are not enough clear. This figure has been changed and also figure 7. Colored lines have been added to make it more legible. Now they correspond to figures 9 and 10.

Comments 7: The statistic results should be elaborated not for all frequencies together, but divided into the groups (as described by analysis of flat fabrics results)

Response 7: Agree. It is a good suggestion to organize the text in a systematic way that aids better reading. It has been introduced into the text starting from line 475