Speaker unit

A speaker unit is realized which directly drives a vibration plate having a low density, light weight, yet sufficient rigidity with a digital audio signal, and can thereby transmit vibration of a voice coil thereof to a carbonaceous acoustic vibration plate without loss. The present invention provides a digital speaker unit including a speaker body (14) comprising a carbonaceous acoustic vibration plate (25), a delta-sigma modulator (11) and a thermometer code conversion section (12) that convert a multi-value bit digital audio signal supplied from a digital sound source (10) to a digital signal with required bits, a plurality of voice coils (24) that cause to vibrate a plurality of the carbonaceous acoustic vibration plates (25) provided in accordance with the number of digital signal bits and a driver circuit (13) that individually drives each voice coil (24) based on the digital signal.

TECHNICAL FIELD

The present invention relates to a speaker unit for sound reproduction, and more particularly, to a speaker unit directly driven by a digital audio signal.

BACKGROUND ART

Conventionally, digital speakers are being developed which reproduce a digital audio signal not by converting it to an analog signal but directly supplying it to a speaker (e.g., see Patent Literature 1). The digital speaker described in Patent Literature 1 assigns weights to a plurality of voice coils wound around a voice coil bobbin respectively so that a drive force corresponding to each bit of the digital signal is generated, the polarity of a certain voltage applied to each voice coil is changed according to the binary value of the respective two bits of the digital signal and the direction of a current flowing through the voice coil is thereby set according to the binary value. This configuration allows a drive force to be generated at a ratio corresponding to quantization of the digital signal.

Furthermore, speaker units are being proposed which apply a digital/analog conversion apparatus that generates an analog signal of high quality from a digital signal to a drive apparatus of a digital speaker to thereby improve quality of reproduced sound and realize circuit scale reduction (e.g., see Patent Literature 2). The speaker unit described in Patent Literature 2 converts an n-bit output of a delta-sigma modulator to a thermometer code through a formatter, performs mismatch shaping processing using a post filter, inputs the output to a buffer circuit, controls a coil with the digital signal outputted from the buffer circuit and adds a magnetic field thereto (see paragraphs 0063 and 0078).

On the other hand, vibration plates of speakers used for mobile devices such as acoustic devices, video equipment and mobile phones are required to have the ability to accurately reproduce clear sound in a wide frequency band, and a high frequency range in particular. Therefore, the material of the vibration plate is required to have a high elastic modulus so as to give sufficient rigidity to the vibration plate and a low density so as to reduce the weight of the vibration plate, which are apparently mutually contradictory characteristics. Especially vibration plates for digital speakers which are becoming a focus of attention in recent years are strongly required to satisfy these characteristics from the standpoint of requirements for vibration response.

CITATION LIST

Patent Literature

Patent Literature 1: Japanese Patent Application Laid-Open No. 4-326291Patent Literature 2: Pamphlet of International Publication No. 2007/135928

SUMMARY OF INVENTION

Technical Problem

It is therefore an object of the present invention to provide a speaker unit capable of directly driving a vibration plate having a low density, light weight, yet sufficient rigidity with a digital audio signal and transmitting vibration of a voice coil to a carbonaceous acoustic vibration plate, thus realizing excellent acoustic characteristics.

Solution to Problem

A speaker unit according to the present invention includes a carbonaceous acoustic vibration plate, a voice coil made up of a cylindrically wound conductive wire, one open end portion of which is fixed in direct contact with the carbonaceous acoustic vibration plate, magnetic flux generating section configured to generate a magnetic flux that penetrates the cylindrical voice coil in a diameter direction, and drive section configured to supply a drive current corresponding to an audio signal to the voice coil.

Since this configuration adopts a structure in which one end portion of the voice coil directly contacts the carbonaceous acoustic vibration plate, vibration excited by the voice coil in response to the audio signal is transmitted to the carbonaceous acoustic vibration plate without loss. Since the vibration of the voice coil can be transmitted to the carbonaceous acoustic vibration plate efficiently, it is possible to realize a speaker capable of outputting a sound that accurately reproduces the audio signal.

Furthermore, in the above-described speaker unit of the present invention, the voice coil is made up of a plurality of unit voice coils corresponding to the number of bits of the digital signal configured by making the plurality of unit voice coils have different diameters and sequentially inserting the unit voice coils such that a unit voice coil of a smaller diameter is inserted into a unit voice coil of a greater diameter and the drive section individually drives the each unit voice coil based on each bit value of the digital signal.

According to this configuration, the speaker body comprising the carbonaceous acoustic vibration plate is directly driven with a digital signal, and it is thereby possible to realize excellent acoustic characteristics by taking advantage of characteristics of the carbonaceous acoustic vibration plate which has a low density, light weight yet sufficient rigidity.

Furthermore, in the above-described speaker unit of the present invention, the each unit voice coil is configured by cylindrically winding a conductive wire having an oblong cross section such that wires neighboring each other in a direction orthogonal to the coil diameter direction are in close contact with each other in the major axis direction of the wire cross section.

According to this configuration, even when a plurality of unit voice coils are multilayered in the diameter direction, it is possible to suppress the coil thickness (one layer or multilayer) in the coil diameter direction of the voice coil as a whole, narrow the gap in which the voice coil is arranged so as to allow a magnetic flux to penetrate the voice coil and reduce magnetic loss.

Furthermore, in the above-described speaker unit of the present invention, the each unit voice coil is configured by cylindrically winding a conductive wire having an oblong cross section such that wires neighboring each other in a direction orthogonal to the coil diameter direction are in close contact with each other in the minor axis direction of the wire cross section.

According to this configuration, since the conductive wire making up the unit voice coil is configured such that the neighboring wires contact each other densely in the minor axis direction of wire cross section, it is possible to further suppress loss when transmitting vibration excited by the voice coil to the carbonaceous acoustic vibration plate.

Furthermore, in the above-described speaker unit of the present invention, the carbonaceous acoustic vibration plate has a first principal surface to which an open end portion of the voice coil is fixed and a second principal surface opposite to the first principal surface, and the voice coil is arranged so that an outermost circumference position of the open end portion is located at a position deviated inward from the vibration plate outer circumferential edge and one end portion of a support member that supports the carbonaceous acoustic vibration plate in a vibratable manner on the vibration plate outer circumferential edge which is on the second principal surface and does not overlap the fixed position of the open end portion of the voice coil.

According to this configuration, since one end portion of the support member which supports the carbonaceous acoustic vibration plate is fixed on the vibration plate outer circumferential edge that does not overlap with the voice coil fixed position in a vibratable manner, it is possible to allow the support member to directly absorb the vibration given by the voice coil to the carbonaceous acoustic vibration plate, thereby avoid a problem that the carbonaceous acoustic vibration plate becomes inflexible, and reduce deterioration of vibration characteristics of the carbonaceous acoustic vibration plate to a minimum.

Furthermore, in the above-described speaker unit of the present invention, the magnetic flux generating section includes a yoke having an end portion facing an outer circumferential surface of the voice coil fixed to the carbonaceous acoustic vibration plate, a centerpiece, inserted into the coil from the other open end portion of the voice coil, that forms a gap between opposed end portions of the yoke and itself, and a permanent magnet located between the centerpiece and the yoke, one magnetic pole of which is faced on the centerpiece side and the other magnetic pole of which is faced on the yoke side, and the carbonaceous acoustic vibration plate has a first principal surface to which an open end portion of the voice coil is fixed, a second principal surface provided opposite to the first principal surface and a convex portion formed at a position at which the open end portion of the voice coil is fixed on the first principal surface wherein the convex portion has a height that a central portion of the voice coil becomes a gap position between the end portion of the yoke and the centerpiece.

According to this configuration, the voice coil is arranged so that its central portion is located at the gap position, which maximizes the number of magnetic fluxes that cross the voice coil and maximizes the force by a current flow through the voice coil. That is, it is possible to vibrate the carbonaceous acoustic vibration plate most efficiently.

In the speaker unit, lead positions of lead wires connected to the respective unit voice coils are preferably distributed uniformly on the outer circumference of the carbonaceous acoustic vibration plate. Since the tension of the lead wires drawn from the unit voice coils has a large influence on the vibration characteristics of the carbonaceous acoustic vibration plate, uniformly distributing the lead positions of the lead wires at locations on the outer circumference of the carbonaceous acoustic vibration plate makes it possible to realize a lead structure that will not deteriorate the vibration characteristics of the carbonaceous acoustic vibration plate.

In the above-described speaker unit of the present invention, the drive section includes a delta-sigma modulator that delta-sigma modulates a multi-value bit digital audio signal supplied from a digital sound source and individually drives the each voice coil based on the digital signal outputted from the delta-sigma modulator.

According to this configuration, the using of the delta-sigma modulator makes it possible to eliminate, through a noise shaping effect, quantization noise produced in the process of converting a multi-value bit digital audio signal supplied from the digital sound source to a digital signal with required bits and reduce quantization errors using an oversampling method.

Furthermore, in the above-described speaker unit of the present invention, the drive section includes a thermometer code conversion section configured to convert a digital signal with predetermined bits outputted from the delta-sigma modulator to a thermometer code with bits corresponding to the number of the voice coils.

According to this configuration, since a binary number outputted from the delta-sigma modulator is a signal, each bit of which is weighted, it is difficult to perform direct drive in digital using the signal as is, but by converting the signal to a thermometer code, each bit of which is not weighted, it is possible to drive directly the speaker body with a digital signal.

In the above-described speaker unit, the carbonaceous acoustic vibration plate may be made of a porous material containing amorphous carbon and carbon powder uniformly dispersed in the amorphous carbon and having a porosity of 40% or above.

Furthermore, in the above-described speaker unit, the carbonaceous acoustic vibration plate may also be configured to include a low-density layer containing amorphous carbon and carbon powder uniformly dispersed in the amorphous carbon and made of a porous material having a porosity of 40% or above, and a high-density layer which contains amorphous carbon, is thinner than the low-density layer and has a higher density than the low-density layer.

In the above-described speaker unit, the speaker body may also be configured to make the voice coil vibrate in contact with the carbonaceous acoustic vibration plate. Alternatively, a configuration may also be adopted in which the carbonaceous acoustic vibration plate is supported by a flexible film and the voice coil is made vibrate in contact with the film.

Advantageous Effects of Invention

The present invention can provide a speaker unit capable of directly driving a vibration plate having a low density, light weight yet sufficient rigidity with a digital audio signal, transmitting vibration of the voice coil to the carbonaceous acoustic vibration plate without loss and realizing excellent acoustic characteristics.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. An embodiment of the present invention is a digital speaker unit including a carbonaceous acoustic vibration plate as a vibration plate of a speaker body, for directly driving a voice coil with a digital signal supplied from a digital sound source to cause the carbonaceous acoustic vibration plate to vibrate. The present invention is suitable for use in a digital speaker unit, but is also applicable to a drive scheme using an analog audio signal.

First Embodiment

FIG. 1is an overall schematic view of a digital speaker unit according to a first embodiment of the present invention. InFIG. 1, a digital sound source10may be comprised of a CD player, DVD player or other digital devices for sound reproducing and outputs a digital audio signal to a digital speaker unit.

The digital speaker unit according to the present embodiment includes a multi-bit delta-sigma modulator11, a thermometer code conversion section12that converts a digital signal outputted from the delta-sigma modulator11to a weightless N-bit thermometer code, a driver circuit13that performs drive control based on the thermometer code and a speaker body14comprising a carbonaceous acoustic vibration plate as principal components.

The structure of the speaker body14will be described with reference toFIG. 2.

The speaker body14comprises a bottomed cylindrical yoke22having a center pole21in the center and a magnet23disposed at a proximal end of the center pole21. The magnet23, yoke22and center pole21constitute a magnetic circuit. Furthermore, in the magnetic circuit, the speaker body14comprises a plurality of voice coils24provided via a coil bobbin (not shown) that surround the outer circumference of the center pole21with a certain space in therebetween, and a carbonaceous acoustic vibration plate25attached at an end portion of the voice coil24. The outer circumferential edge of the carbonaceous acoustic vibration plate25is supported by a frame27via an edge26in a vibratable manner. The number N of coils of the plurality of voice coils24corresponds to the number N of output bits of the thermometer code conversion section12.

FIG. 3toFIG. 5show conceptual diagrams of the speaker drive system. N unit voice coils (24-1to24-N) are independently arranged (FIG. 3) and wound around a coil holding section28, one end of which is connected to the carbonaceous acoustic vibration plate25(FIG. 4). Instead of using the coil holding section28, a structure may also be adopted in which one ends of the unit voice coils (24-1to24-N) are directly connected to one surface of the carbonaceous acoustic vibration plate25. Furthermore, as shown inFIG. 5, lead wires of the N (3inFIG. 5) unit voice coils (24-1to24-N) are connected to their respective driver circuits13(1) to (N) and drive currents independently flow from the corresponding driver circuits13(1) to (N). The unit voice coils (24-1to24-N) are configured so as to be controllable independently of the driver circuits (1) to (N).

In the speaker body14, a current flows through the voice coil24placed in the magnetic circuit made up of the magnet23, yoke22and center pole21and a force generated in a direction orthogonal to a line of magnetic force in the voice coil24is used to cause the carbonaceous acoustic vibration plate25to vibrate to thereby generate a sound wave. A current corresponding to each bit value of the digital signal outputted from the thermometer code conversion section12flows into the voice coil24.

FIG. 6is a circuit configuration diagram of the delta-sigma modulator11. The circuit configuration shown in the figure is an example and a higher-dimension delta-sigma modulator may also be used. Here, suppose a digital audio signal expressed by multi-value input bits has 16 bits and the n-bit output from the delta-sigma modulator11is 4 bits.

The delta-sigma modulator11is basically configured by including an integrator31, a quantizer32, a delayer33and a feedback loop. τ represents a feedback gain. Multi-value bits (e.g., 16 bits) inputted to the delta-sigma modulator11pass through the integrator31and are converted to n bits (e.g., 9 values=4 bits) by the quantizer32. A quantization error generated in quantization is returned to an input end via a feedback loop that passes through the delayer33, a difference is taken and only the quantization error is integrated. Assuming X represents the input, Y represents the output and Q represents the quantization error, the relational expression is expressed by Y=X+(1−Z−1)Q. The transfer function (1−Z−1) by which the quantization error Q is multiplied has a frequency characteristic and decreases in the vicinity of DC, and therefore this characteristic produces a noise shaping effect which will be described later.

In the delta-sigma modulator11, the quantizer32quantizes the digital audio signal with multi-value bits into a number corresponding to the number n of output bits. The quantization error produced by the quantizer32can be corrected by applying an oversampling technique. Oversampling is one of techniques of sampling at a sufficiently higher frequency than a signal band. Furthermore, in the case of delta-sigma modulation, the accuracy of the original signal can be improved through the noise shaping effect. That is, when quantization is performed using the quantizer, quantization noise is uniformly distributed over all frequencies, but through delta-sigma modulation, unnecessary noise components are shifted to a high oversampled frequency domain, which suppresses noise in the vicinity of the original signal and has the effect of improving the accuracy of the original signal.

The thermometer code conversion section12converts n-bit output of the delta-sigma modulator11to an N-bit thermometer code corresponding to the number of voice coils. When, for example, the output is converted to an 8-bit thermometer code, delta-sigma modulator outputs (0010), (0101) and (1000) are converted to thermometer codes (00000011), (00011111) and (11111111) respectively. Since the binary number outputted from the delta-sigma modulator11is a bitwise weighted signal, using the signal as is may make direct drive in digital difficult, but by converting the output to a thermometer code with no bitwise weight, it is possible to directly drive the speaker body14with a digital signal.

The driver circuit13drives the individual unit voice coils24-1to24-N independently based on the thermometer code outputted from the thermometer code conversion section12. To be more specific, each unit voice coil24-1to24-N is associated with each bit value of the thermometer code in a one-to-one correspondence, a 1-bit signal (ON/OFF) as shown inFIGS. 7(a) and (b) is outputted from the thermometer code conversion section12for each bit of the thermometer code. Driving is performed so as to make a current flow to a voice coil24with thermometer code “1” and not to make any current flow to a voice coil24with thermometer code “0.” The voice coil24itself moves in proportion to the current that flows through the voice coil24and the carbonaceous acoustic vibration plate25connected to the voice coil24vibrates to generate voice.

Next, the structure and manufacturing method of the carbonaceous acoustic vibration plate25used in the present embodiment will be described in detail.

The digital speaker unit of the present invention can use a carbonaceous vibration plate including a porous material containing amorphous carbon and carbon powder uniformly dispersed in the amorphous carbon and having a porosity of 40% or above as the carbonaceous acoustic vibration plate25. The carbonaceous acoustic vibration plate25includes the porous material plate as a low-density layer and preferably further includes a high-density layer which contains amorphous carbon, is thinner than the low-density layer and has a higher density than the low-density layer.

Here, with regard to the number of layers, there can be various configurations such as a two-layer structure with a high-density layer and a low-density layer, a three-layer structure with one low-density layer sandwiched by two high-density layers or conversely a three-layer structure with one high-density layer sandwiched by two low-density layers or one-layer structure with only a high-density layer.

The shape of pores of the porous material is preferably spherical and the number average diameter of pores is preferably 5 μm or above and 150 μm or below. The carbon powder preferably contains carbon nanofibers having a number average diameter of 0.2 μm or below and an average length of 20 μm or below. The high-density layer may contain graphite uniformly dispersed in the amorphous carbon. When the carbonaceous acoustic vibration plate is dried and then left in an environment with a temperature of 25° C. and humidity of 60% for 250 hours, its mass increase is preferably 5% or below.

Furthermore, it is possible to manufacture the carbonaceous acoustic vibration plate using a method of uniformly mixing carbon-containing resin with carbon powder, molding the compound into a film shape, heating the compound to form a carbon precursor and carbonizing the carbon precursor in an inert atmosphere. In such a method of manufacturing a carbonaceous acoustic vibration plate, grains of a pore opening member which is solid or liquid at the carbon precursor formation temperature and disappears at the carbonizing temperature and leaves pores, are mixed with the compound beforehand, and in this way, a porous material is produced which contains amorphous carbon and carbon powder after the carbonization.

Before the carbonization, it is preferable to further include a step of creating a carbonaceous acoustic vibration plate including a low-density layer made of the porous material and a high-density layer having a higher density than the low-density layer after the carbonization by forming a layer of carbon-containing resin on at least one surface of the carbon precursor plate. The structure with the high-density layer sandwiched by the low-density layers is obtained, for example, by bonding, with resin, layers of carbon precursors containing a pore opening member to both sides of a carbon precursor containing no pore opening member, uniting the carbon precursors and carbonizing the united body.

The shape of grains of the pore opening member is preferably spherical. The carbon powder preferably contains carbon nanofibers. The layer of the carbon-containing resin may contain graphite uniformly dispersed therein. The carbonization is preferably performed under a temperature of 1200° C. or above.

As described above, by mixing the compound of carbon-containing resin and carbon powder with grains of a pore opening member such as polymethyl methacrylate (PMMA) which is solid or liquid at the carbon precursor formation temperature and disappears at the carbonizing temperature and leaves pores, this pore opening member disappears leaving cubic pores corresponding to the cubic shape thereof in the process of carbonization. Therefore, it is possible to easily control the porosity by controlling the composition ratio of the pore opening member, easily control the cubic shape and size of pores by selecting the cubic shape and size of grains of the pore opening member and realize a porous material having a porosity of 40% or above.

Here, the porosity is a volume percentage of pores with respect to the volume of the whole porous material containing the pores and is defined as a porosity calculated from the volume and mass of the whole porous material assuming a carbon density is 1.5 g/cm3.

Adopting a multilayered structure with a low-density layer and a high-density layer made of the porous material makes it possible to set a porosity of 60% or above while maintaining necessary rigidity and set a density of the whole vibration plate to 0.5 g/cm3or below.

The high-density layer demonstrates its effect when the thickness thereof is on the order of 1 to 30% of the total thickness and plays the role of reproducing a high frequency range with a rigidity of Young's modulus of on the order of 100 GPa.

The low-density layer has Young's modulus of on the order of 2 to 3 GPa, reduces the weight of the whole vibration plate, maintains sound quality of the whole plate and improves vibration response.

These materials are united, sintered and carbonized to form a carbonaceous member having a plurality of layers, and it is thereby possible to realize a multilayered planar speaker vibration plate capable of controlling the characteristics and outputting sound in an audible sound range up to a high frequency range in particular.

Furthermore, it is also possible to provide rigidity by adopting a dome shape and obtain a planar vibration plate with a high reproduction limit frequency by balancing between a compact and high rigidity high-density layer and beam strength of a light weight, low-density layer which becomes the core. Although the sound reproduction range varies depending on the porosity design, the pore diameter has no considerable influence. The handling ability is excellent and shock resistance also improves. Furthermore, by covering one or both sides of the low-density layer of the porous material with the high-density layer, it is possible to prevent absorption of an adhesive when incorporated into the unit.

A characteristic further required for the acoustic vibration plate is to have a low hygroscopic property so that the acoustic characteristic does not change by absorbing water content in the air and becoming heavier. With the carbonization temperature set to 1200° C. or above, it is possible to obtain an acoustic vibration plate with amass increase of 5% or below when left in an environment with a temperature of 25° C. and a humidity of 60% for 250 hours after drying.

Although a structure in which the carbonaceous acoustic vibration plate is supported by a frame via edges has been described above as an example, it is also possible to adopt a structure in which the carbonaceous acoustic vibration plate is supported by a flexible film.

FIG. 8(a) is a cross-sectional view of a speaker body in which the carbonaceous acoustic vibration plate is supported by a flexible film andFIG. 8(b) is a plan view thereof. As shown inFIG. 8(a), a yoke22, magnet23, center pole21, voice coil24and frame27have a structure similar to that of the speaker body14shown inFIG. 2. A carbonaceous acoustic vibration plate41is fixed to the inner surface of a flexible film42. The flexible film42has a shape with a dome-like swollen central portion and is fixed to the top surface of a tabular film base43. A structure is configured such that one end of a voice coil24contacts the undersurface of the outer circumferential edge of the film base43to transmit vibration. The flexible film42is subjected to concavo-convex processing for securing the strength.

A digital drive system as shown inFIG. 1is connected to the speaker body configured above to constitute a digital speaker unit. The method of driving the speaker body using a digital audio signal supplied from a digital sound source is as described above.

By supporting the carbonaceous acoustic vibration plate41by the flexible film42with required rigidity and flexibility, it is possible to realize a high sound pressure compared to the structure in which the carbonaceous acoustic vibration plate is supported by a frame. A verification experiment conducted by the present inventor shows that a peak sound pressure of 90 dBspl could be realized by combining a film and the carbonaceous vibration plate. Therefore, for application requiring a high sound pressure, a configuration as shown inFIG. 8is preferable in which the carbonaceous acoustic vibration plate41is supported by the flexible film42.

Second Embodiment

Next, a second embodiment of the present invention will be described.FIG. 9is a schematic view illustrating a configuration of a digital speaker unit according to a second embodiment of the present invention and shows a cross-sectional structure of the speaker body. The same components as those in the first embodiment will be assigned the same reference numerals and descriptions thereof will be omitted and only differences from the first embodiment will be mainly described.

A speaker body100comprises a yoke121made up of an iron piece and having a U-shaped cross section, a centerpiece122, a magnet123, a cylindrical voice coil124and a carbonaceous acoustic vibration plate125. The yoke121forms a bottomed cylindrical body having a slightly greater inner diameter than the outer diameter of the voice coil124. A yoke wall portion121a(121b) that stands upright from the bottom outer circumferential edge of the yoke121faces the outer circumferential surface of the voice coil124. The centerpiece122is placed in the inner space of the voice coil124.

The magnet123is placed between the undersurface of the centerpiece122and the opposed surface (top surface of the yoke) on the yoke121. The top surface of the magnet123contacting the undersurface of the centerpiece122is polarized to one magnetic pole (e.g., N pole) and the undersurface contacting the top surface of the yoke121is polarized to the other magnetic pole (e.g., S pole). The magnet123, yoke121and centerpiece122together constitute a magnetic circuit.

The shapes of the yoke121and centerpiece122in a plan view are not particularly limited, but when the yoke121has a bottomed cylinder shape or rectangular box shape, the centerpiece122may have the same shape (similar shape), that is, a circular or rectangular shape and may be set to have such a size that allows a gap to be formed between the yoke wall portions121aand121b, and the outer circumferential portion of the centerpiece122.

The voice coil124is placed in the gap formed between the yoke wall portion121a(121b) and the outer circumferential edge of the centerpiece122. The voice coil124is configured by stacking a plurality of unit voice coils124-1,124-2and124-3one on another in the diameter direction. The number N of the plurality of unit voice coils124-1,124-2and124-3corresponds to the number N of output bits of the thermometer code conversion section13. The voice coils124are arranged such that at least some of the voice coils124extend across the gap formed with the yoke wall portion121a(121b) and the outer circumferential edge of the centerpiece122.FIG. 9shows an example where the lower part of the voice coil124extends across the gap. The unit voice coils124-1,124-2and124-3are configured by cylindrically winding a conductive wire crushed so as to have an oblong cross section.

The carbonaceous acoustic vibration plate125is arranged at a predetermined distance L1from the top surfaces of the yoke121and centerpiece122. The carbonaceous acoustic vibration plate125has an outer diameter greater than that of the voice coil124. One open end portion of the voice coil124is bonded and fixed to the undersurface of the carbonaceous acoustic vibration plate125in direct contact therewith. That is, one end portion of the voice coil124is fixed to the carbonaceous acoustic vibration plate125side and the other open end portion of the voice coil124is left open. Furthermore, the voice coil124is mounted such that the outermost circumferential position thereof in the diameter direction is located inward at a predetermined distance L2from the outer circumferential edge of the carbonaceous acoustic vibration plate125.

A frame126is placed so as to surround the outer circumferences of the yoke121, voice coil124and carbonaceous acoustic vibration plate125. The frame126supports the yoke121via a supporting member127of high rigidity and supports the carbonaceous acoustic vibration plate125via an elastic edge128in a vibratable manner. The edge128preferably has a function of supporting the carbonaceous acoustic vibration plate125in a vibratable manner and a damper function of preventing vibration of the carbonaceous acoustic vibration plate125from continuing.

As described above, the outermost circumferential position of the voice coil124in the diameter direction is located inward at the predetermined distance L2from the outer circumferential edge of the carbonaceous acoustic vibration plate125. The present embodiment secures a mounting portion129for fixing a vibration plate side end of the edge128within the range from the outer circumferential edge of the carbonaceous acoustic vibration plate125to the distance L2, which is a region in which the one open end portion of the voice coil124is not in direct contact. That is, the vibration plate side end of the edge128is fixed to the mounting portion129and the frame side end thereof is fixed to part of the frame126.

Here, manufacturing steps of the voice coil124will be described with reference toFIG. 10toFIG. 13. As shown inFIG. 10, a coil wire42wound around a drum41is unreeled and crushed as it passes between a pair of rollers43aand43b. As a result, as shown inFIG. 11, the coil wire42aafter passing between the rollers is deformed from a perfect circular to oblong cross-sectional shape.

Next, as shown inFIG. 12, the coil wire42awhose cross-sectional shape is deformed into an oblong shape is wound around a winding jig44so as to have the cylindrical shape of the voice coil124. In the case of the three-channel (124-1,124-2,124-3) structure shown inFIG. 9, the unit voice coil124-3located innermost is wound around the winding jig44first. The winding section44aof the winding jig44preferably has the same shape as the cross-sectional shape of the voice coil124in the diameter direction. AlthoughFIG. 12schematically illustrates an oblong shape, an arbitrary shape may be adopted using winding sections44ahaving circular, ellipsoidal, rectangular cross sections or the like. The winding width can be adjusted by replacing a plug-in type winding section44a.

FIG. 13is a cross-sectional view when winding using the winding jig44is in progress. The wire is wound by placing the surface crushed into an oblong shape of the coil wire42aset as the winding surface side of the winding section44aand wound densely so that there remain no spaces between the neighboring coil wires42ain the direction of the axis of rotation. This makes it possible to obtain a unit voice coil in which the wire is cylindrically wound such that the wires neighboring in the direction orthogonal to the coil diameter direction are arranged in close contact with each other in the major axis direction of the cross-section of the wire.

When two layers of wire are wounded around the outer circumferential surface of the winding section44aof the winding jig44, the winding operation of the innermost unit voice coil124-3is completed. Both end portions of the coil wire42amaking up the unit voice coil124-3are led out and made connectable to a driver circuit which will be described later. The lead positions of the coil wire42awill be described in detail later.

Next, the coil wire42amaking up the unit voice coil124-2located in the middle is wound around the outer circumferential surface of the innermost unit voice coil124-3in the same way as for the unit voice coil124-3. In this case, since the coil wire42ais crushed so as to have an oblong cross-section and the wires are stacked one on another such that the crushed surfaces contact each other, it is possible to stack the wires one on another without unbalanced wire alignment. When the winding operation of the middle unit voice coil124-2is completed, winding of the outermost unit voice coil124-1is performed likewise.

As described above, winding the coil wire42afor an outer unit voice coil around the outer circumference of an inner unit voice coil results in a structure in which a unit voice coil on a smaller diameter side is sequentially inserted in a unit voice coil on a greater diameter side.

To transmit vibration created in the produced voice coil124to the carbonaceous acoustic vibration plate125efficiently (without loss), it is preferable to densely arrange the coil wire in the direction orthogonal to the diameter direction and also preferable that the unit voice coils be united. Thus, to unite the unit voice coils, it is preferable to harden the entire coil using, for example, hardening resin after winding the coil wire.

Thus, the voice coil124is obtained resulting from uniting the unit voice coils124-1,124-2and124-3corresponding to a plurality of channels. One open end portion of this voice coil124is placed in contact with the undersurface of the carbonaceous acoustic vibration plate125and bonded thereto.

When the unit voice coil is made to vibrate as a single unit, the winding jigs44having the winding sections44acorresponding to the inner diameters of the respective unit voice coils are prepared respectively and unit voice coils of different inner diameters are manufactured one by one. Each unit voice coil is hardened using hardening resin. After that, a unit voice coil of a next smaller diameter is inserted inside a unit voice coil of a greater diameter and a plurality of unit voice coils of different inner diameters are thereby combined into one voice coil124.

In the case of a small speaker unit mounted on a mobile phone or the like, the tension of the lead wires led out from the unit voice coils124-1,124-2and124-3has a great influence on the vibration characteristics of the carbonaceous acoustic vibration plate125. As the size and weight of the carbonaceous acoustic vibration plate125decrease, the influence of the lead wires on the vibration characteristics increases. On the other hand, every time the number of channels (number of unit voice coils N) increments by 1, two lead wires are added, and therefore the number of lead wires increases as the number of channels increases. For this reason, for the lead wires led out from the unit voice coils124-1,124-2and124-3, such a lead structure is required that does not cause the vibration characteristics of the carbonaceous acoustic vibration plate125to deteriorate.

FIG. 14is a schematic perspective view showing a lead arrangement in the voice coil124comprising six unit voice coils. Two lead wires are led out from each of six unit voice coils124-1to124-6. As shown in the same figure, in the case of the rectangular carbonaceous acoustic vibration plate125, two lead wires from each of the unit voice coil sets (124-1,124-2) and (124-4,124-5), a total of four lead wires are led out from each long side and two lead wires are led out from each of the unit voice coils124-3and124-6from each short side. Thus, it is preferable to uniformly distribute lead positions of the lead wires from the carbonaceous acoustic vibration plate125over the total outer circumference of the vibration plate. Since the configuration of the drive system that drives the voice coil124is the same as that of the first embodiment, descriptions thereof will be omitted.

As shown inFIG. 9, the speaker body100of the present embodiment has the structure in which one end of the voice coil124directly contacts the carbonaceous acoustic vibration plate125, and therefore vibration excited by the voice coil124is transmitted to the carbonaceous acoustic vibration plate125in response to a digital audio signal without loss. That is, since vibration excited by the digitally drivable voice coil124is transmitted to the carbonaceous acoustic vibration plate125with high efficiency, it is possible to realize a digital speaker capable of outputting a sound accurately reproducing a digital audio signal.

Furthermore, since one end portion of the voice coil124directly contacts the carbonaceous acoustic vibration plate125, heat (Joule's heat) produced in the voice coil124is transmitted to the carbonaceous acoustic vibration plate125and can be dissipated efficiently. That is, the present embodiment allows the carbonaceous acoustic vibration plate125having excellent thermal conduction characteristics to act as a heat sink of the voice coil124. As a result, it is possible to prevent deterioration of the characteristics due to heat generation in the voice coil124and also simplify the configuration by simplifying heat dissipation measures.

Since the carbonaceous acoustic vibration plate125is supported by the frame126via the edge128having a damper function, the carbonaceous acoustic vibration plate125vibrates in response to digital data, but the vibration corresponding to the digital data is immediately absorbed by the edge128so as not have any adverse influence on the vibration corresponding to the following voice data.

Moreover, the side end portion of the vibration plate of the edge128having the damper function is fixed to the mounting portion129deviated outward from the contacting position of the voice coil124. For this reason, the edge128having the damper function directly absorbs vibration given by the voice coil124to the carbonaceous acoustic vibration plate125, and can thereby solve the problem that the carbonaceous acoustic vibration plate125becomes inflexible and suppress deterioration of the vibration characteristics of the carbonaceous acoustic vibration plate125to a minimum.

Furthermore, since the voice coil124is made up of the coil wire42crushed into an oblong cross-sectional shape and wound multi fold with the planar side stacked one on another in multiple layers, it is possible to reduce the difference between the inner diameter and the outer diameter of the voice coil as a whole to a small size when the plurality of unit voice coils124-1to124-3are stacked one on another in multiple layers. When the gap formed between the yoke ends121aand121band the outer circumferential edge of the centerpiece122is small, magnetic loss can be reduced, and the difference between the inner diameter and outer diameter of the voice coil124arranged in the gap can be reduced to a small size, and therefore it is possible to reduce the size of the gap and realize efficient drive with suppressed magnetic loss.

Next, a modification example of the speaker body1will be described.

FIG. 15shows an example where a convex portion for adjusting the height position of the voice coil is formed in the carbonaceous acoustic vibration plate. The same configuration as the aforementioned embodiment may be applied to the circuit configuration of the drive system.

When at least part of the voice coil124is interposed in the gap formed between the yoke wall portions121aand121band the outer circumferential edge of the centerpiece122, a certain degree of magnetic flux can cross the voice coil124. In particular, such an arrangement that the central portion of the voice coil124comes to a position in the gap causes the number of magnetic fluxes crossing the voice coil124to become a maximum and a current flow through the voice coil124produces maximum force. That is, as shown inFIG. 15, the arrangement that the central portion of the voice coil124comes to a position in the gap allows the carbonaceous acoustic vibration plate51to vibrate most efficiently.

Here, a sufficient space in consideration of a maximum vibration stroke is set between a carbonaceous acoustic vibration plate51(undersurface) and the centerpiece122(top surface) to secure the stroke during vibration of the carbonaceous acoustic vibration plate51. Therefore, there is a limit to adjusting the positional relationship between the voice coil124and the gap position by adjusting the distance between the carbonaceous acoustic vibration plate51(undersurface) and the centerpiece122(top surface). On the other hand, if the voice coil124is extended in length on the side opposite to the vibration plate (downward inFIG. 16(a)), the central portion of the voice coil124can be placed at a position in the gap. However, when the voice coil124is extended in length, the wire distance increases, hence the weight increases. As described above, since the carbonaceous acoustic vibration plate51directly holds the voice coil124, the measure in the direction in which the weight of the voice coil124increases is not desirable.

Thus, a structure is adopted in which a convex portion52from which the voice coil mounting portion protrudes is formed on the carbonaceous acoustic vibration plate51and one end portion of the voice coil124is bonded and fixed to the convex portion52. The height D1of the convex portion52is adjusted to a size in which the central portion of the voice coil124comes to a position in the gap. InFIG. 15, the position at a distance D2from one end portion of the voice coil124corresponds to the central portion.

The formation of the convex portion52on the carbonaceous acoustic vibration plate51causes the weight to increase by the amount corresponding to the convex portion52. Thus, the convex portion52may be hollowed out to suppress the increase in the weight. Alternatively, the thickness d1of the carbonaceous acoustic vibration plate51other than the convex portion52may be reduced to suppress the increase in the total weight.

According to such a modification example, the convex portion52in which the voice coil mounting portion of the carbonaceous acoustic vibration plate51is made to protrude is formed and the central portion of the voice coil124is arranged so as to come to a position in the gap, and it is thereby possible to maximize the number of magnetic fluxes that pass through the voice coil124and allow the carbonaceous acoustic vibration plate51to vibrate most efficiently.

As shown inFIG. 16, the convex portion52is formed on the carbonaceous acoustic vibration plate51and the thickness d1of the carbonaceous acoustic vibration plate51is reduced. This causes the bending strength of the carbonaceous acoustic vibration plate51to reduce, and therefore a rib portion53for reinforcement may be formed on the surface of the vibration plate to increase the strength. Although the rectangular carbonaceous acoustic vibration plate51is illustrated in the same figure, the present invention is also applicable to other shapes.

FIGS. 17(a) and (b) are diagrams illustrating a modification example of the speaker body where the voice coil wire stacking direction is changed.FIG. 17(a) shows the same basic structure as that of the speaker body100shown inFIG. 9andFIG. 17(b) shows the same basic structure as that of the speaker body100shown inFIG. 17.

The speaker body shown inFIGS. 17(a) and (b) is configured by stacking coil wires resulting from crushing each unit voice coil60-1,60-2,60-3making up the voice coil124into an oblong shape and stacking the crushed wires so that their planar surfaces are stacked on one another. Each unit voice coil is created by winding the coil wire around a winding section44aof a winding jig44so that each crushed surface is stacked one on another. Thus, in each unit voice coil, the coil wires are arrayed in close contact with each other, which further suppresses loss when vibration excited by the voice coil124is transmitted to the carbonaceous acoustic vibration plate51.

As shown inFIGS. 17(a) and (b), by reducing the number of stacks (one) of each unit voice coil in the diameter direction, it is possible to prevent the gap between the yoke end portions121aand121band the outer circumferential portion of the centerpiece122from increasing.

Although a structure has been described above where the carbonaceous acoustic vibration plate is supported by a frame via an edge, it is also possible to adopt a structure in which the carbonaceous acoustic vibration plate is supported by a flexible film. The open end portion of the carbonaceous acoustic vibration plate is fixed to the film surface of the flexible film, the flexible film is fixed to the frame via the edge in a vibratable manner. Since the carbonaceous acoustic vibration plate is arranged in the center of the flexible film, this may be called “center plate scheme.”

In the speaker body100according to the center plate scheme, the voice coil124is made to vibrate by causing one end portion of the voice coil124to directly contact the flexible film.

EXAMPLES

Example with Three Layers Covering Both Sides of Low-Density Layer with High-Density Layer

Polyvinyl chloride resin of 35 mass % and carbon nanofibers of 1.4 mass % having an average grain diameter of 0.1 μm and a length of 5 μm as amorphous carbon source and PMMA as a pore opening member to form pores were mixed together to form a composition and diallyl phthalate monomer as a plasticizer was added to this composition, the composition was then dispersed using a Henschel mixer, kneaded repeatedly a sufficient number of times using a pressure kneader to obtain a kneaded composition, which was then pelletized using a pelletizer to obtain a composition for molding. The pellet of this composition for molding was transformed into a sheet-like molded product having a thickness of 400 μm through extrusion molding, both sides of which were coated with furan resin and hardened to be transformed into a multilayered sheet. This multilayered sheet was processed for 5 hours in an air oven at 200° C. to be a carbon precursor. The multilayered sheet was then heated in a nitrogen gas at a temperature rising rate of 20° C./h and left for three hours at 1000° C. The multilayered sheet was naturally cooled and then kept under a vacuum at 1400° C. for three hours, naturally cooled and carbonization was thus completed. Thus, as conceptually shown inFIG. 18, an acoustic vibration plate was obtained which contains a low-density layer116of a porous material having spherical pores114remaining after PMMA grains disappear with carbon nanofiber powder112uniformly dispersed in amorphous carbon110and high-density layers118made of the amorphous carbon110covering both sides thereof.

The porosity of the low-density layer116of the acoustic vibration plate obtained in this way was 70%, the number average pore diameter was 60 μm. The vibration plate as a whole showed excellent properties having a thickness of approximately 350 μm, a bending strength of 25 MPa, Young's modulus of 8 GPa, sound velocity of 4200 m/sec, a density of 0.45 g/cm3and hygroscopic property of 1 mass % or below.

The velocity of sound was calculated from the density and the measured value of Young's modulus (the same will apply hereinafter). The hygroscopic property is mass increase (%) when the vibration plate was dried for 30 minutes at 100° C. and then left in an environment of temperature 25° C. and humidity 60%.FIG. 19shows the relationship between the elapsed time and mass change. As a comparative example 1, the result when the last carbonization temperature was assumed to be 1000° C. is also shown. As is clear fromFIG. 19, assuming the carbonization temperature is 1200° C. or higher, a vibration plate of low hygroscopic property is obtained whose mass increase after 250 hours is 5% or below.

Example where High-Density Layer is Filled with Filler (Graphite)

Polyvinyl chloride resin of 35 mass % and carbon nanofibers of 1.4 mass % having an average grain diameter of 0.1 μm and a length of 5 μm as amorphous carbon source and PMMA as a pore opening member to form pores were mixed together to form a composition and diallyl phthalate monomer as a plasticizer was added to this composition, the composition was then dispersed using a Henschel mixer, kneaded repeatedly a sufficient number of times using a pressure kneader to obtain a kneaded composition, which was then pelletized using a pelletizer to obtain a composition for molding. The pellet of this composition for molding was transformed into a sheet-like molded product having a thickness of 400 μm through extrusion molding, further graphite (SP270 manufactured by Nippon Graphite industries, ltd.) of 5 mass % and having an average grain diameter of on the order of 4 μm was dispersed on furan resin, both sides of which were coated with a liquid containing a hardener and hardened to be transformed into a multilayered sheet. The multilayered sheet was processed in an air oven of 200° C. for five hours to be a carbon precursor. The multilayered sheet was then heated in a nitrogen gas at a temperature rising rate of 20° C./h and left for three hours at 1000° C. The multilayered sheet was naturally cooled and then kept under a vacuum at 1500° C. for three hours, naturally cooled, carbonization completed and a composite carbon vibration plate was thus obtained.

The porosity of the low-density layer of the acoustic vibration plate obtained in this way was 70%, the number average pore diameter was 60 μm. The vibration plate as a whole showed excellent properties having a thickness of approximately 350 μm, a bending strength of 23 MPa, Young's modulus of 5 GPa, sound velocity of 3333 m/sec and a density of 0.45 g/cm3.

Example with Only Porous Material

Polyvinyl chloride resin of 54 mass % and carbon nanofibers of 1.4 mass % having an average grain diameter of 0.1 μm and a length of 5 μm as single-layer molded amorphous carbon source having a porosity of 50% and PMMA as a pore opening member to form pores were mixed together to form a composition and diallyl phthalate monomer as a plasticizer was added to this composition, the composition was then dispersed using a Henschel mixer, kneaded repeatedly a sufficient number of times using a pressure kneader to obtain a kneaded composition, which was then pelletized using a pelletizer to obtain a composition for molding. This pellet was used to perform extrusion molding for a film-like molded product having a thickness of 400 μm through extrusion molding. This film was processed in an air oven heated to 200° C. for five hours to be a carbon precursor. The film was then heated in a nitrogen gas at a temperature rising rate of 20° C./h and left for three hours at 1000° C. The film was naturally cooled and then kept under a vacuum at 1500° C. for three hours, naturally cooled, carbonization completed and a composite carbon vibration plate was thus obtained.

The porous acoustic vibration plate obtained in this way showed excellent properties having a porosity of 50%, a pore diameter of 60 μm, a thickness of approximately 350 μm, a bending strength of 29 MPa, Young's modulus of 7 GPa, sound velocity of 3055 m/sec and a density of 0.75 g/cm3.

Next, the frequency characteristic of a speaker using the vibration plate created in Example 1 above for the aforementioned digital speaker unit will be described. The voice coil24provided for the digital speaker unit is made up of six voice coils, the delta-sigma modulator11converts a 16-bit digital audio signal to a 4-bit signal and the thermometer code outputted from the thermometer code conversion section12is assumed to have a 6-bit configuration.

FIG. 20shows the frequency characteristic when the vibration plate obtained in Example 1 is used. As shown in the same figure, in the case of only the carbonaceous vibration plate, a very flat characteristic from close to 700 Hz to 20 kHz which is said to be an upper limit of the audible frequency band has been realized. With the frequency characteristic shown inFIG. 20, extremely high sound quality can be realized. Furthermore, a peak sound pressure of 85 dBspl or more has been realized.

As described above, the digital speaker unit according to an embodiment of the present invention can realize excellent acoustic characteristics by directly driving, with a digital audio signal, a carbonaceous acoustic vibration plate which has a low density and light weight, yet sufficient rigidity.

The present application is based on Japanese Patent Application No. 2009-057901 filed on Mar. 11, 2009 and Japanese Patent Application No. 2009-111539 filed on Apr. 30, 2009, entire content of which is expressly incorporated by reference herein.