JP3296928B2 - Sound absorbing wall structure - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sound-absorbing wall structure provided at a position facing a noise generating source and reducing noise by absorbing and attenuating noise energy.

[0002]

2. Description of the Related Art FIG. 12 shows an engine undercover (JP-A-58-177781) as an engine noise reduction device provided directly below an automobile engine.
An engine undercover 1 fixed to the vehicle body is provided directly below the engine. The engine undercover 1 has a structure in which noise of the engine is hardly leaked to the outside of the vehicle body. That is, the engine under cover 1
A mat-shaped sound absorbing body 3 in which a sound absorbing material 4 such as glass wool or coarse felt is accommodated in a polyethylene film bag 5 is integrated on an upper surface of a synthetic resin main body 2 in which a number of sound absorbing chambers 2a are formed. In this structure, when the engine sound vibrates the polyethylene film 5 of the sound absorber 3 and passes through the sound absorbing material 4, energy is consumed, and the energy is further reflected by the wall surface of the sound absorbing chamber 2a. And the noise of the engine is attenuated.

FIG. 13 is a perspective view of a sound absorbing wall structure constituting an outer wall of a house. In FIG. 13 , reference numeral a denotes an outer wall board, and a gypsum board c, which is a Helmholtz plate provided with a large number of circular holes d, is provided inside the outer wall board a with an air layer b interposed therebetween. A sound absorbing material 4 such as glass wool is accommodated, and a sheet material e made of vinyl or paper is provided on the side of the gypsum board c facing the sound absorbing material 4 to prevent water from entering between the boards a and c. In the structure,
The energy of the noise is consumed by friction with the inner peripheral surface of the circular hole d, and is further consumed by passing through the sound absorbing material 4 in the air layer b to attenuate the noise.

The outer wall board a is made of aluminum,
In general, a heat insulating sound absorbing wall structure using an aluminum panel instead of the gypsum board c and using an aluminum foil for the sheet material e is also common. However, in the above-mentioned engine undercover, since the sound absorbing material 4 such as glass wool is indispensable, the cost is increased.
As the sound absorbing material 4, glass wool is optimal,
For workers in the manufacturing process,
There is a risk of dust pollution due to inhalation. In order to enhance the sound absorbing effect, the depth (height) of the sound absorbing chamber 2a may be increased, but there is naturally a limit due to the ground height of the vehicle body bottom surface.

In the sound absorbing wall structure constituting the outer wall of a house, a sound absorbing material such as glass wool is indispensable similarly to the above-mentioned problem of the engine under cover, so that the cost is increased and dust pollution is reduced. There is a risk.
From this point of view, a device for reducing the engine noise of an automobile without using any sound absorbing material has been proposed in Japanese Utility Model Application Laid-Open No. 59-1887.
No. 22 was proposed. As shown in FIG. 14 and FIG. 15, the engine under cover is made up of an outer under cover 6 and an inner under cover 7 which is a Helmholtz plate with a circular hole 8 disposed at a predetermined distance from the outer under cover 6. And friction loss on the inner peripheral surface of the circular hole 8;
The engine noise is reduced by the energy loss due to the passage between the covers 6 and 7 thickened by the air layer and the loss due to the reflection at the outer undercover 6. Reference numeral E is the engine body, reference numeral E ₁ denotes an oil pan, reference numeral M ₂ denotes a cross member.

The Helmholtz structure includes a first wall on the side facing the noise source having a large number of circular holes, and a second wall forming an air layer on the back of the first wall. A conventionally known structure in which sound is absorbed by energy loss of sound waves due to air friction on the inner peripheral surface of the hole and energy loss of sound waves due to reflection and transmission on the second wall surface.

[0007]

However, as shown in FIG.
4, the structure shown in FIG. 15 is preferable because it has an effect of reducing the engine noise to some extent without using any sound absorbing material such as glass wool, but the engine noise reduction effect is not sufficient, and the noise is reduced. A more effective means above was sought.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and has as its object to provide a sound-absorbing wall structure having very excellent sound-absorbing properties without using a sound-absorbing material.

[0009]

In order to achieve the above object, a sound absorbing wall structure according to claim 1 is a Helmholtz plate provided with a number of circular holes provided on a side facing a noise generation source. An inner wall, an outer wall which is an air chamber forming plate opposed to the inner wall at a predetermined distance, and a vibration film provided in close contact with a side of the inner wall facing the air chamber. . According to a second aspect of the present invention, in the sound absorbing wall structure according to the first aspect, the vibration film is formed of a sheet material having a low acoustic impedance such as a nonwoven fabric. According to a third aspect of the present invention, the sound absorbing wall structure according to the first aspect is applied to an engine undercover fixed to a vehicle body of an automobile and disposed immediately below an engine. According to a fourth aspect of the present invention, in the sound absorbing wall structure according to the first aspect, the peripheral portion of the circular hole formed in the inner wall is formed to extend toward the noise generation source. Further, in the sound absorbing wall structure according to the fifth aspect, an inner wall which is a Helmholtz plate provided with a number of circular holes provided on a side facing the noise generation source, and a number of circles are formed so as not to overlap the circular holes of the inner wall. A Helmholtz plate having a hole formed therein, the outer wall being an air chamber forming plate opposed to the inner wall at a predetermined distance, and at least one of the inner wall and the outer wall facing the air chamber and provided in close contact with at least one of the sides. And a vibration film having low acoustic impedance and good air permeability such as a nonwoven fabric. In claim 6,
The sound absorbing wall structure according to claim 5 is applied to an engine undercover fixed to a vehicle body of an automobile and disposed directly below an engine. According to a seventh aspect of the present invention, in the sound absorbing wall structure according to the sixth aspect, the outer wall is provided with an outside air intake for introducing outside air into an air chamber inside the outer wall.

[0010]

A large number of circular holes are formed in the inner wall of the Helmholtz plate, and the energy of the sound wave is consumed by the frictional loss of the air in the circular hole with the inner peripheral surface of the circular hole, and the vibration is further increased. It is consumed by vibrating the membrane, and is further consumed by being reflected inside the outer wall or passing through the outer wall. According to the second aspect, since the acoustic impedance of the vibrating membrane (nonwoven fabric) is low, the energy of the sound wave is consumed not only by the vibration of the vibrating membrane (nonwoven fabric) but also when passing through the vibrating membrane (nonwoven fabric). According to the fourth aspect, since the inner peripheral surface of the circular hole extends toward the noise source, friction loss of air in the circular hole with the inner peripheral surface of the circular hole increases, and energy consumption is correspondingly high. According to the fifth aspect, since the inside and the outside of the sound absorbing wall structure communicate with each other through the air chamber by the circular holes provided in the inner wall and the outer wall and the vibrating membrane having good air permeability, the sound absorbing effect is exhibited, And the heat inside is radiated to the outside. In the sixth aspect, the heat in the engine room is radiated through the communication path of the engine undercover. According to the seventh aspect, outside air is introduced from the air intake into the inside of the undercover as the vehicle travels, and the heat is released.

[0011]

Next, an embodiment of the present invention will be described with reference to the drawings. 1 to 5 show an embodiment in which the present invention is applied to an engine undercover of an automobile. FIG. 1 is a plan view of the undercover, and FIG. 2 is a longitudinal sectional view of the undercover (a line II shown in FIG. 1). 3 is an enlarged longitudinal sectional view showing a part of the undercover, FIG. 4 is a plan view showing an adhesion area of the vibration film, and FIG. 5 is a sound absorption of the undercover shown in the present embodiment. FIG. 6 is a graph showing sound absorption characteristics due to normal incidence, showing characteristics compared to a case where a diaphragm is not provided.

In FIGS. 1 and 2, reference numeral E denotes an engine body of an automobile, and an engine under cover 10 fixedly supported on a vehicle body is provided below the engine body E.
Are arranged substantially horizontally. Reference numeral E ₁ is the engine oil pan, reference numeral M ₁ denotes a side member extending in the front of the vehicle body, reference numeral M ₂ is cross member extending across the vehicle body to the left and right, numeral 20 is a unit part of the engine, the engine under cover 10 These It is arranged so as not to interfere with the member of (1).

The undercover 10 has a structure in which a synthetic resin inner cover 14, which is a Helmholtz plate, is integrated and integrated with a flat synthetic resin outer cover 12 by mechanical joining and fixing means such as rivets. An air layer 13 having a predetermined thickness is formed between the inner cover 12 and the inner cover 14. The material of the covers 12 and 14 may be a metal that is easy to process such as aluminum, iron, or brass, in addition to the synthetic resin.

On a substantially entire surface of the inner cover 14, a large number of circular holes 15 are formed at a predetermined pitch (not all the circular holes 15 are shown in FIG. 1; only some of the circular holes 15 are formed). (Shown), sound is absorbed by resonance absorption. That is, the circular hole 15
The air in the air layer 13 oscillates due to the vibration of the air, and energy loss occurs due to air friction on the inner peripheral surface of the circular hole 15 which can be regarded as a thin and short tube, thereby absorbing sound. The inside of the inner cover 14 (the side facing the air layer 13)
The vibrating film 16 attached by an adhesive 17 extends in close contact. That is, the sound wave causes energy loss by vibrating the vibration film 16 together with the air in the air layer 13, thereby absorbing the sound. Furthermore, sound waves transmitted through the air layer 13 are reflected by the outer cover 12 to cause energy loss, or transmitted through the outer cover 12 to cause energy loss and absorb sound.

As the vibration film 16, there are aluminum foil, polyester film, and the like, and other materials are not limited as long as they have excellent weather resistance and are hard to change over time. The vibration film 16 is formed as shown in FIG.
As shown in FIG. 4A, the sound absorption rate is better when the portion away from the peripheral portion of the circular hole 15 is bonded as shown in FIG. 4A than when the entire circular hole peripheral region is bonded. Is good.
This is because when the vibrating film 16 is bonded and fixed to the entire periphery of the circular hole, the rigidity of the vibrating film 16 increases, and the film 16 is less likely to vibrate. It is presumed that the energy loss is also small.

FIG. 5 is a diagram showing the sound absorption coefficient of the Helmholtz structure where no vibrating membrane is provided and the Helmholtz structure where a polyester film, aluminum foil and nonwoven fabric are used as the vibrating membrane. In the figure, the curve A is a 2.5 mm thick polypropylene molded plate having circular holes with an inner diameter of 10 mm provided at intervals of 20 mm, a Helmholtz structure having an air layer with a thickness of 12 mm, and having no vibration film. In the case of the above, the sound absorption characteristic diagram and the B curve show a polypropylene molded plate having a thickness of 1.5 mm in which circular holes having an inner diameter of 5 mm are provided at intervals of 15 mm, an air layer having a thickness of 12 mm, and a vibration film provided. Of the sound absorption characteristics in the case of the Helmholtz structure without
1.5mm thick with m holes at 25mm intervals
The sound absorption characteristics of a Helmholtz structure with an air layer of 24 mm in thickness and an aluminum foil of 25 μm in thickness, and the D curve shows an inner diameter of 12 mm
Sound absorption characteristic diagram of Helmholtz structure in which an air layer of 12 mm thickness is formed of a 1.5 mm thick polypropylene molded plate having circular holes formed at intervals of 25 mm and an aluminum foil of 25 μm thickness is adhered, E curve Is a 1.5 mm thick polypropylene molded plate having 12 mm inner diameter circular holes provided at 25 mm intervals, a sound absorption characteristic of a Helmholtz structure having a 24 μm thick air layer and a 23 μm thick polyester film bonded thereto. The figure and the F curve show that a polypropylene molded plate having a thickness of 1.5 mm, in which circular holes having an inner diameter of 12 mm are provided at intervals of 25 mm, has an air layer having a thickness of 12 mm and a thickness of 2 mm.
A sound absorption characteristic diagram of a Helmholtz structure to which a 3μ polyester film is adhered.
A polypropylene molded plate having a thickness of 1.4 mm provided at intervals of 5 mm, having an air layer of 12 mm, and a nonwoven fabric (14
5g / m ² ) is a sound absorption characteristic diagram of a Helmholtz structure bonded with a double-sided tape.

As shown by the curves A and B, the Helmholtz structure without the vibrating membrane has a low sound absorption coefficient and a frequency range of 1,50.
The sound absorption coefficient between 0 and 2,000 Hz is only about 40% at best. On the other hand, as shown by the C and D curves, the Helmholtz structure to which the aluminum foil is bonded has a frequency range of 500 to 2,000 Hz.
Sound is attenuated well, the C curve shows a sound absorption of approximately 50% to 70%, and the D curve shows a sound absorption of approximately 60%.
In the range of 500 to 2,000 Hz, the sound absorption coefficient does not change so much, and it can be said that the sound absorption characteristics are excellent.

As shown by the E and F curves, the Helmholtz structure provided with the polyester film has particularly excellent sound absorption in a predetermined frequency range. That is, in the E curve, the sound absorption coefficient at a frequency around 500 Hz is approximately 70%, the sound absorption coefficient at a frequency range of 1,000 to 1,250 Hz exceeds 80%, and the sound absorption coefficient at a frequency around 1,700 Hz is also 80%. It turns out that it is particularly effective in reducing the noise in the frequency range of 800 to 1,250 Hz, which is a problem. In the F curve, the frequency is around 1,250Hz and 1,500 ~
2,000Hz sound absorption over 80%, 1,250-2,000Hz
The sound absorption coefficient in the frequency range of
It can be said that this is effective in reducing the absorption of noise at or above Hz.

Further, the Helmholtz structure to which the non-woven fabric is attached has an excellent sound absorption coefficient particularly for sound having a frequency of 1,500 Hz or more, as shown by the G curve, and has a sound absorption coefficient of 20% to 50% in a frequency range of 1,000 to 1,500 Hz. Not so expensive,
Since the nonwoven fabric has a low acoustic impedance, it can be said that the nonwoven fabric has a sound absorbing effect. That is, since the nonwoven fabric has a low acoustic impedance, the energy of the sound wave is consumed not only by vibrating the nonwoven fabric, but also when passing through the nonwoven fabric, which has a sound absorbing effect.

FIGS. 6 and 7 show a second embodiment of the present invention. FIG. 6 is an enlarged plan view showing a part of an engine undercover which is a main part of the second embodiment. FIG. 7 is an enlarged vertical sectional view of the undercover (a sectional view along line VII-VII shown in FIG. 6). In the engine undercover in this embodiment, the outer peripheral portion of about one-fourth of the circular holes 15 formed in the inner cover 14 protrudes upward (noise source) to form a protruding portion (protrusion). Since the inner peripheral area of the circular hole provided with 18 is large, the energy loss due to friction on the inner peripheral surface of the circular hole due to the vibration of the air in the circular hole 15 is larger than in the above-described embodiment. Therefore, it has a structure excellent in sound absorption effect. The other parts are the same as those of the first embodiment, and the description thereof is omitted.

The diameter of the circular hole 15, the pitch of the circular hole 15,
Protrusions 18 relative to the height of protrusions (protrusions) 18 and the number of circular holes
The sound absorption characteristics at a specific frequency can be improved by appropriately adjusting the ratio and the like. 8 and 9 show a third embodiment of the present invention. FIG. 8 is a plan view of an engine undercover which is a main part of the third embodiment, and FIG. 9 is an enlarged vertical sectional view of the undercover (FIG. 2 is a cross-sectional view taken along line IX-IX shown in FIG.

In the undercover 10 according to the first embodiment, only the inner cover 14 is constituted by a Helmholtz plate having a large number of circular holes 15 formed on the entire surface. Then, like the inner cover 15, the outer cover 12 </ b> A has many circular holes 1.
5A is formed by the Helmholtz plate, and the inside of the inner cover 14 (the side facing the air layer 13).
Another feature is that the nonwoven fabric 16A having good air permeability is attached by an adhesive. That is, the inside of the engine room is configured to communicate with the lower portion of the under cover 10A by the circular hole 15, the air chamber 13, and the circular hole 15A of the under cover 10A, so that heat is not stored in the engine room. A circular hole 15 formed in the outer cover 12
A and the circular hole 15 formed in the inner cover 14 are shifted from each other so as not to overlap each other in the vertical direction as shown in an enlarged view in FIG.
As shown in FIG.
The sound that has entered the air chamber 13 from any of the above is reflected on the wall surface of the outer (inner) side cover 12A (14) to lose energy. As the sound indicated by the arrow B, the engine sound, the exhaust sound, and the tire running sound, which have passed through the under cover 10A and attenuated, are reflected on the road surface and are reflected in the circular holes 15A.
It is conceivable that something intrudes into the computer.

The air chamber 13 is opened in the front-rear direction of the vehicle as indicated by reference numerals 13a and 13b.
It is easy for air to flow in. Further, the outer cover 12
In front of the position where the inner cover 14 is disposed, the outside air intake S
Is provided so that outside air can be introduced into the air chamber 13 from the outside air intake S when the vehicle is running. The outside air introduced from the inlet S into the inside of the undercover 10A, as shown by the arrow in FIG.
Flows along the inside (along the inner cover 14 and the air chamber 13), and cools the inside of the air chamber 13 to efficiently radiate heat in the engine room.

In the third embodiment, the nonwoven fabric 1
6A is provided in close contact with the inner cover 14, but may be provided in close contact with the outer cover 12A instead of the inner cover 14, or the inner cover 14 and the outer cover 12A.
May be provided in both. In the above-described embodiment, the present invention is described as an engine undercover. However, the present invention can be applied to a wall of a house, a motor cover of a large blower having a large noise, and the like. FIG. 11 shows an embodiment in which the present invention is applied to a motor cover of a large blower, in which a blower 30 is disposed in a concrete cover body 32,
The motor 31 of the blower 30 is structured to be covered by a motor cover 34 composed of a double Helmholtz plate inside and outside.

[0025]

As is apparent from the above description, according to the sound absorbing wall structure of the present invention, the inner wall provided with a large number of circular holes, and the air layer formed on the back side of the inner wall by the outer wall. The Helmholtz structure, consisting of the following, not only consumes energy of noise, but also consumes energy by vibrating the vibrating membrane provided in close contact with the inner wall facing the air layer. Rate is high,
Noise can be effectively reduced without using a sound absorbing material such as glass wool. According to the second aspect, since the energy of the sound wave is consumed not only by the vibration of the vibrating membrane (nonwoven fabric) but also when passing through the vibrating membrane (nonwoven fabric), a high sound absorption coefficient is obtained, and the noise is more effectively reduced. Can be effectively reduced. According to the third aspect, the noise of the engine can be reduced by using the present invention as an engine undercover. Claim 4
In this case, since the inner peripheral area of the circular hole is large, the friction loss of the air in the circular hole with the inner peripheral surface of the circular hole increases, so that the energy of the sound wave is greatly consumed and the sound absorbing effect is excellent. In claim 5,
In addition to the effect shown in claim 1, since the inside and the outside of the sound absorbing wall structure communicate with each other via the air chamber in the sound absorbing wall structure, heat does not stay inside the sound absorbing wall structure. According to the sixth aspect, the heat in the engine room is radiated through the communication passage in the engine undercover (sound absorbing wall structure).
There is no heat in the engine room. According to the seventh aspect, the outside air is introduced from the air intake into the inside of the undercover and into the air chamber as the vehicle travels, so that the engine room is effectively cooled.

[Brief description of the drawings]

FIG. 1 is a plan view of an engine undercover according to a first embodiment of the present invention.

FIG. 2 is a vertical sectional view of the under cover (II-
Sectional view along II)

FIG. 3 is a partially enlarged longitudinal sectional view of the undercover.

FIG. 4 is a plan view showing an adhesion region of a vibration film.

FIG. 5 is a graph showing the sound absorption characteristics of the under cover according to the present embodiment compared with the case where no vibrating film is provided, in which the sound absorption characteristics at normal incidence are shown.

FIG. 6 is a plan view of an engine undercover according to a second embodiment of the present invention.

FIG. 7 is a sectional view of the undercover (the line VII shown in FIG. 6).
Sectional view along -VII)

FIG. 8 is a plan view of an engine undercover according to a third embodiment of the present invention.

9 is an enlarged vertical sectional view of the undercover (a sectional view taken along line IX-IX shown in FIG. 8).

FIG. 10 is a partially enlarged longitudinal sectional view of the under cover.

FIG. 11 is a sectional view of a blower cover according to a fourth embodiment of the present invention.

FIG. 12 is a longitudinal sectional view of a conventional engine under cover.

FIG. 13 is a perspective view of a conventional sound absorbing wall for a house.

FIG. 14 is a perspective view of an improved conventional engine undercover.

FIG. 15 is a sectional view of the engine under cover.

[Explanation of symbols]

 E Engine as a noise source 10, 10A Engine under cover 12 Outer wall as air chamber forming plate 13 Air chamber 14 Inner wall as Helmholtz plate 15, 15A Circular hole 16 Vibrating film 17 Adhesive 18 Projecting cylindrical portion

Claims (7)

(57) [Claims] 1. An inner wall which is a Helmholtz plate provided with a large number of circular holes provided on a side facing a noise generation source, an outer wall which is an air chamber forming plate opposed to the inner wall at a predetermined distance, and A sound-absorbing wall structure, comprising: a vibration film provided in close contact with a side of the inner wall facing the air chamber. 2. The vibration film according to claim 1, wherein the vibration film is a sheet material having a low acoustic impedance such as a nonwoven fabric.
The sound-absorbing wall structure according to the above. 3. The noise source is an automobile engine,
2. The sound absorbing wall structure according to claim 1, wherein the sound absorbing wall structure is an engine undercover fixed to a vehicle body and disposed directly below an engine. 4. The sound-absorbing wall structure according to claim 1, wherein a peripheral portion of the circular hole formed in the inner wall extends toward the noise source. 5. An inner wall which is a Helmholtz plate provided with a large number of circular holes provided on a side facing a noise generating source, and a Helmholtz plate formed with a large number of circular holes not overlapping with the circular holes of the inner wall. A low acoustic impedance of an outer wall, which is an air chamber forming plate opposed to the inner wall at a predetermined distance, and a nonwoven fabric or the like provided in close contact with at least one of the inner wall and the outer wall facing the air chamber. A sound-absorbing wall structure comprising a vibration film having good air permeability. 6. The noise source is an automobile engine,
6. The sound absorbing wall structure according to claim 5, wherein the sound absorbing wall structure is an engine undercover fixed to a vehicle body and disposed immediately below an engine. 7. The sound-absorbing wall structure according to claim 6, wherein the outer wall is provided with an outside air intake for taking outside air into an air chamber inside the outer wall.