JP2508397B2 - Sound absorber - Google Patents

Description

The present invention relates to a sound absorber used for indoor acoustic design, outdoor noise prevention, etc., and more particularly to a sound absorber exhibiting a high sound absorption coefficient in a low frequency range.

[Conventional technology]

Conventionally, various sound absorbing materials have been used, but all of them have a good sound absorbing coefficient in the high frequency range, and the sound absorbing coefficient is extremely lowered in the low frequency range of 315 Hz or less. Therefore, even if the conventional sound absorbing material is used as it is, it is not so effective in absorbing sound in the low frequency range.

Therefore, the present applicant has previously made a large number of acoustic tubes each having a cylindrical space with one end opened and the other end closed and having different lengths so that the openings are in the same plane and the length is random. , A sound absorbing body preferably arranged in accordance with a quadratic residue series was developed and a patent application has been filed [Patent application 1-
169140 (JP-A-3-33897)]. The typical sound absorber proposed in this patent application has the structure shown in FIGS. 11 and 12. That is, the sound absorbing body indicated by reference No. 1 as a whole has a large number of acoustic tubes 3 each having a cylindrical space formed by a wall 2 and having one end (an upper end in the drawing) opened and the other end closed, and the openings have the same surface. The sound absorbing surface is the surface where the open ends are arranged. Many acoustic tubes 3 have the same cross-sectional area, but their lengths are different, and different lengths are randomly arranged. Furthermore, as shown in FIG. 12,
A part of the long acoustic tube is bent and arranged behind the short acoustic tube, thereby reducing the thickness l of the sound absorber.

Here, as a method of randomly arranging the acoustic tubes 3 having different lengths, the length of the acoustic tubes 3 is set to an integral multiple of this unit length with a certain unit length h as a reference, and the arrangement is performed. It is preferable to determine according to the quadratic residue series.
The sound absorber in Fig. 12 also uses the array of this quadratic residue series. The square residue series will be briefly described below.

Let p be a prime number and a be an integer that is relatively prime with p. When the quadratic congruence equation x ² ≡ a (mod p) has a solution, a is said to be a quadratic residue modulo p.

As an example, the quadratic residue sequence when the prime number p = 7 is as follows.

^{x 2 ≡a (mod 7) x} = 0 → 0 2 ≡0 x = 1 → 1 2 ≡1 x = 2 → 2 2 ≡4 x = 3 → 3 2 ≡2 (9 = 1 × 7 + 2) x = 4 → 4 ² ≡2 (16 = 2 × 7 + 2 ) x = 5 → 5 ² ≡4 (25 = 3 × 7 + 4 ) x = 6 → 6 ² ≡1 (36 = 5 × 7 + 1 ) The square residue series in the case of p = 7 is one-dimensional and becomes 0,1,4,2,2,4,1,0,1,4,2,2,4,1, .... In two dimensions, the 7 × 7 matrix shown in Table 1 is repeated. Here, the horizontal 1st row and the vertical 1st column (the leftmost column) are the numerical values of the above-mentioned one-dimensional array, and the other parts are the sum of the numerical values of the corresponding 1st row and 1st column (from 7 The value is a value obtained by subtracting a multiple of 7 from the case (smaller) or the sum (when larger than 7).

The depth of the acoustic tube 3 in the sound absorber 1 shown in FIG. 11 is determined by the two-dimensional array of the square residue series, and the numerical value described in the opening of the acoustic tube 3 in FIG. These are the numbers in the table. This numerical value indicates the ratio of the depth of the acoustic tube. For example, in Fig. 12, the depth indicated by the numerical value "2" is 2h, the depth indicated by the numerical value "3" is 3h, the numerical value is indicated by "6". The depth is 6h. As a concrete numerical example, for example, the cross section of the acoustic tube 3 may be 70 × 70 mm, h = 70 mm.

In this sound absorbing body 1, the surface on which the openings of a large number of acoustic tubes 3 are arranged acts as a sound absorbing surface, and the sound wave incident on the sound absorbing surface,
In particular, it can absorb sound waves in the low frequency range and is extremely effective as a sound absorber for sound waves in the low frequency range. This is because the phase of the sound wave that is incident on and reflected by the sound absorbing surface of the sound absorber changes depending on the depth of the acoustic tube 3 at the reflection position, so that sound waves having different phases interfere with each other, and this interference causes a sound absorbing effect. It is thought to occur.

However, this sound absorber 1 has a large thickness, for example, in the above example of h = 70 mm, the thickness is about 28 cm, and there is a problem that the internal space is very complicated and is not easy to manufacture. .

[Problems to be Solved by the Invention]

Therefore, it is an object of the present invention to develop a sound absorbing body which has a sound absorbing characteristic equivalent to that of the above developed sound absorbing body, and which has a thin sound absorbing body and is easy to manufacture.

[Means for solving the problem]

As a result of earnest studies to solve the above problems, the present inventors have found that
By attaching a Helmholtz type resonator to the end of an acoustic tube consisting of a cylindrical space with one end open, a relatively low resonance frequency can be obtained while using a short acoustic tube, and the diameter of the thin tube that touches the mouth of the resonator can be obtained. Focusing on the fact that the resonance frequency can be easily changed by adjusting the length and the length, and by using a short acoustic tube equipped with this Helmholtz type resonator instead of the long acoustic tube without a resonator,
The present invention has been completed by finding that a sound absorbing body having the same sound absorbing characteristic as that of the above-described developed sound absorbing body can be formed while reducing the thickness.

That is, the present invention relates to a resonator-equipped acoustic tube having a cylindrical space having one end opened and the other end terminated by a Helmholtz type resonator, and a Schrader type acoustic tube having a cylindrical space having one end open and the other end closed. A number of acoustic tubes without resonators are arranged so that their openings are in the same plane, and the preset first resonance frequencies of a large number of acoustic tubes are arranged randomly on the surface of the openings. The sound absorbing body is

 Hereinafter, the present invention will be described in more detail.

First, the structure and acoustic characteristics of the acoustic tube terminated by the Helmholtz resonator used in the present invention will be described.
FIG. 1 is a sectional view showing an example of an acoustic tube terminated by a Helmholtz resonator (hereinafter referred to as an acoustic tube with a resonator), and FIG. 2 is a front view thereof. The acoustic tube 13 forms a tubular space having a square cross section by the plate material 12, one end (left end) of which is entirely opened to form an opening 14, and the other end is closed by an end wall 15 and a partition wall provided with a hole 16 in the middle thereof. 17 is provided, and the partition wall 17 and its inner part (right part) are Helmholtz type resonators.
Make up eighteen. On the other hand, FIG. 3 shows the sound absorber 1 shown in FIG.
Ordinary acoustic tube that composes the above, that is, a Schrader type acoustic tube not provided with a resonator (hereinafter referred to as an acoustic tube without a resonator) 3
This acoustic tube 3 is formed by simply forming a cylindrical space having a square cross section by the plate material 2, and opening one end (left end) of the whole as an opening 4 and closing the other end by an end wall 5. Is.

The acoustic impedance Z _m ′ seen from the opening 4 of the resonatorless acoustic tube 3 of length L _L as shown in FIG. 3 is given by Z _m ′ = jρc · cot (KL _L ) ... (1) To be Further, the acoustic impedance Z ₁ seen from the resonator inlet of the Helmholtz type resonator 18 located at the end of the acoustic tube 13 with a resonator shown in FIGS. 1 and 2 is Z ₁ = −j [ωm / p + ρc { γcot (kL) -cot (γkL)} ……
It is given by (2).

Here, m = σ (T + δ), γ = ω / ω ₀ , k ₀ = ω ₀ / c, δ = 1.70r (1-0.43r / a), r = (S / π), a = (S _S ), p = S / S _S , L is the length of the resonator, S _S is the cross-sectional area of the acoustic tube, S is the area of the hole 16 in the resonator, k is the wave number, and T is the hole 16 in the resonator. , C is the speed of sound, and ω ₀ is the root of (T + δ) / (pL) = cot (k ₀ L) / k ₀ L.

Impedance of a sound tube whose terminating impedance is Z ₁ ,
That is, the acoustic impedance Z _m seen from the opening of the acoustic tube in which the Helmholtz resonator is attached to the few ends of the acoustic tube of length D is Z _m = ρc {Z ₁ cos (kD) -jρc・ Sin (kD)} / {-jZ ₁ sin (kD) + ρc ・ cos (kD)} (3) where the minimum frequency of Z _m ′ and Z _m , that is, the first resonance frequency is the same. Thus, if T and S are adjusted, a short resonator-equipped acoustic tube having the same characteristics as a long resonator-less acoustic tube is obtained.
You can get 13.

The sound absorbing body of the present invention is a sound absorbing body developed previously, that is, a sound absorbing body in which a large number of resonator-less acoustic tubes having different lengths are randomly arranged. In a long-length acoustic tube without a resonator, a short acoustic tube with a resonator (shown in FIG. 1) having the same first resonance frequency as that of the acoustic tube is used instead. As a result, the length of the acoustic tube that constitutes the sound absorber can be shortened, and the overall thickness can be reduced.

In a preferred embodiment of the present invention, as shown in FIG. 11, in a sound absorbing body in which a large number of resonatorless acoustic tubes are arranged in a two-dimensional array of quadratic residue series, Instead of a long (eg 3 or more) acoustic tube,
A short acoustic tube with a resonator having the same characteristic (for example, a length of 2) is used.

[Action]

In the sound absorber having the above structure, the surface on which the openings of a large number of acoustic tubes are arranged acts as a sound absorbing surface, and sound waves incident on the sound absorbing surface, particularly sound waves in the low frequency range, are absorbed. This is because the phase of the sound wave that is incident on and reflected by the sound absorbing surface of the sound absorbing body changes according to the acoustic impedance of the acoustic tube at the reflection position, so that sound waves with different phases interfere with each other, and this interference causes a sound absorbing effect. It is thought to cause.

Since the sound absorber of the present invention basically does not utilize the sound absorbing effect of the material itself such as the plate material, the end wall, and the partition wall forming the acoustic tube, the material forming the acoustic tube is not limited to the acoustic wave. Highly rigid metal material such as steel plate without internal loss can be used. In this case, there is no weather resistance and hygiene problems, and it can be installed outdoors or used in hospitals and food factories. . However, this constituent material is not limited to a rigid body.

Since the sound absorbing body has a structure in which a large number of acoustic tubes are opened on the sound absorbing surface, dust, dust and the like easily enter the acoustic tube, and cleaning is difficult. Therefore, the sound absorbing surface (opening of the acoustic tube) may be covered with a metal thin film such as stainless steel foil or aluminum foil or a thin film such as a plastic sheet. It was confirmed that the sound absorption effect in the low frequency range is not significantly affected even if such a thin film coating is applied. Since the sound absorbing body coated with a thin film is easy to clean and hygienic, it is particularly suitable for noise prevention in operating rooms of hospitals, food factories, and the like.

Further, instead of the thin film covering the sound absorbing surface of the sound absorbing body, a porous plate such as a glass wool plate, a rock wool plate, a sintered metal plate or a metal fiber plate can be arranged. Arranging this porous plate is preferable because it exhibits a high sound absorption coefficient over all frequencies from the low frequency region to the high frequency region. This porous plate may be placed as it is, but the outer circumference is PV
It is preferable to cover with a plastic film such as F (polyvinyl fluoride film) for easy handling and cleaning.

〔Example〕

 Embodiments of the present invention shown in the drawings will be described below.

FIG. 4 is a schematic plan view of a sound absorbing body according to an embodiment of the present invention, FIGS. 5 and 6 are cross-sectional views taken along the line VV, VI-VI and FIG. 7 of FIG. 4, respectively. Is a schematic perspective view of the sound absorbing body, and FIG. 8 is a schematic perspective view showing the sound absorbing body in a disassembled state. The sound absorber, which is generally designated by the reference numeral 21, is an array of a large number of acoustic tubes 23, each of which is composed of a cylindrical space formed by a plate member 22 and having a square cross section, and is made to have a constant thickness. Each acoustic tube 23 has an opening 24 at one end (upper end) and an end wall 25 at the other end.
A partition wall 27 having a hole 26 is provided in the middle.
The partition wall 27 and the inner portion thereof form a Helmholtz resonator 28. Each acoustic tube 23 is arranged such that its opening 24 is on the same plane, and the surface on which the opening 24 is arranged is a sound absorbing surface. The diameters of the holes 26 of the partition walls 27 provided in the respective acoustic tubes 23 are different so as to give different acoustic impedances. Depending on the acoustic tube 23, the partition wall 27 does not have any holes (such as the acoustic tube at the left end in FIG. 5), or has a hole that is approximately equal to the cross section of the acoustic tube (acoustic sound at the right end in FIG. 5). Tube) and these are acoustic tubes without a resonator. As described above, the sound absorber 21 is an array of acoustic tubes with a resonator and acoustic tubes without a resonator having different acoustic impedances (arrangement will be described later).

As shown in FIG. 8, the sound absorbing body 21 of this structure is composed of an upper unit 31 having a plurality of cylindrical spaces of fixed lengths having both ends opened, and a partition unit having a plurality of holes 26 having a predetermined inner diameter at predetermined positions. It can be easily manufactured by stacking 32 and a lower unit 33 in which a large number of cylindrical spaces having a fixed length whose upper end is opened and whose lower end is closed by an end wall are stacked. As the material of the plate member 22, the end wall 24, the partition wall 26, and the like that form the sound absorbing body 21, a steel plate, a metal material such as aluminum, a plastic material, or the like can be used.

Next, the size of each part of the sound absorber 21 and the arrangement of the acoustic tubes will be described. The sound absorbing body 21 of this embodiment has the same characteristics as a sound absorbing body (sound absorbing body 1 shown in FIG. 11) in which a large number of resonatorless acoustic tubes 3 shown in FIG. 3 are arranged so that their lengths follow a quadratic residue series. It was made to have. Table 2 shows the length (depth) of each acoustic tube of the sound absorbing body having a length according to the quadratic residue series for the prime number 7, and the numerical values in the table are values proportional to the length of the acoustic tube. Shows. The unit depth is 7 cm here.

The sound absorbing body 21 shown in FIGS. 4 to 8 is obtained by implementing the sound absorbing body having the structure shown in Table 2 above with a structure having a depth of 2 (= 2 × 7 = 14 cm). Since the sound absorbing body 21 of this depth can form a resonatorless acoustic tube up to a depth of 2, the depth of 0, 1, 2 in Table 2
The acoustic tube 23 corresponding to the non-resonator acoustic tube need not be terminated with a Helmholtz resonator. That is, the acoustic tube 23 corresponding to the depth 1 may have the partition wall 27 having no hole, and the acoustic tube 23 corresponding to the depth 2 may include
It suffices to make a hole in the partition wall 27 approximately equal to the cross section of the acoustic tube, and these acoustic tubes 23 become acoustic tubes without a resonator. Remaining depth
The acoustic tube 23 corresponding to the acoustic tube without resonator of 3,4,5,6 includes:
In order to obtain the same acoustic impedance in the acoustic tube 23 having a length of 2, a partition wall 27 having a hole 26 is provided in the acoustic tube and is terminated by a Helmholtz type resonator.

Table 3 shows the lengths and the first resonance frequencies of the resonatorless acoustic tubes having the depths of 3, 4, 5 and 6. FIG. 9 (a) shows that impedance is calculated by changing the variable of the resonator so that the resonance frequency of the acoustic tube with the resonator matches the set first resonance frequency of the acoustic tube without the resonator. (B),
This is shown in (c) and (d). Here, the partition wall 27 arranged in the acoustic tube 23 is set to D = L = 7 cm so that a resonatorless acoustic tube having a depth of 1 or 2 can be formed. In addition, the dimensions of other parts are as follows: thickness of partition wall 27 = 1.0 cm, cross section of acoustic tube 7 cm x 7
cm, and its cross-sectional area S _S = 49 cm ² . Table 3 shows the results of determining the area S of the hole 26 of the resonator with respect to the depth (L ₁ ) of the acoustic tube without the resonator, with the dimensions of each part being constant.

From the above results, the depth and hole diameter of the resonator-equipped acoustic tube 23 corresponding to the depth of the resonator-less acoustic tube are as shown in Table 4.

The sound absorbing body 21 of the embodiment shown in FIG. 4 to FIG.
The length D from the opening 24 of the acoustic tube 23 to the partition wall 27 is 7 cm, and the thickness T of the partition wall 27 is based on Table 4.
1 cm, and the length L from the partition wall 27 to the bottom wall 25 is 7 cm.
Then, the inner diameter (diameter) of the hole 26 in each acoustic tube 23 is set to the second
It is determined based on Tables and Table 4, and its outline is shown in FIG. In each of the acoustic tubes shown in FIG. 4, a portion where two diagonal lines intersect is a depth 0 portion, and a portion where one diagonal line is drawn is a depth 1 portion (a hole is formed in the partition wall 27). No.), and the part indicated by a double square is a part of depth 2 (when the partition wall 27 is fully opened).

This sound absorbing body 21 is used to form a soundproof wall by arranging a large number of them. At that time, the sound absorbing surface of the sound absorbing body 21 (the opening 24 of each acoustic tube 23) may be left as it is (the open state), or the sound absorbing surface may be made of a metal thin film such as stainless steel or aluminum, or a plastic sheet. It is also possible to dispose a thin film such as, and cover the opening of the acoustic tube for use. By arranging the thin film in this way, cleaning becomes easy.

It is also possible to cover the sound absorbing surface of the sound absorbing body with a porous plate for use. The porous plate used here is
A glass wool plate, a rock wool plate, a sintered metal plate, a metal fiber plate, etc. can be used as they are or covered with a plastic film. Further, instead of the porous plate, a perforated plate material made of hard board, gypsum board, plywood or the like may be used. When a porous plate is also used in this way, the sound absorbing effect is further enhanced.

Next, the results of a sound absorption test using the sound absorber 21 having the above structure are shown. The sound absorber 21 having the structure shown in FIG. 4 to FIG.
The plate 22 and the end wall 25 are 0.7 mm steel plates, and the partition wall 27 is 10 mm thick.
It was made of acrylic plate. The thickness of this sound absorbing body 21 is about 14 cm.
Is. For comparison, we also manufactured a sound absorber with 0.7 mm steel plates and acoustic tubes without a resonator arranged as shown in Table 2. In this sound absorber, the deepest part of the acoustic tube is 6 (× 7 cm), so its thickness is about 42 cm. For both sound absorbers, JIS-
Figure 10 shows the results of measuring the normal incidence sound absorption coefficient according to A1405. As can be seen from Fig. 10, both the sound absorber with the acoustic tube with resonator (thickness 14 cm) and the sound absorber with only the acoustic tube without resonator (thickness 42 cm) have almost the same sound absorption coefficient above 200 Hz. Have Therefore, although the sound absorbing body according to the embodiment of the present invention has a thickness of only 14 cm, it is possible to obtain the same sound absorbing coefficient as that of the sound absorbing body composed only of the acoustic tube without the resonator, and the porous sound absorbing material is used. It has a very high sound absorption coefficient. The sound absorber consisting of a sound tube without a resonator has a thickest part of 42 cm (sound tube depth 6), and
Even if a part of this long acoustic tube is bent as shown in FIG. 12, its thickness is as large as 28 cm. Therefore, the sound absorbing body 21 of this embodiment can be made extremely thin as compared with these. .

It should be noted that at 80 to 160 Hz, the sound absorbing body of the present embodiment does not show a large sound absorbing coefficient like the sound absorbing body made of the acoustic tube without the resonator. This is probably because the acoustic impedance is high as shown in FIGS. 9 (a) to 9 (d). However, in the actual use, the sound absorbing body 21 of this embodiment also uses a large number of sound absorbing bodies arranged side by side. In that case, it can be expected that a certain large value can be obtained even within this frequency range.

In the above embodiment, the acoustic tube formed in the sound absorbing body has a square cross section, but the cross section is not limited to a square, but may be rectangular, triangular, circular, or the like. Further, the length and other dimensions are merely one example in the above embodiment, and can be changed appropriately.

〔The invention's effect〕

As described above, in the sound absorber of the present invention, by arranging the Helmholtz type resonator at the end of the acoustic tube, it is possible to obtain the same characteristics as the acoustic tube without the resonator while shortening the length of the acoustic tube. It is possible to obtain a high sound absorption coefficient from the low frequency range to the intermediate frequency range in the same way as a sound absorber composed by arranging a number of acoustic tubes without resonators. Since materials can be used, it is easy to manufacture and has excellent weather resistance. It is also possible to reduce the weight by using a plastic material. Therefore, the sound absorbing body of the present invention has an effect that it can be used very effectively in a low-frequency sound absorbing structure indoors and outdoors.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing a resonator-equipped acoustic tube used in the sound absorbing body of the present invention, FIG. 2 is a front view thereof, FIG. 3 is a schematic sectional view of a resonator-less acoustic tube, and FIG. FIG. 5 is a schematic plan view of a sound absorbing body according to an embodiment of the invention, FIG.
7 is a schematic perspective view of the sound absorbing body, FIG. 8 is an exploded perspective view of the sound absorbing body, FIG. 9 (a), ( b), (c), and (d) are graphs showing the acoustic impedance of the acoustic tube, FIG. 10 is a graph showing the measurement results of the normal incidence sound absorption coefficient of the sound absorbing body, and FIG. 11 is a patent application developed earlier. FIG. 12 is a schematic perspective view showing the sound absorbing body, and FIG. 12 is a perspective view of the XII-XII section of FIG. 1 ... Sound absorber, 2 ... Plate, 3 ... Acoustic tube, 4 ... Opening, 5 ...
… End wall, 12 …… Plate, 13 …… Acoustic tube, 14 …… Opening, 15 ……
End wall, 16 ...... hole, 17 ...... partition, 18 ...... Helmholtz type resonator, 21 ...... sound absorber, 22 ...... plate material, 23 ...... sound tube, 24 ...... opening, 15 ...... end wall, 26 …… hole, 27 …… bulk, 28 …… Helmholtz resonator, 31 …… upper unit, 32 …… bulk, 33 …… lower unit.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-2-212895 (JP, A) JP-A-61-185793 (JP, A) JP-A-3-33897 (JP, A) JP-A-49- 110112 (JP, A) Actual opening Sho 60-135409 (JP, U) Actual opening Sho 63-14300 (JP, U)

Claims (2)

(57) [Claims] 1. An acoustic tube with a resonator comprising a cylindrical space having one end opened and the other end terminated by a Helmholtz type resonator,
A large number of Schrader-type resonatorless acoustic tubes, each of which has a cylindrical space with one end open and the other end closed, and whose openings are flush with each other A sound absorber that is arranged so that the resonance frequency is random on the surface of the opening. 2. The sound absorbing body according to claim 1, wherein the plurality of acoustic tubes are arranged so that their first resonance frequencies and the plurality of resonatorless acoustic tubes are arranged so that their lengths follow a quadratic residue series. A sound absorber characterized by being arranged so as to match the first resonance frequency of a corresponding acoustic tube.