Sound absorptive material

Abstract: A sound absorptive material has powder particles (13) showing sound absorbing characteristics at low frequency range and contained generally within pores (12) of a porous support in vibratable state, whereby the sound absorptive material can be increased in the sound absorption coefficient in the low frequency range while maintaining the absorptive material to be thin.

Description: BACKGROUND OF THE INVENTION This invention relates to a sound absorptive material having a sound absorption coefficient in a lower sound frequency range, wherein powder particles are generally contained in a porous support, having at least one supported powder layer. Such sound absorptive material is employed as a wallboard or a like interior material for making a room with excellent acoustic properties, or as a filler material for a hollow part in a double structure installation which is required to have high sound insulating properties, and as an inner liner of a sound absorbing duct, or soundproof covers for use with noise-generating machines and instruments and so on. DESCRIPTION OF RELATED ART The sound absorptive material has been employed as interior material for use in audio-listening rooms, practice rooms for music instruments, hotels, condominiums, conference rooms and so on where excellent acoustic properties are demanded. The absorptive material has also been used as an inner liner of a sound absorbing duct, soundproof covers for use with noise-generating machines and instruments and so on. For the sound absorptive material in these instances, such porous members as foamed urethanes, glasswool and the like have been used so that, upon incidence of sound wave in foams or pores of such sound absorptive material, a continuity of complicated sectional configuration of the foams or pores in the foamed urethanes or glass-wool cause the sound wave to be lowered in its sound pressure due to viscosity friction yielded by walls of the foams or pores while the sound wave propagates through the foams or pores, and eventually the sound wave energy can be absorbed. The porous members including the foamed urethanes and glass-wool show, on the other hand, show a tendency that their sound absorption coefficient becomes higher as the sound wave frequency rises but is lower with respect to the sound wave in lower frequency range. In this event, an increment in the thickness of the porous member may raise the sound absorption coefficient in the low frequency range, whereas the increased thickness of the porous member causes a problem in that the interior material of such porous member reduces the space of the room and that the inner liner of such member renders a smaller inner diameter of the duct too, that is, the intraduct flow path. In U.S. Patent No. 4,386,676 of Villy Gadde et al, further, there has been suggested a sound absorbing mat comprising a flexible porous layer and such particles of a higher specific gravity as a metal concentration, crushed mineral material, sand or the like and filling open pores of the layer over a part of the thickness of the layer. With this arrangement, the mat can be improved in its soundproof effect by an increased surface density and also in its sound absorbing effect by remaining part unfilled with the particles. Further, in Japanese Patent Laid-Open Publication JP-A 61-40941 of Y. Tomoto et al, there has been disclosed a soundproof material in which a base is formed to have a large number of independent voids which are filled with such powdery or particulate member as sand as an attempt to improve the sound insulating properties. However, there has been suggested no measure for improving the sound absorption coefficient in the low frequency range, and the improved sound absorption coefficient in the low frequency range has been a keen demand. CH-A-535.343 teaches a sound absorptive material having particles that are completely enclosed by the support material, said material having pores for giving the material a very low Young's modulus. From GB-A-1.310.355 a heat and acoustic insulating fibre containing material is known which is made from a web of fibrous material wherein anorganic particulate material onto or into the web is introduced, and whereby the web containing the particulate material is consolidated to secure the particulate material in the web. This publication renders no suggestion to improve the sound absorbing characteristics of this material in the low frequency range, which has been in keen demand ever since its publication. SUMMARY OF THE INVENTION Accordingly, a primary object of the present invention is to provide a sound absorptive material which is kept small in thickness and is still capable of increasing the sound absorption coefficient in the low sound frequency range to a sufficiently satisfactory extent. This object can be realized according to the invention by a sound absorptive material which is characterized in that all powder particles are contained in the porous support in a vibratable state, and wherein the porous support has a Young's modulus of less than about 10 5 N/m 2 so as to cause a sound absorption peak in a frequency range below about 500 Hz. Preferred embodiments are specified in the dependent claims. Other objects and advantages of the present invention shall become clear as the description of the invention advances as detailed with reference to embodiments shown in accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a diagram showing schematically an inner structure in an embodiment of the sound absorptive material according to the present invention; FIG. 2 is a diagram showing schematically an inner structure in another embodiment of the sound absorptive material according to the present invention; FIGS. 3, 3A and 4 are diagrams showing characteristics of the sound absorption coefficient with respect to the sound frequency of different powder particle layers employed in the sound absorptive materials according to the present invention; FIG. 5 is an explanatory view for vibratory motion in thickness direction of the powder particle layer employed in the sound absorptive material of the present invention; FIG. 6 shows in a block diagram a system for measuring the Young's modulus; FIG. 7 is an explanatory view for an example of method for manufacturing the sound absorptive material of the present invention; FIG. 8 is a diagram showing Young's modulus with respect to bulk density of various powder particles layers employed in the sound absorptive material according to the present invention; FIG. 9 is a diagram showing characteristics of the sound absorption coefficient with respect to the sound frequency of still another sound absorptive material according to the present invention; FIG. 10 shows in a perspective view with a part removed of the sound absorptive material in another embodiment according to the present invention; FIG. 11 shows in a perspective view with a part removed of the sound absorptive material in a further embodiment according to the present invention; FIG. 12 is a schematic sectioned view of the sound absorptive material shown in FIG. 11; FIG. 13 is a diagram showing the characteristics of the sound absorption coefficient with respect to the sound frequency in the sound absorptive material shown in FIG. 11; FIGS. 14 and 15 show respectively in a perspective view with a part removed further embodiments of the sound absorptive material according to the present invention; and FIG. 16 shows schematically in a perspective view an application example of the sound absorptive material according to the present invention. While the present invention would be now explained in the followings with reference to the embodiments shown in the drawings, it should be appreciated that the intention is not to limit the invention only to these embodiments shown but rather to include all alterations, modifications and equivalent arrangements possible within the scope of appended claims. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS According to a remarkable feature of the present invention, a sufficiently improved sound absorption coefficient is attained in the low frequency range of the sound wave to be absorbed, whereby a sound absorptive material which shows improvement in room acoustic characteristics and sound insulating properties when employed as the interior material as well as effective restraint of noise when utilized as the inner liner is provided, in which the thickness is not increased but is maintained to be thin. In the present instance, a porous support as the base of the sound absorptive material is made to contain powder particles in vibratable state to utilize the vibration of the powder particles. While various types of the powder particles can be employed, a 30mm thick layer of calcium silicate powder particles of, for example, an average particle diameter of 25µm, true density of 2.52g/cm 3 , bulk density of 0.080g/cm 3 and void volume of 0.97 has been subjected to measurement of characteristics of the sound absorption coefficient with respect to the frequency, result of which has been represented by such solid line curve as shown in FIG. 3A. Further, a 30mm thick layer of precipitated silica powder particles of an average particle diameter of 150µm, true density of 2.1g/cm 3 , bulk density of 0.282g/cm 3 and void volume of 0.87 has been subjected to measurement of characteristics of the sound absorption coefficient with respect to the frequency, result of which has been shown by a solid line curve in FIG. 3B. The sound absorption coefficient was measured in accordance with Japanese Industrial Standard (JIS) A 1405. As will be clear from both of FIGS. 3A and 3B, absorption peak has taken place in a lower frequency range than 500Hz in either of them, and it should be appreciated that these powder particles are of a high sound absorption coefficient in the lower frequency range. It has been found that, at the absorption peak, the sound in lower frequency range can be effectively absorbed by the powder particles in such layer L as shown in FIG. 5 which vibrates in longitudinal vibration mode (in thickness direction). For the powder particles, a brass foil powder (z in FIG. 8) of an average flake diameter of 10um and an average thickness of 0.25um may also be employed. It will be also useful to employ powder particles having a substantially constant sound absorption coefficient at frequencies above a predetermined level as shown in FIG. 4. For the powder particles in this case, it is possible to employ vermiculite, silica, mica, calcium carbonate, barium ferrite or the like of a bulk density of 0.1 to 3.0g/cm 3 or, preferably, 0.1 to 1.0g/cm 3 . Further, the center frequency fr at the absorption peak which appearing in the low frequency range, the sound absorption coefficient value at this center frequency fr, frequency width of the absorption peak and so on are variable depending on the particular diameter and shape of the powder particles employed, the bulk density for the layered powder particles, agglomerating state of the powder particles and so on. Here, a shift of the center frequency fr of the absorption peak, if possible, to further lower side may allow a further useful sound absorptive material to be provided. An empirical confirmation has been made in respect that the center frequency fr of the sound absorption peak in the low frequency range at the powder particle layer can be obtained basically by means of a following formula (1): fr = 0.25t -1 (E/ρ) 0.5 in which t denotes the thickness of the powder particle layer, E is the Young's modulus of the powder particle layer, and ρ is the bulk density of the layer. While it is seen from this formula (1) that the center frequency fr may be lowered by enlarging either the thickness t or the bulk density ρ of the powder particle layer, such measures is undesirable since the enlargement of the thickness causes the room or flow path space to be decreased and the enlargement of the bulk density ρ results in a deterioration of the sound absorption coefficient due to that the vibration of the powder particles is restrained. On the other hand, it will be clear from the formula (1) that the center frequency fr can be lowered by reducing the Young's modulus E instead of the enlargement of either the thickness t or the bulk density ρ of the powder particle layer. In other words, it is possible to lower the center frequency fr by combining the Young's modulus of the powder particle layer which is normally over 10 5 N/m 2 with a porous support of a lower Young's modulus, i.e., below 10 5 N/m 2 . Since it has been found that the Young's modulus E of the powder particle layer is largely influenced by the spring constant between mutually contacting powder particles, the Young's modulus can be optimumly reduced for the entire powder particle layer by preparing this powder particle layer with the powder particle contained in a porous support of the Young's modulus less than 10 5 N/m 2 which is employed as the base of the sound absorptive material. Accordingly, a porous support 11 in such aspect of fibers mutually entangling complicatedly as shown in FIG. 1 is made to contain powder particles 13 in a vibratable manner, so as to prepare the powder particle layer and eventually the sound absorptive material. Further, the sound absorptive material can be prepared with the powder particle layer formed, as shown in FIG. 2, with a porous support 21 in which many pores 22 disposed in vertical and horizontal arrays and with powder particles 23 respectively contained in all or at least part of the pores 22 of the porous support 21. For the porous support to be employed in the sound absorptive material according to the present invention of these arrangements, it is possible to employ, for example, a nonwoven fabric, fibrous structure, artificial pulp, filter paper or the like which is optimumly in the form of sheet about 5-40mm thick. When the thickness is insufficient, the porous support may be a stack of a plurality of such sheets. For the porous support, further, it is also possible to employ such fibrous structure as glass-wool, rock wool or the like, such foamed resin material as foamed urethane or the like, and so on, which may also be in the form of sheet. In an event where the nonwoven fabric is employed as the porous support employed in the present invention, it is possible to employ one of polyester 25µm in the fibrous diameter, 0.011g/cm 3 in the bulk density and 2.1x10 3 N/m 2 in the Young's modulus, one of polyester 25µm in the fibrous diameter, 0.0013g/cm 3 in the bulk density and 2.5x10 3 N/m 2 , or one of polyester 25µm in the fibrous diameter, 0.014g/cm 3 in the bulk density and 9x10 3 N/m 2 . For the powder particles in this case, it is also possible to effectively employ mica powder particles, vermiculite powder particles, ultrafine acrylic powder particles, spherical silica powder particles or the like. More specifically, talc powder particles of 2.3 to 9.4µm in the average particle diameter, 2.75g/cm 3 in the true density, 0.25 to 0.45g/cm 3 in the bulk density and 0.84 to 0.91 in the void volume, the spherical silica powder particles of 3 to 28µm in the average particle diameter, 2.23g/cm 3 of the true density, 0.31 to 0.92g/cm 3 in the bulk density and 0.59 to 0.86 in the void volume, and the like may be employable. In measuring the Young's modulus of the foregoing porous supports, as shown in FIG. 6, the porous support 31 subjected to the measurement is placed within a vibration plate or pot 30, and an additional mass member 32 is disposed on the support 31. Then the pot 30 is vibrated with an exciter 33, and a transmission function between the vibration acceleration of an impedance head 35 and the vibration acceleration of the additional mass member 32 is taken through charge amplifiers 34a and 34b by an FET amalyzer 36. At last, the Young's modulus can be computed at a personal computer 37 from the frequency of peaks of the transmission function. By the way, this measuring arrangement of FIG. 6 may be used not only for the measurement of the Young's modulus for the porous support alone but also for the Young's modulus measurement for the powder particle layer or the porous support into which the powder partilces are introduced. For a method of manufacturing the sound absorptive material, such an arrangement as shown in FIG. 7 may be employed, in which, so long as the outer dimensions are not demanded to be too large, the porous support 41 is placed in a container 40, the powder particles 43 are placed on the support 41, the whole is vibrated with the vibrator 46 and the powder particles 43 are caused to be contained within the pores of the porous support 41. Further, the sound absorptive material according to the present invention is demanded to sufficiently contain the powder particles therein, but it is inconvenient that the powder particles exceed 90 volume%, in maintaining the powder particles to be vibratable. Here, the porous support is made to be of an arrangement in which the support is 5 volume% and the rest will be the void. In FIG. 8, there are shown the Young's moduli of various powder particle layers employed in the present invention, in which diagonal axses denote the frequencies at sound absorption peaks. Further in the drawing, dot a denotes electrofused magnesia powder particles, dot b calcium carbonate powder particles, dots c and f polymethacrylate resin powder particles, dots d and y polytetrafluoroethylen fine powder particles, dot e whole milk powder particles, dot g barium ferrite powder particles, dots h and m Shirasu baloon powder particles, dot i oil furnace black powder particles, dot j hard wheat flour, dots k and l chloride process titanium dioxide powder particles, dot n crystalline cellulose powder particles, dot o soft wheat flour, dot p lycopodium spores, dots q and s nylon powder particles, dot r bentonite powder particles, dots u and v expanded perlite powder particles, dots t and w silicon powder particles, and dot x natural amorphous graphite. Here, the respective powder particle layers were made to be 30mm thick. Various sound absorptive materials were prepared in accordance with the foregoing manufacturing method of FIG. 7, with various nonwoven fabrics respectively of the Young's modulus below 10 5 N/m 2 and in the form of sheet of 5 to 40mm thick as the porous support and with various powder particles of precipitated silica and so on of 10 to 500µm in the average particle diameter as the powder particles, while keeping the powder particle amount not to exceed 90 valume%. Observation of sound absorbing characteristics in respect of these various sound absorptive materials thus obtained has proved that they were excellent in the sound absorbing characteristics in the low frequency range. EXAMPLE 1: In accordance with the manufacturing method of FIG. 7, a polyester nonwoven fabric of 25µm in the average fibrous diameter, 0.011g/cm 3 in the bulk density 2.1x10 3 N/m 2 in the Young's modulus as the porous support was made to contain vermiculite of 180 to 250µm in the particle diameter, 0.171g/cm 3 in the bulk density and 2.8x10 5 N/m 2 in the Young's modulus, keeping the powder particle amount not to exceed 90 volume%, and a sound absorptive material of 0.180g/cm 3 in the bulk density and 1.62x10 5 N/m 2 in the Young's modulus was obtained. As the result of observation of the sound absorbing characteristics in respect of this sound absorptive material obtained through the above Example 1, it has been found, as shown in FIG. 9, that the center frequency was lowered to be closer to 250Hz in the case of Example 1 (denoted by a solid line curve) in contrast to a sound absorptive material relying only on a layer of vermiculite (denoted by a broken line curve), and the sound absorptive material of Example 1 was made excellent in the sound absorbing characteristics in the low frequency range. According to another feature of the present invention, the sound absorptive material may be prepared by having the powder particle layer 53 contained in the nonwoven fabric as in the foregoing, filling a box 50 opened fully on one side with the powder particle layer 53, and covering the open side of the box 50 by a sheet 55 acoustically transparent, as shown in FIG. 10. For the material of this box 50, employable one will be wood, gypsum board, calcium silicate plate, cemented wood-wool plate, cemented wood chips or the like, and, for the sheet 55, air-permeable fabric of saran cloth, glass cloth or the like, or a synthetic resin sheet less than about 0.05mm in the thickness of polyethylene, vinyl or the like. Further, according to another working aspect of the present invention as shown in FIG. 11, three powder particle layers 63, 63a and 63b, for example, are stacked with sheet-shaped porous supports 61 and 61a interposed between the respective layers, the stacked layers 63-63b and supports 61 and 61a are accommodated within the box 60 opened on one side, the open side of the box 61 is closed by the acoustically transparent sheet 65, and a sound absorptive material is thereby completed. In this case, it is possible to arrange the porous support and powder particle layer in a set, and to increase the number of the powder particle layer as occasion demands, as shown in FIG. 12. EXAMPLE 2: A sound absorptive material was formed by stacking four powder particle layers on each other with three porous supports interposed respectively between the powder particle layers, so that the total thickness was 30mm. For the powder particle layer, silica powder of an average particle diameter of 150µm was employed, while keeping the respective layer to be about 5mm thick. For the porous support, the ones of the glass wool of 3mm thick each were used. In the case of this sound absorptive material thus formed through this Example 2, the characteristics of the sound absorption coefficient with respect to the sound frequency attained by means of the sets of the powder particle layers and porous supports were as shown in FIG. 13, in which there was proved that the sound absorptive material was excellent in the sound absorbing characteristics in the low frequency range. According to a further feature of the present invention, there can be formed a powder particle layer in which the powder particles present a remarkable peak value in the frequency characteristics. In this case, the porous support employed is one in which thin fibers of nylon, for example, are entangled about the periphery of polyurethane fiber, for example, and the powder particles of silica or the like are caused to be contained in this porous support. In this arrangement, the porous support shows relatively flexible spring properties, whereby the powder particle layer as a whole is caused to be smaller in the spring action and the sound absorbing frequency range can be effectively shifted to lower side. EXAMPLE 3: A porous support was prepared by irregularly stacking fibers of 1.19g/cm 3 true density and consisting of nylon of about 10µm diameter entangled about polyurethane fibers of about 100µm diameter. This porous support showed the void volume of 98.6% and the Young's modulus of 7.56x10 3 N/m 2 , a powder particle layer was formed with such porous support made to contain therein silica powder particles of 150µm in the average particle diameter, and the same box as that shown in FIG. 10 was used to prepare a sound absorptive material. In respect of the sound absorptive material prepared through this Example 3, it has been found that the same was excellent in the sound absorbing characteristics more in the lower frequency range. According to still another feature of the present invention, a sound absorptive material can be provided, which sound absorbing characteristics are stabilized by means of a stack of a plurality of layers of the powder particles performing the sound absorbing action through their vibration, with sheet members interposed between them, which sheet members having such an enough denseness as not allowing the powder particles to pass therethrough. That is, as shown in FIG. 13, the sheet members 71 and powder particle layers 73 are alternately stacked on each other, so that the sheet members 71 which do not allow the powder particles to pass therethrough will effectively prevent in particular the powder particles from being deviately concentrated to one part area, and being thereby caused to vary in the sound absorbing characteristics. For the sheet member 71, nonwoven fabric, woven fabric, knitted web or the like may be employed, so long as the member has surface openings very fine and smaller than the powder particles. And for the powder particles, for example, a particle diameter in a range of 0.1 to 1,000µm a bulk density in a range from about 0.1g/cm 3 to about 1.5g/cm 3 may be employed. It should be appreciated that such stack as shown in FIG. 14 should preferably be housed within such box as shown in FIG. 11, so as to prepare the sound absorptive material. In order to further reduce the fluidity of the powder particles forming the stack of the powder particle layers, there may be provided an arrangement such as shown in FIG. 15, in which partitions 81 extending in directions perpendicular to the sheet members 82 arranged within the box 80 and the powder particle layers 83 are accommodated within the spaces defined by the partitions 81 and the sheet members 82. EXAMPLE 4: A sound absorptive material was prepared by stacking a plurality of nonwoven fabrics about 15µm in the average fibrous diameter 0.13g/cm 3 in the bulk density and 0.4mm in the thickness, alternately with the powder particle layers respectively 2.2mm thick and consisting of vermiculite of the average particle diameter of 63µm, so that the powder particle layers were eleven and the total thickness was 31mm, and placing the stack into such box as shown in FIG. 11. Investigation of the characteristics of the sound absorption coefficient with respect to the sound frequency in respect of this sound absorptive material prepared through this Example 4 has proved that constant sound absorption characteristics were attained throughout the measurement repeated many times. Further, it should be readily appreciated that, while a use of the sound absorptive material containing the powder particles in normal arrangement as an inner liner 103 of a duct or conduit member 100 such as shown in FIG. 16 will apt to cause the powder particles to deviatingly concentrate to a partial area, the sound absorptive material shown in FIGS. 14 and 15 in particular should be useful when used in such duct member.

Claims: A sound absorptive material having a sound absorption coefficient in a lower sound frequency range, wherein powder particles are generally contained in a porous support, having at least one supported powder layer, characterized in that all powder particles are contained in the porous support in a vibratable state, and the porous support has a Young's modulus of less than about 10 5 N/m 2 so as to cause a sound absorption peak in a frequency range below about 500 Hz. The sound absorptive material of claim 1, characterized in that additionally the material is constituted by a plurality of supported powder layers and a plurality of porous supports, the layers and supports being disposed alternately. The sound absorptive material of claim 1 or 2, characterized in that the material comprises at least one supported powder layer disposed within a box, at least one side of which box is covered by an acoustically transparent member. The sound absorptive material of any one of claims 1 to 3, characterized in that the powder particles are selected from the group consisting of talc, calcium silicate, silica, vermiculite, mica, calcium carbonate, barium ferrite, acrylate and polymethacrylate resin. The sound absorptive material of any one of claims 1 to 4, characterized in that the powder particles have a bulk density of 0.1 to 3.0 g/cm 3 . The sound absorptive material of any one of claims 1 to 5, characterized in that the powder particles occupy no more than about 90 volume % of the sound absorptive material, and the porous support occupies about 5 volume % of the material, the rest being void. The sound absorptive material of any one of claims 1 to 6, characterized in that the powder particles have an average particle diameter of about 10 µ m to about 500 µ m. The sound absorptive material of claim 2 or 3, characterized in that the porous supports respectively have a thickness of no more than about 3 mm and being of such density and pore size as to prevent passage of the powder particles therethrough, and the supported powder layers respectively have a thickness of no more than 5 mm. The sound absorptive material of claim 8, characterized in that the material further includes a plurality of partitions respectively disposed perpendicular to the supported powder layers and porous supports, the partitions defining thus a plurality of divisions of the material.

Inventors: ANDO HIDEYUKI, KURIHARA YOSHITAKA, NAKAI TAKASHI, OKUDAIRA YUZO, UMEOKA KAZUNORI, YAMADA WAKIO, ANDO, HIDEYUKI, KURIHARA, YOSHITAKA, NAKAI, TAKASHI, OKUDAIRA, YUZO, UMEOKA, KAZUNORI, YAMADA, WAKIO, Ando, Hideyuki, c/o Matsushita Electric, Kurihara, Yoshitaka, c/o Matsushita Electric, Nakai, Takashi, c/o Matsushita Electric, Okudaira, Yuzo, c/o Matsushita Electric, Umeoka, Kazunori, c/o Matsushita Electric, Yamada, Wakio, c/o Matsushita Electric
Applicants: MATSUSHITA ELECTRIC WORKS LTD, MATSUSHITA ELECTRIC WORKS, LTD.
Application year: 1992
Main classification: G10K11
IPC codes: E04B1, B32B5, G10K11