Sound absorbing arrangement using a porous material

Abstract: A sound absorbing arrangement using a porous material has a sound absorbing plate (2) of a thin plate porous material made by partially heating for welding plastic particles and a supporting member (1) for supporting the sound absorbing plate and forming a back air space. Plural reflecting members (40) are disposed to be opposed to the surface of the sound absorbing plate. The sound absorbing arrangement has a superior sound absorption characteristic from lower frequencies to higher frequencies.

Description: BACKGROUND OF THE INVENTION Field of the Invention: This invention relates to an improvement of a sound absorbing arrangement to be placed around a noise generating source or in a propagation path of a noise, and more particularly relates to a sound absorbing arrangement using a porous material. Description of the Prior Art: PRIOR ART 1. Fig. 18 is a sectional view showing the construction of a conventional sound absorbing arrangement using a hard porous material as a first prior (prior art 1), and the figure also has an explanatory diagram for showing a sound pressure distribution of a sound wave to be input into the sound absorbing plate thereof. In Fig. 18 reference numeral 1 designates a sound insulator such as a wall; and numeral 2 designates a sound absorbing plate of a hard porous material made of plastic particles, a ceramic, foam metal or the like, for example. Reference numeral 11 designates a back air space of the sound absorbing plate 2; numeral 11a designates the thickness of the back air space 11; numeral 81 designates an input sound; reference character β designates an average input angle of the input sound 81; and character λ designates a wavelength of a sound wave having the highest sound pressure level among the input sounds 81. In the explanatory diagram showing a sound pressure distribution, mark + designates the operation of positive pressure on the sound absorbing plate 2; and mark - designates the operation of negative pressure on the sound absorbing plate 2. Arrows 85 and 86 designate directions of an input sound wave operating on the back air space 11 through the sound absorbing plate 2. Next, the operation thereof will be described. The input sound 81 passes through the sound absorbing plate 2 to be input into the back air space 11. The sound absorbing plate 2 has acoustic mass m and acoustic resistance r as the acoustic characteristics thereof, and the back air space 11 has acoustic capacity c as the acoustic characteristic thereof. The acoustic equivalent circuit according to the acoustic characteristics of the sound absorbing plate 2 and the back air space 11 can be expressed as a series resonance of r - m - c . According to this series resonance circuit, the resonance frequency thereof f 0 is expressed as the following formula. f 0 = ( 1 / 2 π ) x √ (1 / mc) When a sound wave having a frequency close to this resonance frequency f 0 is input into the sound absorbing plate 2, the input impedance observed from the sound source side becomes minimum. Accordingly, only the acoustic resistance r of the sound absorbing plate 2 should be considered. If the acoustic resistance r of the sound absorbing plate 2 is tuned to be a value close to the characteristic impedance p x a (p: density of air; a: sound velocity) of air, the sound absorption coefficient becomes 1.0 at the resonance frequency f 0 . Consequently, the sound wave having the frequency close to the resonance f 0 penetrates into the sound absorbing arrangement most efficiently. The penetrated sound wave forces the air existing in the back air space 11 and having an acoustic characteristic of acoustic capacity c to vibrate. The vibrated air goes in and out through gaps in the sound absorbing plate 2, and the sound wave is transformed into thermal energy by the acoustic resistance r of the gaps. That makes it possible to radiate energy. This means that the energy of the input sound wave was absorbed in the sound absorbing arrangement, namely sound absorption has been performed. In the aforementioned sound absorption arrangement, it is known that the efficiency of sound absorption is highest in the case where the input sound 81 is input into the sound absorption plate 2 perpendicularly. That is to say, in the case where a sound wave is input perpendicularly, the phase relation of the sound wave on the top surface of the sound absorbing plate 2 is equal at any place on the top surface, and the whole of the sound absorbing plate 2 and the whole of the back air space 11 are unified consequently, so that the effective operation of resonance and sound absorption is performed. On the other hand, the case where the input sound 81 is input into the sound absorbing plate 2 not perpendicularly but at a certain input angle β will be considered as an ordinary case. As shown in Fig. 18, when a sound wave having a wavelength λ is input the sound absorbing plate 2 at an input angle β, a phase difference having a period of λ /cos (β) or sound pressure distribution is generated on the sound absorbing plate 2. A sound wave is basically absorbed by utilizing a resonance phenomenon. But, if a distribution of the strength of sound pressure is generated along a direction on a surface of the sound absorbing plate 2, pressures 85 and 86 having reverse directions to each other operate on the back air space 11, so that adjoining parts of the back air space 11 is acoustically oscillated reversely. Then, pressures are balanced in the back air space 11, and consequently it becomes difficult that air vibrations synchronized with input sound waves are generated. That is to say, it becomes difficult that resonance phenomena are generated between the sound absorbing plate 2 and the back air space 11, so that sound absorption effect is extremely checked. PRIOR ART 2. Fig. 19 is a longitudinal sectional view showing a sound absorbing arrangement utilizing a sound absorbing material and a resonance phenomenon by combining them as a second prior art (prior art 2), which is shown, for example, in the Japanese Patent Gazette No. 76116/ 1992 (Tokko-Hei 4-76117). Fig. 20 is a sound absorption characteristic diagram of the sound absorbing arrangement shown in Fig. 19. In Fig. 19, reference numeral 91 designates a wall; numerals 92 and 93 designate air spaces; numeral 94 designates a small opening or a slit; numeral 95 designates a nozzle; numeral 96 designates a porous plate; and numeral 97 designates a sound absorbing material. Next, the operation thereof will be described. The aforementioned sound absorbing arrangement of the prior art 2 is provided with a porous plate 96 apart from the wall 91 with the air space 92 between. The porous plate 96 has a large number of small openings or slits 94, which are provided with nozzles 95 connected to them. Across the porous plate 96, the sound absorbing material 97 which is made of a fibrous material or a material made of a large number of particles is set over the whole plane at the tips of the nozzles 95 with the air space 93 between. In this connection, the air space 92, the small openings or slits 94 and the nozzles 95 comprise sound absorbing arrangements utilizing a resonance phenomenon, and the sound absorbing material 97 and the air spaces 93 comprise sound absorbing arrangements utilizing sound absorbing materials. The aforementioned elements of the sound absorbing arrangements utilizing a resonance phenomenon are connected to each other through the air space 92, and the elements of the sound absorbing arrangements utilizing sound absorbing materials are connected to each other through the air space 93. The sound absorbing arrangement of the prior art 2 has a sound absorption characteristic of the curved line 3 shown with a solid line in Fig. 20. A sound absorption characteristic of a sound absorbing arrangement utilizing only a resonance phenomenon is shown with a dotted line (curved line 2) in Fig. 20, which sound absorbing arrangement has large sound reduction effects at lower frequencies. A sound absorption characteristic of a sound absorbing arrangement utilizing only sound absorbing materials is shown with a dashed line (curved line 1) in Fig. 20, which sound absorbing arrangement has large sound reduction effects at higher frequencies. PRIOR ART 3. Fig. 21 is a partially cutaway perspective view showing the construction of a conventional sound absorbing arrangement as a third prior art (prior art 3), which utilizes both the slits and a porous material and is shown, for example, at pp. 245 - 250 and pp. 351 - 356 of Kenchiku Onkyo Kogaku Hando Bukku (Architectural Acoustics Handbook) ed. by Nippon Onkyo Zairyo Kyokai (Japan Acoustical Materials Association) (Gihodo, Tokyo, 1963). Fig. 22 is a sound absorption characteristic diagram of the sound absorbing arrangement shown in Fig. 21. In Fig. 21 reference numeral 91 designates a wall; numerals 92 and 93 designate air spaces; numeral 98 designates a porous material; and numeral 99 designates a slit plate. Next, the operation thereof will be described. The aforementioned sound absorbing arrangement of the prior art 3, which uses a structure utilizing slits and a porous material, raises the sound absorption characteristics of the porous material 98 and the air space 92 by means of the resonance phenomena of the slit plates 99 and the air spaces 93. As shown in Fig. 22, the raised sound absorption characteristics are particularly effective at lower frequencies around 200 to 500 Hz due to the resonance phenomena at the slit parts. Since the sound absorbing arrangement of the prior art 1 is constructed as mentioned above, the resonance frequency f 0 is determined in accordance with the thickness 11a of the back air space 11 if the sound absorbing plate 2 is specified. The sound absorption coefficient becomes maximum at the resonance frequency f 0 , and the sound absorption characteristic has large values in a narrow frequency band with the resonance frequency f 0 as a 1/3 octave band center frequency. Since some sound pressure distributions are generated in some directions on the sound absorbing plate 2 when sound waves are input the sound absorbing plate 2 at angles other than a right angle, the prior art 2 has a problem that the interference of input sound waves is generated at some frequencies according to phase differences to bring about the reduction of the sound absorption coefficient. Since the sound absorbing arrangement of the prior art 2 is constructed as mentioned above so that a sound absorbing arrangement utilizing a resonance phenomenon to be generated by elements connected to each other and a sound absorbing arrangement utilizing sound absorbing materials connected to each other are combined to absorb sound waves, the prior art 2 has problems that some sound pressure distributions are generated in some directions on the sound absorbing material 97 when sound waves are input into the sound absorbing material 97 at angles other than a right angle similarly in the prior art 1, so that the interference of input sound waves is generated at some frequencies according to phase differences to bring about the reduction of the sound absorption coefficients at lower frequencies as shown in, for example, Fig. 20. The sound absorbing arrangement of the prior 3, which utilizes slits and a porous material, has a problem that the sound absorption coefficients at lower frequencies around 200 Hz to 500 Hz are large due to sound resonance phenomena at the slits but the sound absorption coefficients at higher frequencies more than 500 Hz are small. Further, EP-A-46559 and EP-A-246464 each disclose a sound absorbing arrangement using a porous material to be placed on a sound insulator such as a wall, which comprises a sound absorbing plate made of a thin plate of a porous material and a supporting member for supporting the sound absorbing plate above the sound insulator. The supporting member forms plural adjacent back air spaces by subdividing the space between the sound absorbing plate and the sound insulator. SUMMARY OF THE INVENTION It is an object of the present invention to provide a sound absorbing arrangement using a porous material which has a superior sound absorption characteristic from lower frequencies to higher frequencies. This object is solved by the sound absorbing arrangement according to claim 1. Claims 2 and 3 provide further improvements of this sound absorbing arrangement. It is a further object of the present invention to provide a sound absorbing arrangement using a porous material which has a superior sound absorption characteristic from lower frequencies to higher frequencies by forming a sound absorbing plate of a porous material and equipping plural reflecting members. It is a further object of the present invention to provide a sound absorbing arrangement using a porous material which has a superior sound absorption characteristic from lower frequencies to higher frequencies by disposing a protecting plate having an opening in front of reflecting members. According to the first aspect of the present invention, there is provided a sound absorbing arrangement using a porous material which sound absorbing arrangement comprises plural reflecting members disposed in front of a sound absorbing plate with a space from the sound absorbing plate. As stated above, the sound absorbing arrangement using a porous material according to the first aspect of the present invention makes it easy to bring about a resonance phenomenon and improves the sound absorbing performance thereof by reflecting members disposed in front of a sound absorbing plate with a space from the sound absorbing plate, and consequently, a sound absorbing arrangement having a superior sound absorption characteristic from lower frequencies to higher frequencies can be obtained. According to the present invention, there is provided a sound absorbing arrangement using a porous material which sound absorbing arrangement comprises plural reflecting members disposed in front of a sound absorbing plate with a space from the sound absorbing plate, and a protecting plate disposed in front of the reflecting members for fixing the reflecting members which protecting plate has an opening. The sound absorbing arrangement using a porous material according to the present invention improves the sound absorbing performance thereof by comprising plural reflecting members disposed in front of a sound absorbing plate and a protecting plate disposed in front of the reflecting members which protecting plate has an opening, and consequently, a sound absorbing arrangement having a superior sound absorption characteristic from lower frequencies to higher frequencies can be obtained. As stated above, the sound absorbing arrangement using a porous material according to the third aspect of the present invention improves the sound absorbing coefficients thereof at higher frequencies by comprising a sound absorbing plate made of a thin plate of a porous material and disposed above a sound insulator with a back air space between, and plural reflecting members disposed in front of the sound absorbing plate with a space from the sound absorbing plate, and consequently, a sound absorbing arrangement having a superior sound absorption characteristic from lower frequencies to higher frequencies can be obtained. According to the fourth aspect of the present invention, there is provided a sound absorbing arrangement using a porous material which sound absorbing arrangement comprises a protecting plate disposed in front of reflecting members for fixing the reflecting members, which protecting plate has an opening. As stated above, the sound absorbing arrangement using a porous material according to the fourth aspect of the present invention improves the sound absorbing performance thereof by comprising a protecting plate disposed in front of reflecting members, which protecting plate has an opening, and consequently, a sound absorbing arrangement having a superior sound absorption characteristic from lower frequencies to higher frequencies can be obtained. The above and further objects and novel features of the present invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly unterstood, however, that the drawings are for purpose of illustration only and are not intended as a definition of the limits of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view showing the construction of a sound absorbing arrangement using a porous material according to the embodiment 1; Fig. 2 is a longitudinal sectional view showing the construction of a sound absorbing arrangement using a porous material according to the embodiment 1; Fig. 3 is a longitudinal sectional view showing the construction of a sound absorbing arrangement using a porous material according to the embodiment 2; Fig. 4 is a perspective view showing the construction of a sound absorbing arrangement using a porous material according to the embodiment 3; Fig. 5 is a longitudinal sectional view showing the construction of a sound absorbing arrangement using a porous material according to the embodiment 3; Fig. 6 is a longitudinal sectional view showing the construction of a sound absorbing arrangement using a porous material according to the embodiment 3; Fig. 7 is a perspective view showing the construction of a sound absorbing arrangement using a porous material according to the embodiment 4; Fig. 8 is a sound absorption characteristic diagram of a sound absorbing arrangement using a porous material according to the embodiment 4 in conformity with the method for measurement of sound absorption coefficients in a reverberation room; Fig. 9 is a characteristic diagram showing an effect of a sound absorbing arrangement using a porous material according to the embodiment 4; Fig. 10 is a longitudinal sectional view showing the construction of a sound absorbing panel using a porous material according to the embodiment 5 of the present invention; Fig. 11 is a longitudinal sectional view showing the construction of a sound absorbing arrangement using a porous material according to the embodiment 6 of the present invention; Fig. 12 is a perspective view showing the construction of a sound absorbing arrangement using a porous material according to the embodiment 7; Fig. 13 is a longitudinal sectional view showing the construction of a sound absorbing arrangement using a porous material according to the embodiment 7; Fig. 14 is a perspective view showing the construction of a sound absorbing arrangement using a porous material according to the embodiment 8 of the present invention; Fig. 15 is a longitudinal sectional view showing the construction of a sound absorbing arrangement using a porous material according to the embodiment 8 of the present invention; Fig. 16 is a longitudinal sectional view showing the construction of a sound absorbing arrangment using a porous material according to the embodiment 9 of the present invention; Fig. 17 is a sound absorption characteristic diagram of a sound absorbing panel using a porous material according to the embodiment 9 of the present invention in conformity with the method for measurement of sound absorption coefficients in a reverberation room; Fig. 18 is a longitudinal sectional view showing the construction of a conventional sound absorbing arrangement using a porous material including an explanatory diagram showing a sound pressure distribution of a sound wave to be input into the sound absorbing plate thereof; Fig. 19 is a longitudinal sectional view showing the construction of a conventional sound absorbing arrangement utilizing a sound absorbing material and a resonance phenomenon by combining them; Fig. 20 is a sound absorption characteristic diagram of the conventional sound absorbing arrangement utilizing a sound absorbing material and a resonance phenomenon by combining them; Fig. 21 is a partially cutaway perspective view showing the construction of a conventional sound absorbing arrangement utilizing both slits and a porous material; and Fig. 22 is a sound absorption characteristic diagram of the conventional sound absorbing arrangement utilizing both slits and a porous material. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. EMBODIMENT 1 Fig. 1 is a perspective view showing the construction of a sound absorbing mechanism using a porous material according to a first embodiment (embodiment 1); This embodiment is not an embodiment of the invention but helpful to understand certain aspects of the invention and Fig. 2 is a longitudinal sectional view showing the a sound absorbing arrangement using a porous material shown in Fig. 1. In Figs. 1 and 2, reference numeral 1 designates a sound insulator such as a wall. Reference numeral 2 is a sound absorbing plate. Reference numeral 11 designates a back air space of the sound absorbing plate 2; and numerals 11a designates the thickness of the back air spaces 11. Reference numerals 40 designates plural reflecting members disposed in front of the sound absorbing plate 2 so as to be opposed to the sound absorbing plate 2 with a space. Reference numeral 80 designates input sounds into a back air space 11, which input sounds 80 having evaded the reflection members 40; numeral 81 designates an input sound into the back air space 11; and numeral 81a designates a re-input sound into the back air space 11 which re-input sound 81a is the input sound 81 having been reflected by the sound absorbing plate 2 and a reflecting member 40. Such materials as polypropylene resin, polyvinyl chloride resin, ABS resin and polycarbonate resin can be used as the reflecting members 40. The shapes of the reflecting members 40 may be a hollowed pipe or a solid rod. Next, the operation thereof will be described. The resonance frequency f 0 of the back air space 11 is determined in accordance with the thickness 11a thereof. Sound absorption coefficients become maximum when the frequencies of the input sounds 80 and 81 are equal to the respective resonance frequencies f 0 . Many sounds do not pass through the sound absorbing plate 2 but are reflected on the surface thereof in the case where the sound absorbing coefficient thereof is small. Accordingly, when the reflecting members 40 are placed so as to be opposed to the sound absorbing plate 2, the reflected sounds are reflected by the reflecting members 40 again and are input into the sound absorbing plate 2 to be absorbed by it Because sounds having a shorter wavelength become re-input sounds 81a more efficiently, the sound absorption coefficients at frequencies higher than the resonance frequency f 0 are increased, and thereby sound absorption coefficients can be improved from lower frequencies to higher frequencies as compared with those of the prior arts. Because the re-input sounds 81a have propagation paths longer than those of the input sounds 81, their phases are shifted. Consequently, resonance phenomena are reinforced at some frequencies, which brings about the increase of sound absorption coefficients. The input sounds 80 are essentially reflected on the top surfaces of the reflecting members 40, but some sound waves of them are pulled into the spaces between the reflecting members 40 owing to the phenomena such as diffraction. Because the impedance of them is matched and their input angles become close to be perpendicular, they are absorbed efficiently. EMBODIMENT 2 Fig. 3 is a longitudinal sectional view showing the construction of a sound absorbing arrangement using a porous material according to a second embodiment (embodiment 2). This embodiment is not an embodiment of the invention but helpful to understand certain aspects of this invention. In Fig. 3 reference numeral 41 designates a plural reflecting member disposed in front of the sound absorbing plate 2 with a space from the sound absorbing plate 2 and having a sectional form of an inverted trapezoid. Because the reflecting members 41 can utilize also the side surfaces of them to reflect sound waves, re-input sounds 81a can be obtained more efficiently. Consequently, the sound absorption coefficients at frequencies higher than the resonance frequency f o are increased, and thereby sound absorption coefficients can be improved from lower frequencies to higher frequencies as compared with those of the prior art 1. EMBODIMENT 3 Fig. 4 is a perspective view showing the construction of a sound absorbing arrangement using a porous material according to a third embodiment (embodiment 3); This embodiment is not an embodiment of the invention but helpful to understand certain aspects of this invention and Figs. 5 and 6 are longitudinal sectional views showing the construction of the sound absorbing arrangement using a porous material shown in Fig. 4. In Figs. 4, 5 and 6, reference numeral 1 designates a sound insulator such as a wall. Reference numeral 2 designates a sound absorbing plate. Reference numerals 11 and 12 designate back air spaces of the sound absorbing plate 2; and numerals 11a and 12a designate the respective thickness of the back air space 11 and 12. Reference numerals 20a and 20b designate latticed supporting members for supporting the sound absorbing plate 2 so as to be opposed to the sound insulator 1 with the space of the thickness 12a of the back air spaces 11. Reference numeral 30 designates resonators fixes to the insulator 1 side of the sound absorbing plate 2 with the space of the thickness 12a of the back air spaces 12; numeral 30a designates hollow members for forming the resonators 30. The resonators 30 are disposed so as to be parallel to the supporting members 20b. Reference numeral 40 designates plural reflecting members disposed in front of the sound absorbing plate 2 so as to be opposed to the sound absorbing plate 2 with a space and parallel to the resonators 30. Reference numeral 81 designates an input sound into a back air space 11; numeral 81a designates an re-input sound into a back air space 11 which re-input sound 81a is the input sound 81 having been reflected by the sound absorbing plate 2 and a reflecting member 40; numeral 81b designates a re-input sound into a back air space 12 which re-input sound 81b is the input sound 81 having been reflected by the sound absorbing plate 2 and a reflecting member 40; numeral 82 designates an input sound into a back air space 12; and numeral 82b designates a re-input sound into a back air space 11 which re-input sound 82b is the input sound 82 having been reflected by the sound absorbing plate 2 and a reflecting member 40. Such materials as polypropylene resin, polyvinyl chloride resin, ABS resin and polycarbonate resin can be used as the materials of the reflecting members 40. Since the sound absorbing plate 2 is supported by the supporting members 20a and 20b, the strength of the sound absorbing plate 2 is increased. The shapes of the reflecting members 40 may be a hollowed pipe or a solid rod. Next, the operation thereof will be described. The resonance frequency f 0 of the input sound 81 is determined mainly in accordance with the thickness 11a of the back air spaces 11. The resonance frequency f 0 of the input sound 82 is also determined mainly in accordance with the thickness 12a of the back air spaces 12. The sound absorption coefficients respectively become maximum at the resonance frequencies f 0 of them. Since each sound absorbing arrangement is independent of the other, the total sound absorption characteristic is the sum of respective sound absorption characteristic. Since the back air spaces 11 are separated by the supporting members 20a, 20b and the back air spaces 12 are separated by the resonators 30 and the supporting members 20b respectively, each back air space 11 and each back air space 12 respectively operate independently, thereby it becomes easy to generate resonance phenomena, which brings about the improvement of the sound absorption performance thereof. Since the interference of sound waves due to phase differences is thus little, the present sound absorbing arrangement has larger sound absorption coefficients as compared with those of the prior arts 1 and 2. Furthermore, many sounds do not pass through the sound absorbing plate 2 but are reflected on the surface thereof in the case where the sound absorbing coefficient thereof is small. Accordingly, when the reflecting members 40 are placed so as to be opposed to the sound absorbing plate 2, the reflected sounds are reflected by the reflecting members 40 again and are input into the sound absorbing plate 2 as the re-input sounds 81a, 81b and 82b to be absorbed by it. Because sounds having a shorter wavelength become re-input sounds 81a, 81b and 82b more efficiently, the sound absorption coefficients at frequencies higher than the resonance frequency f 0 are increased, and thereby sound absorption coefficients can be improved from lower frequencies to higher frequencies as compared with those of the prior arts 1 to 3. In Figs. 4, 5 and 6, the embodiment 3 has latticed supporting members 20a and 20b, but the present invention comprises the use of the supporting members 20a alone or the supporting members 20b alone. By such usage, a part of the effects of the present embodiment can be obtained. EMBODIMENT 4 Fig. 7 is a perspective view showing the construction of a sound absorbing arrangement using a porous material according to a fourth embodiment (embodiment 4); This embodiment is not an embodiment of the invention but helpful to understand certain aspects of the invention. Fig. 8 is a sound absorption characteristic diagram in conformity with the method for measurement of sound absorption coefficients in a reverberation room; and Fig. 9 is a characteristic diagram showing an effect of the reflecting members 40. Fig. 9 shows the ratios of the sound absorpting coefficients in the case where the sound absorbing arrangement shown in Fig. 7 is equipped with the reflecting members 40 to the sound absorption coefficients in the case where the sound absorbing mechanism is not equipped with the reflecting members 40. The reflecting members 40 are opposed to the top surface of the sound absorbing plate 2, and disposed to be crossed with the resonators 30 perpendicularly. The dispositions of the reflecting members 40 shown in Figs. 4 to 7 also bring about the sound absorption effects shown in Figs. 8 and 9 basically. The directions of the dispositions of the reflecting members 40 to the resonators 30 are not limited to the shown perpendicular and parallel directions, but they ma be arbitrary. And, similar sound absorption effects can be obtained in the arbitrary direction dispositions. Next, the operation thereof will be described. The sound absorbing arrangement is constructed by placing, for example, a sound absorbing plate 2 having the thickness of 3,5 mm so that the thickness 11a of the back air spaces 11 becomes about 35 mm, to which sound absorbing plate 2 hollow members 30a are fixed so that the thickness 12a of the back air spaces 12 becomes about 9 mm for forming the resonators 30. And then, square pipes made from ABS resin and having the width of about 33 mm and the height of about 15 mm are disposed with the space of about 10 mm from the sound absorbing plate 2 as the reflecting members 40. The sound absorption characteristic of the sound absorbing arrangement thus constructed is improved in the sound absorption coefficients at frequencies higher than about 1,5 kilo-Hz owing to the effect of reflection and at frequencies lower than about 600 Hz owing to the effect of slit resonation phenomena as compared to the sound absorption characteristic in a case of having no reflecting members, and the former is totally improved at a wider frequency band, as shown in Figs. 8 and 9. According to the results of some experiments, sound absorption coefficients are furthermore improved at the thickness 12a of the back air spaces 12 being about 15 mm and at the space between the reflecting members 40 and the sound absorbing plate 2 being 15 mm. EMBODIMENT 5 Fig. 10 is a longitudinal sectional view showing the construction of a sound absorbing panel using a porous material according to a fifth embodiment (embodiment 5) of the present invention. In Fig. 10, reference numeral 1a designates a sound insulating plate also serving as a housing of the sound absorbing panel. Reference numeral 4 designates a protecting plate made of a punching metal or the like, which protecting plate 4 has at least one opening and is fixed to the insulating plate 1a so as to cover the opened part of the sound insulating plate 1a. Reference numeral 21a designates a supporting member for disposing the reflecting members 40. The directions of the reflecting members 40 may be parallel or perpendicular to the resonators 30. This sound absorbing panel has the same effects as those of the embodiments 3 and 4. EMBODIMENT 6 Fig. 11 is a longitudinal sectional view showing the construction of a sound absorbing arrangement using a porous material according to a sixth embodiment (embodiment 6) of the present invention. In Fig. 11 reference numeral 1 designates a sound insulator such as a wall. Reference numeral 2 designates a sound absorbing plate; and numeral 4 designates a protecting plate made of a punching metal or the like, which protecting plate 4 has at least one opening and is disposed so as to be opposed to the top surface of the sound absorbing plate 2. Reference numeral 11 designates the back air space of the sound absorbing plate 2; and numeral 11a designates the thickness of the back air space 11. Reference numeral 42 designates plural reflecting members fixed to the protecting plate 4 and disposed in front of the sound absorbing plate 2 with a space from the sound absorbing plate 2. Reference numeral 81 designates an input sound into the back air space 11; and numeral 81a designates a re-input sound into the back air space 11 which re-input sound 81a is the input sound 81 having been reflected by the sound absorbing plate 2 and a reflecting member 42. Such materials as polypropylene resin, polyvinyl chloride resin, ABS resin and polycarbonate resin can be used as the material of the sound absorbing plate 2. The shapes of the reflecting members 42 may be a hollowed pipe or a solid rod. Next, the operation thereof will be described. The resonance frequency f 0 of the input sound 81 is determined in accordance in accordance wit the thickness 11a of the back air space 11. Sound absorption coefficients become maximum at the resonance frequencies f 0 . Many sounds do not pass through the sound absorbing plate 2 but are reflected on the surface thereof in the case where the sound absorbing coefficient thereof is small. Accordingly, when the reflecting members 42 are placed so as to be opposed to the sound absorbing plate 2, the reflected sound is reflected by a reflecting member 42 again and is input into the sound absorbing plate 2 as the re-input sound 81a to be absorbed by it. Because sounds having a shorter wavelength become re-input sounds 81a more efficiently, the sound absorption coefficients at frequencies higher than the resonance frequency f o are increased, and thereby sound absorption coefficients can be improved from lower frequencies to higher frequencies as compared with those of the prior art 1. Besides, the damage of the sound absorbing plate 2 can be prevented by the protecting plate 4. Since the reflecting members 42 are fixed to the protecting plate 4 in advance, the efficiency of fitting operation of the protecting plate 4 at fitting sires is high. The reflecting members 42 serves also as a reinforcement material of the protecting plate 4. EMBODIMENT 7 Fig. 12 is a perspective view showing the construction of asound absorbing arrangement using a porous material according to a seventh embodiment (embodiment 7; This embodiment is not an embodiment of the invention but helpful to understand certain aspects of this invention and Fig. 13 is a longitudinal sectional view showing the sound absorbing arrangement using a porous material shown in Fig. 12. In Figs. 12 and 13 reference numeral 4 designates a protecting plate made of a punching metal or the like, which protecting plate 4 is formed by bending its portions corresponding to the reflecting members 42 described in the embodiment 6 and has openings in the portions other than the portions corresponding to the reflecting members 42 and furthermore is disposed so as to be opposed to the top surface of the sound absorbing plate 2. The sound absorbing mechanism thus constructed has also the same effects as those of the embodiment 6. EMBODIMENT 8 Fig. 14 is perspective view showing the construction of a sound absorbing mechanism using porous material according to an eighth embodiment (embodiment 8) of the present invention; and Fig. 15 is longitudinal sectional view showing the sound absorbing arrangement using a porous material of Fig. 14. In Figs. 14 and 15, reference numeral 1 designates a sound insulator such as a wall. Reference numeral 2 designates a sound absorbing plate; and reference numeral 4 designates a protecting plate made of a punching metal or the like, which protecting plate has openings and disposed in front of the sound absorbing plate 2. Reference numerals 11 and 12 designate back air spaces of the sound absorbing plate 2; and numerals 11a and 12a designate respective thicknesses of the back air spaces 11 and 12. Reference numerals 20a and 20b designate latticed supporting members for supporting the sound absorbing plate 2 so as to be opposed to the sound insulator 1 above the sound insulator 1 with the space of the thickness 11a of the back air spaces 11. Reference numeral 30 designates resonators equipped to the insulator 1 side of the sound absorbing plate 2 with the space of the thickness 12a of the back air spaces 12; and numeral 30a designates hollow members for forming the resonators 30. The resonators 30 are disposed so as to be parallel to the supporting members 20a and perpendicular to the supporting members 20b. Reference numeral 42 designates plural reflecting members fixed to the protecting plate 4, and disposed so as to be opposed to the sound absorbing plate 2 and parallel to the resonators 30. Reference numeral 81 designates an input sound into a back air space 11; numeral 81b designates a re-input sound into a back air space 12 which re-input sound 81b is the input sound 81 having been reflected by the sound absorbing plate 2 and a reflecting member 42; numeral 82 designates an input sound into a back air space 12; and numeral 82b designates a re-input sound into a back air space 11 which re-input sound 82b is the input sound 82 having been reflected by the sound absorbing plate 2 and a reflecting member 42. Such materials as polypropylene resin, polyvinyl chloride resin, ABS resin and polycarbonate resin can be used as the material of the sound absorbing plate 2. Since the sound absorbing plate 2 is supported by the supporting members 20a and 20b, the strength of the sound absorbing plate 2 is increased. The shapes of the reflecting members 42 may be a hollowed pipe or a solid rod. Next, the operation thereof will be described. the resonance frequency f 0 of the input sound 81 is determined mainly in accordance with the thickness 11a of the back air spaces 11. The resonance frequency f 0 of the input sound 82 is also determined mainly in accordance with the thickness 12a of the back air spaces 12. Sound absorption coefficients respectively become maximum at the resonance frequencies f 0 of them. Since each sound absorbing arrangement is independent of the other, the total sound absorption characteristic is the sum of the respective sound absorption characteristics. Since the back air spaces 11 are separated by the supporting members 20a and 20b and the back air spaces 12 are separated by the resonators 30 and the supporting members 20b respectively operate independently, and thereby it becomes easy to generate resonance phenomena, which brings about the improvement of the sound absorption performance thereof. Since the interference of sound waves due to phase differences is thus little, the present sound absorbing arrangement has larger sound absorption coefficients as compared with those of the prior arts 1 and 2. Furthermore, many sounds do not pass through the sound absorbing plate 2 but are reflected on the surface thereof in the case where the sound absorbing coefficient thereof is small. Accordingly, when the reflecting members 42 are placed so as to be opposed to the sound absorbing plate 2, the reflected sounds are reflected by the reflecting members 42 again and are input into the sound absorbing plate 2 as the re-input sounds 81b and 82b to be absorbed by it. Because sounds having a shorter wavelength become re-input sounds 81a and 82b more efficiently, sound absorption coefficients at frequencies higher than the resonance frequency f 0 are increased, and thereby sound absorption coefficients can be improved from lower frequencies to higher frequencies as compared with those of the prior arts 1 to 3. Besides, the damage of the sound absorbing plate 2 can be prevented by the protecting plate 4. Since the reflecting members 42 are fixed to the protecting plate 4 in advance, the reflecting members 42 also serves as reinforcement materials of the protecting plate 4, and the efficiency of fitting operation of the protecting plate 4 at fitting sites is high. In Figs. 14 to 15, the embodiment 8 has latticed supporting members 20a and 20b, but the present invention comprises the use of the supporting members 20a alone or the supporting members 20b alone. By such usage, a part of the effects of the present embodiment can be obtained. The similar effects can be expected in the case where the reflecting members 42 are disposed perpendicularly to the resonators 30. EMBODIMENT 9 Fig. 16 is a longitudinal sectional view showing the construction of a sound absorbing panel using a porous material according to a ninth embodiment (embodiment 9) of the present invention; and Fig. 17 is a sound absorption characteristic diagram in conformity with the method for measurement of sound absorption coefficients in a reverberation room. In Fig. 16, reference numeral 1a designates a sound insulating plate also serving as a housing of the sound absorbing panel. Reference numeral 4 designates a protecting plate made of a punching metal or the like, which protecting plate 4 has at least one opening and is fixed to the sound insulating plate 1a. Reference numeral 42 designates plural reflecting members fixed to the protecting plate 4 and disposed so as to be opposed to the sound absorbing plate 2. The reflecting members 42 are disposed to be perpendicular to the resonators 30. Next, the operation thereof will be described. since the back air spaces 11 are separated by the supporting members 20a and 20b and the back air spaces 12 are separated by the hollow members 30a and the supporting members 20b respectively, each back air space 11 and each back air space 12 respectively operate independently, and thereby it becomes easy to generate resonance phenomena, which brings about the improvement of the sound absorption performance thereof. Since the interference of sound waves due to phase differences is thus little, the present sound absorbing panel has larger sound absorption coefficients as compared with those of the prior arts 1 and 2. Furthermore, many sounds do not pass through the sound absorbing plate 2 but are reflected on the surface thereof in the case where the sound absorbing coefficient thereof is small. Accordingly, when the reflecting members 42 are placed so as to be opposed to the sound absorbing plate 2, the reflected sounds are reflected by the reflecting members 42 again and are input into the sound absorbing plate 2 again to be absorbed by it. Because sounds having a shorter wavelength are input more efficiently, sound absorption coefficients at frequencies higher than the resonance frequency f 0 are increased, and thereby sound absorption coefficients can be improved from lower frequencies to higher frequencies as compared with those of the prior arts 1 to 3. The sound absorbing panel is constructed by forming, for example, a galvanized steel plate having the thickness of 1,6 mm into a box sized to be about 500 mm x 1960 mm x 50 mm as the sound insulating plate 1a, and by placing the sound absorbing plate 2 having the thickness of about 3,5 mm in the box so that the thickness 11a of the back air spaces 11 becomes about 35 mm, to which sound absorbing plate 2 the hollow members 30a are fixed so that the thickness 12a of the back air spaces 12 becomes about 9 mm for forming the resonators 30. And then, square bars made from ABS resin and having the width of about 27 mm and the height of about 15 mm are fixed to the protecting plate 4 made of an aluminium plate having the thickness of 0,8 mm and the rate of opened area of about 40% as the reflecting members 40. And then, the protecting plate 4 is fixed to the sound insulating plate 1a. The sound absorption characteristic of the sound absorbing panel thus constructed is improved in the sound absorption coefficients at frequencies higher than about 1,5 kilo-Hz as compared to the sound absorption characteristic in case of having no reflecting members, and the former is totally improved at a wider frequency band, as shown in Fig. 17. Similar effects can be expected in the case where the reflecting members 42 are disposed to be parallel to the resonators 30.

Claims: A sound absorbing arrangement using a porous material to be placed on a sound insulator (1) such as a wall comprising a sound absorbing plate (2) made of a thin plate of a porous material and disposed above said sound insulator (1) with a back air space between; characterized by plural reflecting members (40, 42) disposed in front of said sound absorbing plate (2) with a space from the absorbing plate (2), and a protecting plate (4) disposed in front of said reflecting members (42) for fixing the reflecting members (42), the protecting plate (4) having an opening. The sound absorbing arrangement using a porous material according to claim 1, wherein said sound absorbing plate (2) is partially made of welded plastic particles. The sound absorbing arrangement using a porous material according to claim 1, which is formed as a sound absorbing panel equipping a sound insulating plate corresponding to said sound insulator at a back of a sound absorbing arrangement.

Inventors: IMAI TOSHIHISA, KURASHINA MASAYUKI, OOTSUTA KATSUHISA, TANI SHUICHI, TSUKAMOTO KOUJI, IMAI, TOSHIHISA, KURASHINA, MASAYUKI, OOTSUTA, KATSUHISA, TANI,SHUICHI, TSUKAMOTO, KOUJI, Imai, Toshihisa, c/o Mitsubishi Electric Home, Kurashina, Masayuki, c/o Mitsubishi Denki K.K., Tani,Shuichi, c/o Mitsubishi Denki K.K., Tsukamoto, Kouji, c/o Mitsubishi Denki K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI ELECTRIC HOME APPL, MITSUBISHI DENKI KABUSHIKI KAISHA, MITSUBISHI ELECTRIC HOME APPLIANCE CO., LTD
Application year: 1995
Main classification: G10K11
IPC codes: G10K11