US3603427A

US3603427A - Loudspeaker

Info

Publication number: US3603427A
Application number: US821803A
Authority: US
Inventors: Hiromi Sotome
Original assignee: Nippon Gakki Co Ltd
Current assignee: Nippon Gakki Co Ltd
Priority date: 1968-11-19
Filing date: 1969-05-05
Publication date: 1971-09-07
Anticipated expiration: 1988-09-07
Also published as: GB1270032A; DE1923769A1

Abstract

A loudspeaker including a diaphragm of rigid, self-supporting, foamed plastic material clamped at its peripheral portion to a speaker frame, without providing increased compliance at said peripheral portion, to generate sound by bending motion of said diaphragm, said diaphragm having a soft material such as foamed or unfoamed rubberlike material (e.g. natural rubber, synthetic rubber or rubberlike plastic material) applied on at least a part of the peripheral portion at at least one side thereof, said soft material having relatively high specific gravity sufficient to suppress divided vibration of the diaphragm in low frequency sound range.

Description

United States Patent [72] Inventor HiromiSotome 2,863,520 12/1958 Manley et al. 181/31 Hamamatsu-shi, Shizuoka-ken, Japan 3,247,925 4/1966 \Varnaka 181/31 {2]} Appl. No. 821,803 3,308,904 3/1967 Kelly 181/32 1 Filed g n 32 FOREIGN PATENTS [45] Patented pt. l 1 Assignee pp Gum Seize Kabushiki Kaisha 626,876 2/1963 Belgium 181/32 Hamamatsu-shi, Shizuoka-ken, Japan Primary Examiner-Stephen .1. Tomsky Attorney-Stevens, Davis, Miller & Mosher [54] LOUDSPEAKER C alms rawmg ABSTRACT: A loudspeaker including a diaphragm of rigid, [52] 11.8. Cl 1. 181/32 lfl ni f d plastic material clamped at its 1 Int. peripheral portion to a peaker frame providing in- H047 7/00 creased compliance at said peripheral portion, to generate [50] Field 01 Search l8l/3l.l, Sound by bending motion f said diaphragm Said diaphragm 31, 32 having a soft material such as foamed or unfoamed rubberlike material (e.g. natural rubber, synthetic rubber or rubberlike [56] References cited plastic material) applied on at least a part of the peripheral UNITED STATES PATENTS portion at at least one side thereof, said soft material having 2,716,462 8/1955 Brennan 181/32 relatively high specific gravity sufficient to suppress divided 2,815,823 12/1957 Olson et a] 181/32 vibration of the diaphragm in low frequency sound range.

PATENTED SEP H97! 315033427 sum 3 or 5 F|G.4 FIG.5

PATENTED SEP 7 l9?! SHEET l; UF 5 FIG.1O

m wt 83 5&501

FREQUENCY Hz) FIG.11

m w 63 5050 x! FREQUENCY HZ) FIG. 16

FIG. 18

LOUDSPEAKER BACKGROUND OFTHE INVENTION The present invention relates to a loudspeaker having a diaphragm which is clamped at its peripheral portion to a speaker frame, without providing increased compliance at the peripheral area, so that it makes a bending motion at its whole vibrating area to produce sounds, and more particularly it pertains to a flat-plate type loudspeaker having a diaphragm made of rigid, self-supporting, foamed plastic material, such as foamed polystyrene.

FIGS. 1a and 1b show examples ofa conventional cone-type loudspeaker, which includes a diaphragm I made of conical paper, a speaker frame 2, a voice coil 3, a magnetic circuit 4 and a damper 5. Such a cone-type loudspeaker is designed to produce sound by piston motion of the diaphragm which is formed at its peripheral portion with a flexible suspension 6 having increased compliance and fixed to the speaker frame by means ofa gasket 7.

In such a cone-type loudspeaker it has been proposed to coat the flexible suspension 6 with softening paint 8, as shown in FIG. 1a, or to line the suspension 6 with sound-absorbing material 9 such as polyurethane foam, as shown in FIG. lb, whereby dips in frequency characteristics produced in medium frequency sound range due to opposite-phase resonance of the suspension are reduced and reflection at the suspension produced in the high-frequency sound range is also reduced.

FIGS. 2a and 2b show examples of a flat-plate type loudspeaker having a substantially flat plate of rigid, self-supporting, foamed plastic material such as polystyrene foam which is fixed at its peripheral portion to a speaker frame, so that it makes a bending motion at its vibrating part to produce sounds. This type of loudspeaker comprises a diaphragm 11 made of foamed plastic material, a speaker frame 12, a voice coil 13, a magnetic circuit 14 and a damper 15. In general, the flat-plate type loudspeaker is directly clamped at its peripheral portion to the speaker frame, without providing increased compliance area at its peripheral portion, as shown in FIG. 2a. Such a loudspeaker is designed to provide a large surface area of the diaphragm so that it can satisfactorily reproduce lowfrequency sound.

In order to reproduce lower frequency sound by smaller surface area of the diaphragm, it has been proposed to form .a peripheral portion 16 having reduced thickness, as shown in FIG. 2b.

In principle, the effect to be obtained by providing such a thin peripheral portion 16 is substantially similar to that obtained by the flexible suspension of the cone-type loudspeaker as mentioned above. That is, the diaphragm tends to make a piston motion rather than a .bending motion in the lowfrequency sound range, and consequently the thin peripheral portion 16 is likely to 'produce opposite-phase resonance, in the same manner as in the flexible suspension of the cone-type loudspeaker, in the medium frequency sound range. In order to avoid such a disadvantage, it has been proposed to apply light sound-absorbing felt 17 or damping material such as liquid rubber to the thin portion 17, which can improve the frequency characteristics to some degree by reducing the occurrence of the opposite-phase resonance and also absorbing the vibration of the peripheral area in the medium and highfrequency sound range.

It will be understood from the above explanation that in the conventional loudspeaker having a peripheral portion of increased compliance, such as shown in FIGS. la, lb and 2b, means should be provided to damp and absorb the vibration of the said portion, in order to avoid the adverse effect of the vibration of the peripheral portion. In the loudspeaker of the kind as shown in FIG. 2a, in which the diaphragm is directly clamped to the speaker frame without providing increased compliance at its peripheral portion, the diaphragm is adapted to make bending motion over the whole area thereof and includes no portion corresponding to the flexible suspension in the cone-type loudspeaker, so that there is no problem of oppositephase resonance. However, the diaphragm of this kind acts over its whole area like the suspension portion of the cone-type loudspeaker, so that turbulence in frequency characteristic occurs at a lot of frequency points where resonant divided vibrations of the diaphragm occur, like the turbulence produced by the opposite-phase resonance of the suspension portion. The term resonant divided vibration" as used in the specification means second or higher order mode of the resonant vibration. In general, it is very difficult to decrease such a turbulence in the frequency characteristic. In order to improve the frequency characteristic it has been proposed to form the diaphragm in asymmetric form as seen in front view so that the vibration mode may be divided in complicated form thereby avoiding the occurrence of large peaks and dips in frequency characteristic. It has been also proposed to form the central portion of the diaphragm in a conical form in order to increase flexural rigidity at said portion, thereby improving the frequency characteristics.

In the loudspeaker having rnultiresonance characteristic as mentioned above, the peaks and dips can be reduced, but there still remains appreciable peaks and dips in some range of frequency characteristics. The peaks and dips can be further reduced by making the diaphragm from material having relatively high damping property (material having high internal loss) or applying damping agent onto the surface of the diaphragm. However such a measure necessarily results in increase of weight of the diaphragm and considerable decrease of radiation efficiency. Furthermore, even if the internal loss of the material of the diaphragm is increased, the peaks and dips produced in the frequency range having long wavelength cannot be decreased.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a loudspeaker of the above kind which suppresses excessive resonant divided vibration of the diaphragm, thereby decreasing the peaks and dips in frequency characteristics and obtaining relatively flat frequency characteristic curve.

It is another object of the invention to provide a loud speaker of the above kind which particularly reduces dips produced in the low-frequency sound range.

It is another object of the present invention to provide a loudspeaker of the above kind which can obtain a flat frequency characteristic curve, without reducing radiation efficiency, and reproduce welldamped and clear sounds.

It is a further object of the present invention to provide a loudspeaker of the above kind which can obtain the above effects in simple, stable and efficient manner.

In accordance with the present invention there is provided a loudspeaker including a diaphragm of rigid, self-supporting, foamed plastic material (e.g. foamed polystyrene) clamped at its peripheral portion to a speaker frame, without providing increased compliance at said peripheral portion, to generate sound by bending motion of said diaphragm, said diaphragm having a soft material such as foamed or unfoamed rubberlike material (e.g. natural rubber, synthetic rubber such as neoprene foam or rubberlike plastic material) applied on at least a part of the peripheral portion at at least one side thereof, said soft material having relatively high specific gravity sufficient to suppress resonant divided vibration of the diaphragm in low-frequency sound range.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. I and 2 illustrate examples of loudspeakers according to prior art, in which:

FIGS. la and lb are sectional views of a cone-type loudspeaker, and

FIGS. 2a and 2b are sectional views of a flat-plate type loudspeaker.

FIGS. 3a-c to 18 illustrate several embodiments of the present invention, in which:

FIG. 3a is a sectional view showing an embodiment of the present invention;

FIGS. 3b and 3c are fragmentary sectional views showing alternate forms of the embodiment of FIG. 3a.

FIG. 4 is a rear view of FIG. 3;

FIGS. 5, 6 and 7 are rear views showing modifications, respectively;

FIG. 8 is a sectional view taken along line VllI-VIII in FIG.

FIG. 9 is a sectional view showing another embodiment of the present invention;

FIG. 10 is a graph showing frequency characteristic of the flat-plate type loudspeaker of prior art;

FIG. 11 is a graph showing frequency characteristic of the loudspeaker according to the present invention;

FIG. 12 is a fragmentary sectional view of another embodiment of the present invention;

FIG. 13 is a fragmentary sectional view, similar to FIG. 12, showing a further embodiment of the invention;

FIG. 14 is a rear view showing another embodiment of the invention;

FIG. 15 is a perspective view, in enlarged scale, showing a peripheral portion of the diaphragm shown in FIG. 14;

FIG. 16 is a rear view, similar to FIG. 14, showing another embodiment of the invention;

FIG. 17 is a sectional view showing another embodiment of the invention; and

FIG. 18 is a sectional view showing the soft material in the embodiment shown in FIG. 17.

PREFERRED EMBODIMENTS OF THE INVENTION FIGS. 3a-l8 illustrate several embodiments of the present invention.

Referring to FIGS. 3a and 4, the loudspeaker comprises a diaphragm 21 made of rigid, self-supporting, foamed plastic material such as foamed polystyrene, a speaker frame 22, a voice coil 23, a magnetic circuit 24 and a damper 25. The diaphragm 21 is fixed at its peripheral portion to the speaker frame 22 by means of a thin cushion tape 26. A soft material 27 having a relatively high specific gravity sufficient to suppress divided vibration of the diaphragm in low-frequency sound range is applied on the rear side of the diaphragm around the whole peripheral portion of the vibrating part of the diaphragm. The soft material 27 is preferably made of the above-mentioned foamed or unfoamed rubberlike elastomer, and the soft material having adhesive agent previously applied on one surface thereof is most preferred.

It should be understood that the soft material 27 used in the present invention must have a sufficiently high specific gravity and that the specific gravity of the liquid rubber or like damping agent, felt or low molecular-weight polyurethane foam (such as moltprene, specific gravity 0.025) used in the loudspeaker shown in FIGS. la, lb and 2b is too low to accomplish the object of the present invention. Accordingly, the specific gravity of thesoft material should be sufficiently higher than that of the diaphragm to suppress the divided vibration in the low-frequency sound range.

The position, thickness or width of the soft material 27 is selected, depending on shape and size of the loudspeaker and materialand thickness of the diaphragm. The test has shown that the frequency characteristic can be considerably improved by applying the soft material on the whole peripheral portion of the diaphragm, as shown in FIG. 4. However, the soft material 27 may be applied onto a part of the peripheral portion of the diaphragm. It may be applied onto the rear side and/or front side of the diaphragm. Even when the soft material is applied as shown in FIG. 4, the thickness and the width should be carefully selected and if it is too small the sufficient effect cannot be accomplished, while if it is too large, deep dips tend to occur.

By applying the soft material on the peripheral portion of the vibrating part, the adverse effect produced by the reflection at the fixed peripheral portion of the vibration in the medium and high-frequency sound range can be decreased, without decreasing radiation efficiency of the loudspeaker. If the soft material were applied on the central portion rather than the peripheral portion, more effect could be obtained to reduce peaks and dips due to resonance of divided vibrations in the low-frequency sound range, but there would be considerable decrease in radiation efficiency, which is of course disadvantageous. Therefore the soft material should be applied on the peripheral portion of the diaphragm so that the decrease in radiation efficiency is negligible.

A satisfactory result could be obtained by the loudspeaker as shown in FIGS. 30 and 4, with the following specifications:

Diaphragm (21) material foamed polystyrene specific gravity 0.07

durometer hardness (ASTM designation: D224064T about 50 A-type durometer hardness) size minor diameter 320 mm. major diameter 440 mm.

thickness about 2 mm.

Soft material (27) material foamed neoprene specific gravity 0.20

durometer hardness (ASTM designation; D2240-64T less than 3 A-type durometer hardness) thickness 5 mm.

width about 30 mm.

Preferably the total weight of the soft material is selected to about 20-200 percent of the total weight of the diaphragm and in general the former is selected to be approximately the same as the latter. In the above example, the diaphragm having specific gravity 0.020.20 and the soft material having specific gravity about 0. l0l .25 may be used, and the diaphragm having thickness l-IO mm. may be used. A further sheet, such as vinyl chloride sheet may be applied on the front surface of the diaphragm to improve its mechanical properties, such as flexural rigidity and internal friction loss.

In the embodiment shown in FIGS. 3a and 4, the soft material has a uniform sectional shape throughout the whole peripheral portion of the diaphragm, but it may partly vary its thickness and/or width or it may be applied on the diaphragm, with small space being left inside of the inner periphery of the speaker frame.

FIGS. 5-9 illustrate several modifications. FIG. 5 shows a diaphragm 31 of asymmetric shape having a soft material 37 which has wider width in the direction of the major radius and narrower width in the direction of the minor radius.

FIG. 6 shows a diaphragm 41 having a soft material 47 applied thereon, with small space 49 being left inside of the inner periphery of the speaker frame 42.

FIGS. 7 and 8 show a diaphragm 51 having a soft material 57 applied on the peripheral portion thereof and a weight-adding material 58 made of foamed or unfoamed rubberlike elastomer such as natural rubber or synthetic rubber, superim' posed on the soft material 57 to further increase the weight thereof.

In these embodiments the soft material may be applied on the front surface rather than the rear surface of the diaphragm as shown in FIG. 3b wherein like parts are designated by the same numerals as in FIG. 3a or may be applied on both surfaces as shown in FIG. 3c wherein like parts are designated by the same numerals as in FIGS. 3a and 3b In the latter case, the soft material may be applied in different manner on the front side and the rear side.

FIG. 9 shows a diaphragm 61 which is fixed at its peripheral portion to a speaker frame 62 by means of a soft material 67 itself which acts also as an adhesive tape.

It will be understood that in accordance with the present invention the frequency characteristic can be considerably improved by addition of the soft material having high specific gravity, on the following reasons: (I) When producing sound by bending motion of a diaphragm, in a first-order (flexural, but not divided) vibration mode, the vibration mainly occurs in the vicinity of the central portion of the diaphragm, so that the bending angle of the diaphragm at the peripheral portion thereof is relatively small, but in the lower-order mode of divided vibration the amplitude is relatively large since the divided vibration of lower order occurs in the lower frequency sound range and the maximum amplitude is located closer to the peripheral portion, so that the bending angle of the diaphragm at the peripheral portion is relatively large. In divided vibration of higher order, the amplitude is smaller and the vibration predominantly occurs in the central area, so that the bending angle is smaller. According to the present invention the material having relatively heavy weight attached to the peripheral portion of the diaphragm serves to damp the divided vibration of lower order, and thus reduce the peaks and dips in the frequency characteristic within the low-frequency sound range. (2) The material applied to the peripheral portion is sufficiently soft that the flexural rigidity at the peripheral portion of the diaphragm is not substantially increased thereby even if the soft material is applied to the whole peripheral portion of the diaphragm. Consequently, lowest resonance frequency f0 is not substantially affected by the soft material. (3)

The soft material is applied to the periphery of the diaphragm, and therefore the effective mass of the diaphragm is not substantially increased by the addition of the soft material even if it has relatively high specific rigidity. Accordingly the radiation efficiency is not substantially lowered.

By applying the foamed or unfoamed rubber like elastomer on the peripheral portion of the diaphragm, the peaks and dips owing to the resonance at the lower frequency having long wavelength is damped by the weight of said elastomer, while the peaks and dips produced in the medium and high frequency range are absorbed by the elastomer and the reflection at the fixed periphery of the diaphragm is reduced, without the reduction of the efficiency.

FIG. shows a frequency characteristic of the loudspeaker which is essentially identical with that shown in FIGS. 3 and 4 but has no soft material applied on the diaphragm. FIG. 11 shows a frequency characteristic of the loudspeaker which is identical with the above and has the soft material applied on the diaphragm as shown in FIGS. 3 and 4.

FIG. 12 shows another embodiment of the invention including a diaphragm 7i, a speaker frame 72 and a soft material 77. The soft material 77 is formed as a supporting plate which is fixed to the speaker frame 72 and has an inner periphery projecting toward the center of the loudspeaker, to which the diaphragm 71 is fixed at its periphery. As shown in FIG. 12, the outer peripheral surface 73 is substantially aligned with the inner periphery 74 of the diaphragm attaching surface 75 of the speaker frame 72.

In the construction of the diaphragm in which the diaphragm extends to the outer periphery of the diaphragm attaching surface of he speaker frame, flexural rigidity of the peripheral portion of the diaphragm is relatively high and consequently lowest resonance frequency f0 is of relatively high value. In the construction as shown in FIG. 12, in which the outer periphery 73 of the diaphragm terminates at the inner periphery 74 of the attaching surface 75 of the speaker fame and is fixed through the soft material 77 to the attaching surface 75, the flexural rigidity at the peripheral portion of the diaphragm is relatively low and consequently the lowest resonance frequency is decreased, whereby the characteristic in the low-frequency sound range is considerably improved.

It should be understood that the outer periphery of the diaphragm is aligned with the inner periphery of the speaker frame, so that the diaphragm cannot make piston motion and the force applied to the diaphragm acts to produce bending motion of the diaphragm which is fixed at its peripheral portion to the speaker frame,

FiG. 13 shows an embodiment of the invention including a diaphragm 31, a speaker frame 82 and a soft material 87. The soft material 37 is formed as a supporting plate which is similar to the supporting plate 77 shown in FIG. 12. In this form, the outer periphery 83 of the diaphragm 81 extends beyond the inner periphery 84 of the attaching surface 85 of the speaker frame 82 and terminates at the midway of said surface 85. An adhesive retaining member 86 made of soft foamed plastic material having durometer hardness (ASTM designation: D240-64T A-type durometer hardness) of less than 3, such as foamed polyurethane is adhered onto the peripheral portion of the diaphragm and the soft material 87 so as to hold the peripheral portion of the diaphragm. Thus the diaphragm can be tightly held on the attaching surface of the speaker frame.

FIG. 14 and FIG. 15 show an embodiment which includes a diaphragm 91, a speaker frame 92, a magnetic circuit 94 and a soft material 97 made of rubberlike elastomer, such as natural rubber, synthetic rubber such as chloroprene, rubberlikc plastic or foam of such material or the like. The soft material 97 has an inner periphery 95 in serrated form. FIG. 16 shows a modification in which the soft material 97 has an inner periphery 9% in sinuous form.

In the construction shown in FIGS. 14 and 15 or FIG. 16, the inner periphery of the soft material 97 is formed in serrated or sinuous shape and consequently propagating waves reflected at the inner periphery of the soft material irregularly change their phases and directions so that standing waves due to reflection substantially disappear. Thus the reflection as a whole at the edge of the soft material is decreased and the peaks and dips in the frequency characteristic owing to the interference of the propagating waves in the medium and high frequency sound range are decreased, whereby the frequency characteristic in the medium and high-frequency sound range is considerably improved. The soft material of this form can be manufactured in effective manner by forming a straight strip of the soft material and then dividing it into two pieces by cutting a serrated or sinuous slit along the centerline of said strip.

FIGS. 17 and 18 show an embodiment including a diaphragm I01, a speaker frame I02 and a soft material 103. The soft material 103 has a tapered inside surface 104 and gradually decreases its thickness toward the center of the diaphragm. In such a construction, the boundary condition at the peripheral portion of the diaphragm, to which the tapered soft material is attached, varies gradually and there is not formed a well-defined reflecting line. Accordingly the whole reflection at the soft material is reduced, whereby the frequen cy characteristic in the medium and high frequency sound range is improved.

While it will be apparent that the preferred embodiments of the invention disclosed are well calculated to fulfill the objects above stated, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope of the appended claims.

Iclaim:

I. A loudspeaker comprising a speaker frame having an attaching surface, a diaphragm of rigid, self-supporting, foamed plastic material within said frame, said diaphragm having a vibrating portion for generating sound by bending movement thereof, and a soft material having opposing adhesive surfaces fixed at its one surface to the outer edge and part of the vibrating portion of the diaphragm and at its other surface to the attaching surface of the speaker frame, said soft material having relatively high specific gravity sufficient to suppress resonant divided vibration of the diaphragm in the low frequency sound range.

2. A loudspeaker according to claim I in which the soft material which is fixed to the speaker frame has an inner portion projecting inwardly of said speaker frame and the diaphragm has an outer edge aligned with the inner edge of the attaching surface of said speaker frame.

3. A loudspeaker according to claim 1 in which the soft material which is fixed to the speaker frame has an inner por tion projecting inwardly of said speaker frame, the diaphragm has an outer edge terminating at about the middle of the attaching surface of said speaker frame and an adhesive retainmaterial has an inner periphery in serrated form. in member is attached to said diaphragm and to said soft 6 A loudspeaker according to claim 1 in which the soft material at the outer portions thereof. material has an inner periphery in sinuous form.

4 A loudspeaker a o din t l i 1 i hi h h 7. A loudspeaker according to claim l,in which the the soft diaphr gm has an uter ge aligned with the outer ed f 5 material has a tapered inside surface and gradually decreases the attaching Surface f the Speaker f its thickness toward the center of the diaphragm.

5. A loudspeaker according to claim 1 in which the soft

Claims

1. A loudspeaker comprising a speaker frame having an attaching surface, a diaphragm of rigid, self-supporting, foamed plastic material within said frame, said diaphragm having a vibrating portion for generating sound by bending movement thereof, and a soft material having opposing adhesive surfaces fixed at its one surface to the outer edge and part of the vibrating portion of the diaphragm and at its other surface to the attaching surface of the speaker frame, said soft material having relatively high specific gravity sufficient to suppress resonant divided vibration of the diaphragm in the low frequency sound range.

2. A loudspeaker according to claim 1 in which the soft material which is fixed to the speaker frame has an inner portion projecting inwardly of said speaker frame and the diaphragm has an outer edge aligned with the inner edge of the attaching surface of said speaker frame.

3. A loudspeaker according to claim 1 in which the soft material which is fixed to the speaker frame has an inner portion projecting inwardly of said speaker frame, the diaphragm has an outer edge terminating at about the middle of the attaching surface of said speaker frame and an adhesive retaining member is attached to said diaphragm and to said soft material at the outer portions thereof.

4. A loudspeaker according to claim 1 in which the diaphragm has an outer edge aligned with the outer edge of the attaching surface of the speaker frame.

5. A loudspeaker according to claim 1 in which the soft material has an inner periphery in serrated form.

6. A loudspeaker according to claim 1 in which the soft material has an inner periphery in sinuous form.

7. A loudspeaker according to claim 1, in which the the soft material has a tapered inside surface and gradually decreases its thickness toward the center of the diaphragm.