Patent Description:
In general, boards used while objects are placed thereon, such as sound absorbing boards installed in a trunk of a vehicle or the like, are configured to support the load of the objects even while having sound absorption performance by itself. To this end, sound absorbing boards are manufactured in a multi-layered structure or are made in a honeycomb structure that not only can reinforce structural rigidity, but also is lightweight and can be easily manufactured and used.

<CIT> discloses a load floor having sound absorption and insulation performances for a vehicle, including: a honeycomb structure manufactured in a sheet form; two glass mesh mats respectively installed on both side surfaces of the honeycomb structure; polyurethane sheets formed of a hard material and respectively provided to overlap the glass mesh mats; and two wall papers respectively provided to overlap the polyurethane sheets, wherein punched holes having a predetermined gap may be formed in at least one of the polyurethane sheets, wherein the punched hole may have a diameter in a range of <NUM> to <NUM>.

According to <CIT>, noise of a specific frequency band that enters into a trunk and is different according to a model of a vehicle can be removed through changing the number of gaps between and the diameter of the punched holes in the polyurethane sheets.

<CIT> discloses a method of forming an undercover for a vehicle using a honeycomb according by sequentially stacking polyurethane foam and glass mat on both sides of a honeycomb structure, and forming a plurality of through holes in the polyurethane foam and the glass mat provided on the opposite side so as to communicate with the honeycomb structure.

Further examples of the prior art are disclosed in Patent Document <NUM> to Patent Document <NUM>.

An objective of this invention is to provide a luggage board for a vehicle having sound absorption and insulation performance, in which since the luggage board is manufactured by producing a honeycomb structure using paper and adding polyurethane sheets to both surfaces thereof, lightweightness, sufficient structural rigidity, and a sound absorption effect can be obtained through the honeycomb structure, and the polyurethane sheet is formed on each of the both surfaces of the honeycomb structure so that a sound absorption and insulation effect can be further improved. Further, another objective of this invention is to provide a luggage board for a vehicle, in which since a plurality of perforations are formed in at least one of the two polyurethane sheets, the non-perforated polyurethane sheet blocks external noise introduced from the outside of a vehicle, sounds are absorbed through the perforated polyurethane sheet and the honeycomb structure, and thus the sound absorption and insulation performance can be improved. In particular, still another objective of this invention is to provide a luggage board for a vehicle having sound absorption and insulation performance, in which a frequency range of other specific bands which enter a trunk from the outside of the trunk for each vehicle model can be removed through the number of perforations, an interval between the perforations, and the diameter of respectively provided to overlap the glass mesh mats; and two wall papers respectively provided to overlap the polyurethane sheets, wherein punched holes having a predetermined gap may be formed in at least one of the polyurethane sheets, wherein the punched hole may have a diameter in a range of <NUM> to <NUM>.

An objective of this invention is to provide a luggage board for a vehicle having sound absorption and insulation performance, in which since the luggage board is manufactured by producing a honeycomb structure using paper and adding polyurethane sheets to both surfaces thereof, lightweightness, sufficient structural rigidity, and a sound absorption effect can be obtained through the honeycomb structure, and the polyurethane sheet is formed on each of the both surfaces of the honeycomb structure so that a sound absorption and insulation effect can be further improved. Further, another objective of this invention is to provide a luggage board for a vehicle, in which since a plurality of perforations are formed in at least one of the two polyurethane sheets, the non-perforated polyurethane sheet blocks external noise introduced from the outside of a vehicle, sounds are absorbed through the perforated polyurethane sheet and the honeycomb structure, and thus the sound absorption and insulation performance can be improved. In particular, still another objective of the perforations, and thus optimal noise improvement performance can be obtained depending on the vehicle model.

This invention relates to a sandwich panel for a luggage board of a vehicle and a method of manufacturing the same, in which as first and second bonding reinforcement layers are formed between first and second skin layers and a core layer, and an adhesive force between the first and second skin layers and the core layer is improved to prevent the first and second skin layers and the core layer from peeling, the lifetime of the luggage board is increased so that maintenance costs are reduced, and as the first and second rigidity reinforcement layers and the support part aim to reduce the weight of the sandwich panel for a luggage board while reinforcing the rigidity, the safety and reliability of the vehicle can be improved, fuel efficiency of the vehicle can be enhanced, and user convenience can be achieved.

This invention relates to a luggage board for a vehicle and relates to a luggage board for a vehicle, which is installed in a trunk of a vehicle as one of the interior panels for the vehicle. Such a luggage board for a vehicle includes: a lower panel installed in the trunk of the vehicle; a raising and lowering panel located on the lower panel; and an a raising and lowering device that raises and lowers the raising and lowering panel, wherein each of the lower panel and the raising and lowering panel includes a paper honeycomb member, chop mats laminated on both surfaces of the honeycomb member, polyurethane layers foam-molded on the surfaces of the chop mats, and surface materials adhering to the surfaces of the polyurethane layers.

However, in order to improve sound absorption performance, a sound absorbing board used for the existing luggage board or the like is made of, in addition to the honeycomb, various materials such as a natural fiber reinforced board, melt-blown polypropylene (PP), and a bubble sheet used for suppressing the dissipation of hydration heat generated inside poured concrete when the concrete is curing while the surface of the concrete is covered. Accordingly, the following problems occur.

In consideration of the above problems, an objective of the present disclosure is to provide a method of manufacturing a sound absorbing board using a honeycomb and a sound absorbing board using a honeycomb manufactured using the same, in which when a sound absorbing board, which manufactured by laminating glass fiber mats on both sides of a honeycomb structure that is light and has structural rigidity and foaming polyurethane sheets, is manufactured, a sound absorption coefficient is calculated using the diameter, the number, and the porosity of perforations, and the perforations having a diameter at which a sound absorption effect is obtained in a predetermined frequency band are formed in the polyurethane sheets, thereby further improving sound absorption performance in the predetermined frequency band and reducing the weight by making the thickness of the sound absorbing board small.

To achieve the objective, a method of manufacturing a sound absorbing board using a honeycomb, in which the sound absorbing board is formed by laminating glass fiber mats (<NUM>) on both surfaces of the honeycomb (<NUM>) and foam-molding polyurethane sheets (<NUM>) on surfaces, which are exposed to the outside, of the glass fiber mats (<NUM>) according to the present disclosure, includes: a first step of setting a frequency band; a second step of setting the cell size (S) and the thickness (t) of the honeycomb (<NUM>); a third step of setting a range of the thickness (t) of the polyurethane sheets (<NUM>), a range of the diameter d of perforations (<NUM>) to be formed in the polyurethane sheets (<NUM>), and the porosity of the perforations (<NUM>); a fourth step of obtaining a sound absorption coefficient according to a frequency change by calculating an inherent acoustic impedance of the perforations (<NUM>) using variables in the third step while at least the diameter (d) among the thickness (t) of the polyurethane sheets (<NUM>) and the diameter (d) of the perforations (<NUM>) is input differently; and a fifth step of marking positions of the perforations (<NUM>) in the polyurethane sheets (<NUM>) with at least one diameter d of the perforations (<NUM>) at which the sound absorption coefficient is high in the frequency band set in the first step among sound absorption coefficients obtained in the fourth step.

The honeycomb (<NUM>) may be made of paper, aluminum, or synthetic resin.

The cell size (S) may be in the range of <NUM> to <NUM>.

The glass fiber mats (<NUM>) may have a surface density of <NUM>/m<NUM> to <NUM>,<NUM>/m<NUM>.

The polyurethane sheets (<NUM>) may have a surface density of <NUM>/m<NUM> to <NUM>,<NUM>/m<NUM>.

The diameter (d) of the perforations (<NUM>) may be in the range of <NUM> to <NUM>.

An interval between the perforations (<NUM>) may be the same as the cell size (S).

The present disclosure includes a sound absorbing board using a honeycomb, which is manufactured by perforating the polyurethane sheets (<NUM>) with the marked diameter (d) of the perforations (<NUM>) in the above-described method of manufacturing a sound absorbing board using a honeycomb.

The perforations (<NUM>) may be formed using a laser, a roller or press equipped with several needles, or needles equipped with a servo or cylinder.

The sound absorbing board using a honeycomb may be a luggage board for a vehicle.

The above and other objects, features and advantages of the present disclosure will become more apparent to those of ordinary skill in the art by describing exemplary embodiments thereof in detail with reference to the accompanying drawings, in which:.

Hereinafter, exemplary embodiments of the present disclosure will be described in more detail with reference to the accompanying drawings. Prior to this, terms or words used in the present specification and the appended claims should not be interpreted as being limited to usual or dictionary meanings and should be interpreted as meanings and concepts corresponding to the technical spirit of the present disclosure according to the principle that the inventor may properly define the concepts of the terms in order to describe his/her own invention in the best way.

Thus, since the embodiments described in the present specification and configurations illustrated in the drawings are merely the most exemplary embodiments of the present disclosure and do not represent all the technical spirit of the present disclosure, it should be understood that various equivalents and variations that may replace the embodiments and the configurations are present at filling of the present application.

As illustrated in <FIG>, the objective of a manufacturing method according to the present disclosure is to perforate polyurethane sheets <NUM> to achieve optimal sound absorption performance in a set frequency band when a sound absorbing board B formed by laminating glass fiber mats <NUM> on both surfaces of a honeycomb <NUM> and foam-molding the polyurethane sheets <NUM> on the surfaces of the glass fiber mats <NUM>, the surfaces being exposed to the outside, is designed.

In this case, a sound absorption coefficient is calculated using a cell size S and a thickness T of a cell <NUM> of the honeycomb <NUM>, the number, a diameter d, and the porosity of perforations <NUM> formed in the polyurethane sheets <NUM> to pass through the cell <NUM>, a position having excellent sound absorption performance in a preset frequency band is perforated, and thus the sound absorption performance can be optimized. In this case, the perforations <NUM> are designed to have the same diameter d or at least two types of perforations <NUM> having different diameters are designed in some cases, and thus the optimal sound absorption performance can be obtained in the preset frequency band.

Hereinafter, these configurations will be described in more detail with reference to the accompanying drawings. Here, since the manufacturing method includes five steps, the manufacturing method will be described in detail for each step. Here, the sound absorbing board may be used wherever sound absorption performance and structural rigidity are required. However, here, an example of the sound absorbing board used as a luggage board will be described.

A first step is a step of setting a frequency band as illustrated in <FIG>. That is, since a frequency band varies depending on a place where the sound absorbing board B manufactured according to the present disclosure is to be used, the frequency band for enhancing a sound absorption effect is set among frequency bands generated at the place of use. For example, when the sound absorbing board B is applied to and used in a trunk lid, a frequency band to be removed among frequencies generated in a trunk is the same or similar when the vehicle model is the same but is different when the vehicle model is different. Thus, in this way, the frequency band for obtaining the sound absorption effect in the other frequency bands is set so that the sound absorption performance obtained as the perforations <NUM>, which will be described below, are formed is optimized in this frequency band.

A second step is a step of setting a cell size S and a thickness T of the honeycomb <NUM> as illustrated in <FIG>. This is a step of setting the size and the height of each cell <NUM> constituting the honeycomb <NUM>. Here, as illustrated in <FIG>, the cell size S represents a length between facing sides of the cell <NUM>, and the thickness T of the cell <NUM> represents the thickness of the honeycomb <NUM>.

This is for determining the structural rigidity, the thickness of the sound absorbing board B, and the like, which are required in the place of use, by limiting the size and the thickness of the cell <NUM> of the honeycomb <NUM> to be used in the sound absorbing board B according to the present disclosure. For example, as the thickness of the sound absorbing board B used for the luggage board is limited, the thickness T of the honeycomb <NUM> constituting the sound absorbing board B is also limited. Thus, in the second step, the cell size S and the thickness T are set such that the honeycomb <NUM> is configured to have a thickness T that meets a constraint having occurred in this way. Further, the cell size S is used as an interval between adjacent perforations <NUM> when the perforations <NUM>, which will be described below, are formed.

Meanwhile, in the exemplary embodiments of the present disclosure, although the honeycomb <NUM> may be made of various materials and used, it is preferable that the honeycomb <NUM> is manufactured using paper, aluminum, or synthetic resin that is light, is easily formed and may be manufactured and used in a desired size.

Further, in the exemplary embodiments of the present disclosure, it is preferable that the cell size S is in the range of <NUM> to <NUM>. This is for obtaining sufficient sound absorption performance in a specific frequency band, for example, medium frequency pattern noise (<NUM>), through an inner space of each cell <NUM> without weakening the structural rigidity of the honeycomb <NUM>.

In a third step, as illustrated in <FIG> and <FIG>, the range of the thickness t of the polyurethane sheet <NUM>, the range of the diameter d of the perforation <NUM> formed in the polyurethane sheet <NUM>, and the porosity of the perforations <NUM> are set.

This is for calculating the sound absorption coefficient in a fourth step which will be described below. In particular, it is preferable that by setting the thickness t of the polyurethane sheet <NUM> and the diameter d of the perforation <NUM> to a certain range, when the sound absorption coefficient is calculated in the fourth step, the optimal sound absorption coefficient may be calculated while changing the thickness t and the diameter d. For example, it is preferable that the diameter d of the perforation <NUM> is in the range of <NUM> to <NUM>. As described above, this is for further increasing the sound absorption effect by dissipating incident sound energy into thermal energy in each perforation <NUM> as at least one perforation <NUM> having a small diameter d is formed in the cell <NUM> corresponding to one space. In this case, it is preferable that an interval between adjacent perforations <NUM> is formed to have the cell size S of the cells <NUM> constituting the honeycomb <NUM>.

A fourth step is a step of calculating and obtaining the sound absorption performance, as illustrated in <FIG>, <FIG>. The sound absorption performance in this case is obtained through an inherent acoustic impedance of the perforation <NUM> used for calculating the sound absorption coefficient based on the variables input in the above-described steps.

Here, as illustrated in <FIG>, the inherent acoustic impedance of the perforation <NUM> may be obtained by Equation <NUM> based on Rao & Munjal's impedance model.

Here, ζ denotes an impedance, R denotes a real part of the impedance, X denotes an imaginary part of the impedance, M denotes the Mach number, σ denotes the porosity, t denotes the thickness (the thickness of the polyurethane sheet) of the perforation, and d denotes the diameter of the perforation.

Here, as described above, the sound absorption coefficient using the inherent acoustic impedance ζ of the perforation <NUM> is calculated by repeatedly inputting at least a different diameter d among the thickness t of the polyurethane sheet <NUM> and the diameter d of the perforation <NUM>. This is, for example, for forming only the perforation <NUM> having the diameter d of <NUM> in the polyurethane sheet <NUM> having a constant thickness t of <NUM>, for forming perforations <NUM> having diameters d of <NUM> and <NUM>, or forming the perforation <NUM> having a different larger diameter d. Thus, when the sound absorption coefficient obtained while changing the diameter d can produce the sound absorption effect in the above-described frequency band, the perforation <NUM> having this diameter d should be marked in a design process such that the perforation <NUM> may be processed in the polyurethane sheet <NUM>.

Further, as illustrated in <FIG>, the perforation <NUM> is formed in the surface of one of the polyurethane sheets <NUM> formed on both sides based on the honeycomb <NUM>. The perforation <NUM> is formed to pass through the polyurethane sheet <NUM>, and at least one perforation <NUM> is formed to communicate with the above-described cell <NUM>. This is for sometimes forming at least one perforation <NUM> in one cell <NUM> since even in a space having the same porosity, the sound absorption performance becomes more excellent as the number of fine perforations becomes larger. When the incident sound entering the perforation <NUM> passes through an edge (neck) of the perforation <NUM>, the perforation <NUM> causes the incident sound energy to be dissipated into the thermal energy due to friction, thereby achieving the sound absorption effect.

Meanwhile, as illustrated in <FIG>, the polyurethane sheets <NUM> are formed by foaming on both sides of the above-described honeycomb <NUM>, preferably, by foaming in a spraying method, and in this case, the sound absorption performance varies depending on the thickness t. Accordingly, the present disclosure is designed such that the range of the thickness t of the polyurethane sheet <NUM> is determined in advance, and the polyurethane sheet <NUM> is manufactured with a selected value within the range.

In this case, as illustrated in <FIG>, the polyurethane sheets <NUM> are formed by foam-molding on surfaces of the glass fiber mats <NUM> integrally laminated on both sides of the above-described honeycomb <NUM>, the surfaces being exposed to the outside. Here, the glass fiber mats <NUM> are formed in a long fiber state by melting glass in a platinum furnace and dropping the melted glass into a small hole and are made of, in a mat shape, glass fibers which are well known to be used as insulators, air filter media, electrical insulation materials, and sound absorbing materials because the glass fibers have excellent heat resistance, excellent durability, excellent sound absorption, and excellent electrical insulation. It is most preferable that the glass fiber mats <NUM> having a surface density of <NUM>/m<NUM> to <NUM>,<NUM>/m<NUM> are used.

Meanwhile, in the exemplary embodiments of the present disclosure, it is preferable that the perforations <NUM> are formed in a desired number in a desired position by using a laser, a roller or press equipped with several needles, or a needle equipped with a servo or cylinder when the sound absorbing board B according to the present disclosure is manufactured.

Further, in the exemplary embodiments of the present disclosure, it is preferable that the polyurethane sheet <NUM> having the surface density of <NUM>/m<NUM> to <NUM>,<NUM>/m<NUM> is used so that a sound absorbing board B that is lightweight and has maximized sound absorption performance is manufactured.

A fifth step is a step of marking and processing the perforation <NUM> in the polyurethane sheet <NUM> based on the above-described sound absorption coefficient obtained in the fourth step, as illustrated in <FIG> and <FIG>. In this case, in the position marking of the perforation <NUM>, the position of the perforation <NUM> having a diameter d at which the sound absorption performance is high in the frequency band set in the above-described first step is marked. With regard to the diameter d of the perforation <NUM>, as described above, one kind of perforation <NUM> having the same diameter d may be formed or two kinds of perforations <NUM> having different diameters d may be formed, depending on the frequency band, the thickness t of the polyurethane sheet <NUM>, and the layer configuration of the sound absorbing board B. <FIG> illustrates an example where one perforation <NUM> having the same diameter d is formed at the center of each cell <NUM>.

As described above, in the present disclosure, the sound absorption coefficient is calculated in advance using the variables such as the diameter of the perforation, the thickness of the polyurethane sheet, and the porosity, and the diameter and the thickness of the perforation are determined such that this sound absorption coefficient can increase the sound absorption efficiency in a predetermined frequency band. Thus, the sound absorbing board having optimal sound absorption performance can be designed.

The present disclosure includes a sound absorbing board manufactured based on the sound absorption performance obtained by the above-described method of manufacturing a sound absorbing board using a honeycomb. In this case, the sound absorbing board is manufactured with a variable value having the most excellent sound absorption performance in the predetermined frequency band. Further, it is preferable that such a sound absorbing board is used as a luggage board for a vehicle.

Meanwhile, a result of calculating the performance and the sound absorption coefficient for the sound absorbing board manufactured in this way will be described below.

<FIG> is a graph illustrating a result of calculating the sound absorption coefficient through input of a variable using the method of manufacturing a sound absorbing board using a honeycomb according to the present disclosure, wherein a horizontal axis represents a frequency (Hz) and a vertical axis represents the sound absorption coefficient. Further, the graph in <FIG> illustrates that the sound absorption coefficient varies depending on the variable used in design.

<FIG> illustrate a result of comparing the sound absorption coefficients of three types of luggage boards having different thicknesses according to a frequency change. <FIG> illustrates a luggage board. Further, <FIG> is an enlarged image of a bright part of the center of the luggage board of <FIG>, wherein the thickness of an edge was <NUM>, the thickness of a small square part in the center was <NUM>, and the thickness of the remaining part was <NUM>. In this case, the part having the thickness of <NUM> was not perforated, and the part having the thickness of <NUM> and the part having the thickness of <NUM> were perforated with diameters of <NUM> and <NUM>, respectively.

<FIG> is a graph illustrating the comparison between the sound absorption coefficients according to the example in which the actually used luggage board manufactured to have the thicknesses of <NUM>, <NUM>, and <NUM> is perforated as in <FIG> and a comparative example in which the luggage board is not perforated, wherein a horizontal axis represents the frequency, a vertical axis represents the sound absorption coefficient, a solid line represents a graph of the sound absorption coefficient according to the embodiment, and a dotted line represents a graph of the sound absorption coefficient according to the comparative example. As shown in <FIG>, it can be seen that the sound absorption coefficient according to the embodiment is higher than that according to the comparative example in a specific frequency range of about <NUM> to <NUM>. This means that, by setting the frequency band, the sound absorption coefficient can be improved in the corresponding frequency band.

<FIG> is a graph obtained by measuring the sound absorption coefficient of the luggage board as in <FIG> according to the frequency change and is a graph illustrating a comparison between an actual vehicle test and the sound absorption coefficient obtained by applying three peaks using the perforations having the diameters of <NUM> and <NUM> for the parts having the thickness of <NUM> and the thickness of <NUM>. Here, a horizontal axis represents the frequency, a vertical axis represents the sound absorption coefficient, dots marked in light blue corresponds to a graph of the sound absorption coefficient directly detected using sound absorption coefficient measurement equipment (Alpha Cabin), a red line graph and a light green line graph are obtained by theoretically calculating the sound absorption coefficient, wherein the thickness was <NUM>, and a pink line graph was obtained by theoretically calculating the sound absorption coefficient, wherein the perforation had the thickness of <NUM>. As a result, as shown in <FIG>, it can be seen that the theoretically calculated sound absorption coefficient and the actually measured sound absorption coefficient are substantially similar in the intended frequency range.

<FIG> illustrates a result a result of an actual vehicle test in a state in which the perforated luggage board according to the example and the non-perforated luggage board according the comparative example are actually mounted in a vehicle. In this case, a horizontal axis represents a frequency (Hz), a vertical axis represents a power based noise reduction (PBNR) (dB), a dotted line represents a graph according to the comparative example, and a solid line represents a graph according to the embodiment. In this case, an excitation point was set as a volume source (Q-SOURCE) in the trunk, and a measurement position was the center of a rear seat. As a result, as in <FIG>, it can be seen that the PBNR is improved by <NUM>-<NUM> dB at <NUM>-<NUM>,<NUM>.

<FIG> is a graph obtained by comparing the sound absorption coefficients according to the presence or absence of the perforation in a state in which the perforated luggage board according to the example and the non-perforated luggage board according to the comparative example are manufactured to have the same thickness of <NUM>. Here, a horizontal axis represents the frequency (Hz), a vertical axis represents the sound absorption coefficient, a solid line represents the embodiment, and a dotted line represents the comparative example. As a result, it can be seen that the sound absorption coefficient of the luggage board perforated through optimum perforation design according to the embodiment was improved by <NUM>% or more in an intended section.

<FIG> is a graph obtained by measuring the sound absorption coefficient in a state in which the perforated luggage board according to the example and the non-perforated luggage board according to the comparative example have the same thickness (<NUM>), but the diameters of the perforations are different from each other. Here, a horizontal axis represents the frequency (Hz), a vertical axis represents the sound absorption coefficient, a dotted line represents the actually measured sound absorption coefficient, a blue line represents an example where the luggage board was perforated with a diameter of <NUM>, a light red line represents an example where the luggage board was perforated with a diameter of <NUM>, and a green line represents an example where the luggage board was perforated with a diameter of <NUM>. As a result, it can be seen that the sound absorption coefficient obtained through the theoretical perforation calculation and the actual sound absorption result are similar to each other in the intended frequency section.

<FIG> is a picture illustrating the sound absorbing board manufactured according to the present disclosure. In this picture, a small perforation had a diameter of <NUM> and a large perforation had a diameter of <NUM>.

Claim 1:
A method of manufacturing a sound absorbing board using a honeycomb (<NUM>) in which the sound absorbing board is formed by laminating glass fiber mats (<NUM>) on both surfaces of the honeycomb (<NUM>) and foam-molding polyurethane sheets (<NUM>) on surfaces, which are exposed to the outside, of the glass fiber mats (<NUM>), the method comprising:
a first step of setting a frequency band for sound absorption performance;
a second step of setting a cell size (S) and a thickness (T) of the honeycomb (<NUM>);
a third step of setting a range of a thickness (t) of the polyurethane sheets (<NUM>), a range of a diameter (d) of perforations (<NUM>) to be formed in the polyurethane sheets (<NUM>), and a porosity of the perforations (<NUM>);
a fourth step of obtaining a plurality of sound absorption coefficients according to a frequency change by calculating a respective inherent acoustic impedance of the perforations (<NUM>) using the following Equation <NUM>: <MAT> <MAT> <MAT> Where ζ denotes the inherent acoustic impedance, R denotes a real part of the impedance, X denotes an imaginary part of the impedance, M denotes a Mach number, σ denotes the porosity of the perforations, t denotes a thickness of the polyurethane sheets and thus of the perforations, and d denotes the diameter of the perforations,
while, in order to obtain said plurality of sound absorption coefficients, at least one of the following variables: the thickness (t) of the polyurethane sheets (<NUM>) and the diameter (d) of the perforations (<NUM>), is input differently in the respective range set in the third step; and
a fifth step of marking positions of the perforations (<NUM>) on at least one of the polyurethane sheets (<NUM>) with at least one diameter (d) of the perforations (<NUM>) at which the sound absorption coefficient is high in the frequency band set in the first step among sound absorption coefficients obtained in the fourth step.