Improved Sound Radiation of Flat Panel Loudspeakers Using the Local Air Spring Effect
Round 1
Reviewer 1 Report
The manuscript is well written and proceeds logically. It is easy to follow and comes to a reasonable conclusions. I have some comments to be addressed in the manuscript before it could be considered for publication.
- The analyses of frequency responses are fine and typical to evaluate the quality of a loudspeaker. However, the response in time is also very important for the quality and I would like to see the impulse response of the loudspeaker, at least in the "standard", "separation", and "optimised" versions. As the frequency response is more flat in the optimised version I assume that the time response is also shorter (indicating better transient response), but it would be nice to see the measured response.
- The directivity is not discussed at all (maybe it was mentioned in the introduction). At least some discussion on the directivity and the effect of proposed modifications/optimisation to directivity could be elaborated.
- Could you find any reason on the differences between the measurement and the simulation (Fig. 5)? I agree that they are "close enough", but could it be a diff in speed of sound or some other parameter in the simulation? I think simulations could be even better matched, although if no better match is found the current one is probably enough.
- Figure 11 is great and helps a lot of understanding of the behaviour of the whole system.
- What are k and m in Equation (1)? They are not explained and I do not understand what they are.
- There are several times written:"The following Figure xx...". The word "following" is there pointless and should be removed.
Author Response
Dear MDPI Team, thank you for this very detailed and comprehensive feedback. In the following document, we have addressed the main critical points of the paper and modified them in a newer version of this paper.
Please see the attachment
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript presents an alternative approach for optimizing the modal behavior through the additional air spring effect adding a suitably shaped back panel to form a shaped enclosure. At certain vibrational frequencies, the air compliance suppresses the front panel’s anti-phase components, thus minimizing minima in the frequency response. The approach is validated with the measured and simulated results for a simple prototype.
The main idea seems to be good and it works, at least to some extent.
However, the manuscript should be improved for better conciseness, clarity, and for a more solid scientific sound:
- Using the proposed technique, it appears that the air trapped between the front panel and the added rear panel reacts when it is compressed by the regions of the front panel coming backwards and squeeze it, but the same air can do nothing when the same regions go out inward direction outward.
So, there is a sort of direction-dependent counter reaction. In my modest experience, this produces a non-linear behavior that turns out in a not negligible harmonic distortion. Authors should at least comment about it.
- The material presented is quite extensive, and could be profitably divided into two manuscripts: the first could present the main idea, some simulations and the experimental validation of those simulations. The second could present the design of a flat panel speaker with the related optimization procedure and accurate validation, this time on the entire expected spectrum of the prototype optimized con the proposed procedure.
Minor:
- FEM, FEA, BEM, h-refinement, QQUAD8 TET10 etc. MUST be defined and, at the occurrence explained to the broader reader, BEFORE used.
- Figure 1, 5, 6, 7, 9, 10, 12, 14, 15, 18 better show the same frequency axis span.
-Please refrain to use bullet style “introduction”, local “conclusions” or similar.
- what is NFS in fig. 4?
- in Fig. 6, the modal zones should be indicate as later done in Fig. 7, and arrows should link deformed sections of the panel to the related modal region.
-Fig. 7 is difficult to read/understand. Moreover, what are SPL-Total, AAL-In, etc?
- in the vibration/sound field maybe better use the term “plate” instead of “panel”.
- subsection 4.3: 1) it misses any experimental section; 2) how equations (1) and (2) are related with the anyway not-explained assessment tool ACF?
- Citation style doesn’t include first author in the text before [].
-what does it mean “in-homogeneous enclosure shape”
-what are “mass points”?
-is it possible to “validate” by means of computer simulations? IMHO “validation” is given by experiments.
- by the way, an 855x467 panel does not have a ratio of 2 but instead of 1.83.
- in the manuscript, whenever a result is said to be sufficiently accurate, it would be useful to first specify the numerical limits of that sufficiency.
-in Fig. 12 it seems that the air stiffening attenuates all the vibration at certain frequencies (see e.g. 164 e 193 Hz), not only that of the regions in counter phase. At 82 Hz we even pass from the plate all in phase to regions in counter phase. The authors should comment on the result.
- what about the apex cross in the symbols list after the first author?
Author Response
Dear MDPI Team, thank you for this very detailed and comprehensive feedback. In the following document, we have addressed the main critical points of the paper and modified them in a newer version of this paper.
Please see the attachment
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The manuscript has been significantly improved and can now be published in its present form.
