JP5490429B2 - Speaker unit - Google Patents

Description

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

  2. Description of the Related Art Conventionally, a digital speaker has been developed in which a digital audio signal is directly supplied to a speaker and reproduced without being converted into an analog signal (see, for example, Patent Document 1). In the digital speaker described in Patent Document 1, each of a plurality of voice coils wound around a voice coil bobbin is weighted so that a driving force corresponding to each bit of a digital signal is generated, and is applied to each voice coil. The direction of the current flowing through the voice coil is set according to the binary value by switching the polarity of the voltage according to the binary value of each 2 bits of the digital signal. With this configuration, it is possible to generate a driving force at a ratio corresponding to the quantization of the digital signal.

  In addition, a speaker unit has been proposed in which a digital-to-analog conversion device that generates a high-quality analog signal from a digital signal is applied to a digital speaker driving device to improve reproduction sound quality and reduce circuit scale (for example, Patent Document 2). The speaker unit described in Patent Document 2 converts the n-bit output of the delta-sigma modulator into a thermometer code by a formatter, performs mismatch shaping processing by a post filter, and inputs the output to the buffer circuit. It describes that a magnetic field is added by controlling a coil with an output digital signal (see paragraphs 0063 and 0078).

  On the other hand, speaker diaphragms used in various audio equipment, video equipment, mobile devices such as mobile phones, and the like are required to be able to faithfully reproduce clear sound in a wide frequency band, particularly in a high sound range. . For this reason, the material of the diaphragm is required to have seemingly contradictory properties such that the elastic modulus is high to give the diaphragm sufficient rigidity and the density is low to reduce the weight of the diaphragm. In particular, diaphragms for digital speakers, which have been attracting attention in recent years, are strongly demanded for these properties due to the demand for vibration response.

JP-A-4-326291 International Publication No. 2007/135928

  Accordingly, an object of the present invention is to provide a speaker unit that realizes good acoustic characteristics by directly driving a diaphragm having low rigidity and light weight but sufficient rigidity with a digital audio signal.

The digital speaker unit of the present invention includes a speaker body having a carbonaceous acoustic diaphragm, a conversion circuit for converting a digital audio signal of multi-valued bits supplied from a digital sound source into a digital signal of required bits, and the conversion circuit. A plurality of voice coils that are provided corresponding to the number of bits of the output digital signal and that respectively vibrate the carbonaceous acoustic diaphragm, and each voice coil individually based on the digital signal output from the conversion circuit A low-density layer comprising a porous body having a porosity of 40% or more, comprising amorphous carbon and carbon powder uniformly dispersed in the amorphous carbon. comprised of amorphous carbon, said low density thin thickness than layer, characterized by including the said low density higher density than layer dense layer To.

  According to this configuration, since the speaker main body equipped with the carbonaceous acoustic diaphragm is directly driven by a digital signal, it is preferable to utilize the characteristics of the carbonaceous acoustic diaphragm having sufficient rigidity while being low density and light weight. Acoustic characteristics can be realized.

  According to the present invention, in the digital speaker unit, the conversion circuit includes a delta-sigma modulator that delta-sigma-modulates a multi-level bit digital audio signal supplied from the digital sound source.

  With this configuration, by including the delta sigma modulator, it is possible to eliminate the quantization noise generated in the process of converting the digital audio signal of multi-level bits supplied from the digital sound source into the digital signal of the required bits by the noise shaping effect, It is also possible to adopt a configuration in which the quantization error is suppressed by the oversampling method.

  According to the present invention, in the digital speaker unit, the conversion circuit converts a predetermined bit digital signal output from the delta-sigma modulator into a thermometer code having a bit number corresponding to the number of voice coils. A code conversion unit is provided.

  With this configuration, since the binary number output from the delta-sigma modulator is a signal having a weight for each bit, it is difficult to directly drive the digital signal if the signal is used as it is. By converting to a meter code, the speaker body can be directly driven by a digital signal.

  In the digital speaker unit, the speaker body may be configured to vibrate by bringing the voice coil into contact with the carbonaceous acoustic diaphragm. Alternatively, the carbonaceous acoustic diaphragm may be held by a flexible film body, and the voice coil may be brought into contact with the film body to vibrate.

  ADVANTAGE OF THE INVENTION According to this invention, the speaker unit which implement | achieves a favorable acoustic characteristic can be provided by directly driving the diaphragm which is low density and lightweight, but has sufficient rigidity with a digital audio signal.

1 is a schematic overall view of a digital speaker unit according to an embodiment of the present invention. Typical sectional drawing which shows the structure of the speaker main body in the said one Embodiment Schematic diagram showing a plurality of voice coil arrangements in the above embodiment Schematic diagram showing the relationship among the voice coil, carbonaceous acoustic diaphragm and driver circuit Circuit diagram showing relationship between voice coil and driver circuit Circuit configuration diagram of the delta-sigma modulator in the above embodiment Conceptual diagram of carbonaceous acoustic diaphragm with low density layer and high density layer Characteristic diagram of carbonaceous acoustic diaphragm showing the relationship between elapsed time and mass change rate (A) Entire waveform diagram of digital signal for direct driving of speaker, (b) Enlarged waveform diagram of part of digital signal (A) Sectional view of the speaker body that supports the carbonaceous acoustic diaphragm with a flexible film, (b) Plan view of FIG. Frequency characteristics of digital speaker with only carbonaceous acoustic diaphragm

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
One embodiment of the present invention is a digital speaker unit that includes a carbonaceous acoustic diaphragm as a diaphragm of a speaker body, and vibrates the carbonaceous acoustic diaphragm by directly driving a voice coil with a digital signal supplied from a digital sound source. It is.

FIG. 1 is a schematic overall view of a digital speaker unit according to an embodiment of the present invention.
In FIG. 1, the digital sound source 10 can be constituted by a CD play, a DVD player, or other digital audio reproduction device, and outputs a digital audio signal to the digital speaker unit.

  The digital speaker unit according to the present embodiment includes a multi-bit delta sigma modulator 11, a thermometer code converter 12 that converts a digital signal output from the delta sigma modulator 11 into an N-bit thermometer code without weight. The driver circuit 13 that controls driving based on the thermometer code and the speaker main body 14 that includes a carbonaceous acoustic diaphragm are the main components.

The structure of the speaker body 14 will be described with reference to FIG.
The speaker main body 14 includes a bottomed cylindrical yoke 22 including a center pole 21 having a plate shape at the center, and a magnet 23 disposed at a base end portion of the center pole 21. The magnet 23, the yoke 22, and the center pole 21 constitute a magnetic circuit. The speaker main body 14 is attached to a plurality of voice coils 24 and a distal end portion of the voice coil 24 via a coil bobbin (not shown) that surrounds the center pole 21 with a gap in the magnetic circuit. A carbonaceous acoustic diaphragm 25. The outer peripheral edge portion of the carbonaceous acoustic diaphragm 25 is supported by the frame 27 via the edge 26 so as to vibrate. The number N of coils of the plurality of voice coils 24 corresponds to the number N of output bits of the thermometer code conversion unit 12.

  3 to 5 are conceptual diagrams of the speaker drive system. N voice coils (24-1 to 24-N) are independently arranged (FIG. 3), and one ends are wound around coil holding portions 28 connected to the carbonaceous acoustic diaphragm 25, respectively. (FIG. 4). In addition, without using the coil holding part 28, it can also be set as the structure which connects the edge part of a voice coil (24-1 to 24-N) directly to one surface of the carbonaceous acoustic diaphragm 25. FIG. Moreover, as shown in FIG. 5, each voice coil (24-1 to 24-N) is configured to be controllable independently from the driver circuits (1) to (N).

  In the speaker main body 14, a current is passed through a voice coil 24 placed in a magnetic circuit constituted by a magnet 23, a yoke 22, and a center pole 21, and a force generated in a direction perpendicular to the magnetic field lines is used for the voice coil 24. Then, the carbonaceous acoustic diaphragm 25 is vibrated to generate sound waves. A current is passed through the voice coil 24 in accordance with each bit value of the digital signal output from the thermometer code converter 12.

  FIG. 6 is a circuit configuration diagram of the delta-sigma modulator 11. The circuit configuration shown in the figure is an example, and a higher-order delta-sigma modulator can also be used. Here, the digital audio signal expressed by multi-valued input bits is 16 bits, and the n-bit output from the delta-sigma modulator 11 is 4 bits.

The delta sigma modulator 11 basically includes an integrator 31, a quantizer 32, a delay unit 33, and a feedback loop. τ is a feedback gain. Multilevel bits (for example, 16 bits) input to the delta sigma modulator 11 pass through the integrator 31 and are converted into n bits (for example, 9 values = 4 bits) by the quantizer 32. The quantization error generated in the quantization is returned to the input terminal by a feedback loop passing through the delay unit 33, and the difference is taken, whereby only the quantization error is integrated. When the input is X, the output is Y, and the quantization error is Q, the relational expression is expressed as Y = X + (1-Z ⁻¹ ) Q. Since the transfer function (1-Z ⁻¹ ) multiplied by the quantization error Q has a frequency characteristic and becomes small near the direct current, this characteristic becomes a noise shaping effect described later.

  In the delta-sigma modulator 11, the quantizer 32 quantizes the multi-bit digital audio signal into a number corresponding to the number n of output bits. The quantization error caused by the quantizer 32 can be eliminated by applying an oversampling method. Oversampling is one of techniques for performing sampling at a frequency sufficiently higher than the signal band. In the case of delta-sigma modulation, the original signal accuracy can be improved by a noise shaping effect. In other words, when quantization is performed using a quantizer, quantization noise is evenly distributed over all frequencies, but by delta-sigma modulation, unnecessary noise components are shifted to an oversampled high frequency region. Noise in the vicinity of the original signal is suppressed, and the accuracy of the original signal can be improved.

  The thermometer code converter 12 converts the n-bit output of the delta sigma modulator 11 into an N-bit thermometer code corresponding to the number of voice coils. For example, when converting to an 8-bit thermometer code, the delta-sigma modulator outputs (0010), (0101), and (1000) are transferred to the thermometer codes (00000011), (00011111), and (11111111), respectively. Convert. Since the binary number output from the delta-sigma modulator 11 is a signal having a weight for each bit, it is difficult to perform direct digital driving if the signal is used as it is. By converting, the speaker body 14 can be directly driven by a digital signal.

  The driver circuit 13 independently drives the individual voice coils 24-1 to 24-N based on the thermometer code output from the thermometer code conversion unit 12. Specifically, each of the voice coils 24-1 to 24-N and each bit value of the thermometer code have a one-to-one correspondence, and each bit of the thermometer code from the thermometer code conversion unit 12 is shown in FIG. A 1-bit signal (ON / OFF) as shown in 9 (a) and 9 (b) is output. A current is supplied to the voice coil 24 with the thermometer code “1”, and the voice coil 24 with the thermometer code “0” is driven so that no current flows. The voice coil 24 itself moves in proportion to the current flowing through the voice coil 24, and the carbonaceous acoustic diaphragm 25 coupled to the voice coil 24 vibrates to generate sound.

Next, the structure and manufacturing method of the carbonaceous acoustic diaphragm 25 used in the present embodiment will be described in detail.
In the digital speaker unit of the present invention, a diaphragm including amorphous carbon and carbon powder uniformly dispersed in the amorphous carbon and having a porous body having a porosity of 40% or more can be used as the carbonaceous acoustic diaphragm 25. . The carbonaceous acoustic diaphragm 25 includes the porous plate as a low density layer, contains amorphous carbon, has a high density layer that is thinner than the low density layer and higher in density than the low density layer. Furthermore, it is preferable to comprise.

  Here, the number of layers is a two-layer structure of a high-density layer and a low-density layer, a three-layer structure in which both sides of the low-density layer are sandwiched by high-density layers, and conversely, both sides of the high-density layer are sandwiched by low-density layers. Various configurations such as a three-layer structure or a single-layer structure having only a high-density layer are possible.

  It is desirable that the pores of the porous body have a spherical shape, and the number average pore diameter is 5 μm or more and 150 μm or less. The carbon powder preferably includes carbon nanofibers having a number average diameter of 0.2 μm or less and an average length of 20 μm or less. The high-density layer may include graphite that is uniformly dispersed in the amorphous carbon. This carbonaceous acoustic diaphragm desirably has an increase in mass of 5% or less when dried for 250 hours in an environment of temperature 25 ° C. and humidity 60%.

  In addition, carbon powder is uniformly mixed with a carbon-containing resin, the mixture is formed into a film, heated to form a carbon precursor, and a carbonaceous acoustic diaphragm is obtained by carbonizing the carbon precursor in an inert atmosphere. Can be manufactured. In this method for producing a carbonaceous acoustic diaphragm, particles of a drilling material that are solid or liquid at the carbon precursor temperature and disappear at the carbonization temperature and leave pores are premixed in the mixture. Thus, a porous body containing amorphous carbon and carbon powder is obtained after the carbonization.

  Before the carbonization, by forming a carbon-containing resin layer on at least one surface of the carbon precursor plate, after the carbonization, than the low-density layer and the low-density layer made of the porous body It is preferable to further include a carbonaceous acoustic diaphragm including a high-density layer having a high density. The structure in which both sides of the high-density layer are sandwiched between the low-density layers is obtained by, for example, integrating a carbon precursor layer that includes a punching material on both surfaces of a carbon precursor that does not include a punching material, and integrating them with a resin. Can be obtained.

  The particles of the hole punching material are preferably spherical. The carbon powder preferably includes carbon nanofibers. The carbon-containing resin layer may include graphite dispersed uniformly therein. The carbonization is desirably performed at a temperature of 1200 ° C. or higher.

  As described above, a mixture of a carbon-containing resin and carbon powder is a solid or liquid at the temperature at which the carbon precursor is formed, and a hole forming material that disappears at the carbonization temperature and leaves pores, for example, polymethyl By mixing the particles of methacrylate (PMMA), in the process of carbonization, the perforated material disappears leaving pores having a three-dimensional shape corresponding to the three-dimensional shape. Therefore, the porosity can be easily controlled by controlling the blending ratio of the drilling material, and the three-dimensional shape and size of the pores can be easily selected by selecting the three-dimensional shape and size of the drilling material particles. It can be controlled, and a porous body having a porosity of 40% or more can be realized.

The porosity is a percentage of the volume of the pores with respect to the entire volume of the porous body including the pores, and is defined as a porosity calculated from the volume and mass of the entire porous body, assuming that the density of carbon is 1.5 g / cm ^3. To do.

If a multi-layer structure of the low-density layer and the high-density layer made of the porous body is used, the porosity can be set to 60% or more while maintaining the necessary rigidity, and the density of the entire diaphragm can be reduced to 0.5 g / cm ³ or less.
The high-density layer exhibits an effect at about 1 to 30% of the total thickness, and plays a role of high-frequency reproduction with rigidity of Young's modulus of about 100 GPa.

  The Young's modulus of the low-density layer is about 2 to 3 GPa, making the entire diaphragm lightweight, maintaining the overall sound quality, and improving the vibration response.

  Since these are integrated and fired and carbonized to form a multi-layered carbonaceous material, a multilayer flat speaker diaphragm capable of controlling characteristics, particularly outputting audible sound up to the high frequency range, is possible. .

  Rather than imparting rigidity to the dome shape, a flat diaphragm having a high reproduction limit frequency can be obtained by balancing the dense strength of the dense and high-rigidity high-density layer and the low-density light-weight core layer. Although the reproduced sound range varies depending on the porosity design, the pore diameter does not greatly affect. Handleability is improved and impact resistance is improved. Further, by covering one surface or both surfaces of the low-density layer of the porous body with the high-density layer, it is possible to prevent the suction of the adhesive during the incorporation into the unit.

  A further required characteristic of the acoustic diaphragm is that the hygroscopic property is low so that the acoustic characteristics are not changed by absorbing moisture in the air and becoming heavy. By setting the temperature of carbonization to 1200 ° C. or higher, a product having an increase in mass of 5% or less when dried for 250 hours in an environment having a temperature of 25 ° C. and a humidity of 60% can be obtained.

  In the above description, the structure in which the carbonaceous acoustic diaphragm is held by the frame via the edge is exemplified, but a structure in which the carbonaceous acoustic diaphragm is supported by a flexible film may be used.

  FIG. 10A is a cross-sectional view of a speaker body that supports a carbonaceous acoustic diaphragm with a flexible film, and FIG. 10B is a plan view thereof. As shown in FIG. 10A, the yoke 22, the magnet 23, the center pole 21, the voice coil 24, and the frame 27 have the same structure as the speaker body 14 shown in FIG. The carbonaceous acoustic diaphragm 41 is fixed to the inner surface of the flexible film 42. The flexible film 42 has a shape in which a central portion bulges out in a dome shape, and is fixed to an upper surface of a film base 43 having a plate shape. The end of the voice coil 24 is in contact with the outer peripheral edge of the lower surface of the film base 43 so as to transmit vibration. The flexible film 42 is provided with uneven processing for ensuring strength.

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

  Thus, by holding the carbonaceous acoustic diaphragm 41 with the flexible film 42 having the required rigidity and flexibility, a higher sound pressure can be obtained compared to the structure in which the carbonaceous acoustic diaphragm is held by the frame. realizable. In a verification experiment by the present inventor, 90 dBspl was realized as a peak sound pressure by combining a carbonaceous diaphragm with a film. Therefore, in applications that require high sound pressure, it is desirable that the carbonaceous acoustic diaphragm 41 be held by the flexible film 42 as shown in FIG.

(Example 1) Example of three layers in which both sides of a low-density layer are covered with a high-density layer 35% by mass of a vinyl chloride resin as an amorphous carbon source, carbon nanofiber 1.4 having an average particle diameter of 0.1 μm and a length of 5 μm After adding diallyl phthalate monomer as a plasticizer to a composition in which PMMA as a hole forming material for pore formation for mass% is added and dispersed using a Henschel mixer, it is sufficient to use a pressure kneader. Kneading was repeated to obtain a composition, which was pelletized by a pelletizer to obtain a molding composition. The pellets of the molding composition were formed into a sheet-like molded product having a thickness of 400 μm by extrusion molding, and further, furan resin was coated on both sides and cured to obtain a multilayer sheet. This multilayer sheet was treated in an air oven at 200 ° C. for 5 hours to obtain a precursor (carbon precursor). Thereafter, the temperature was increased in nitrogen gas at a temperature increase rate of 20 ° C./h, and the temperature was maintained at 1000 ° C. for 3 hours. After natural cooling, after holding in vacuum at 1400 ° C. for 3 hours, natural cooling was performed to complete firing. Accordingly, as conceptually shown in FIG. 7, the low density of the porous body having the spherical pores 114 remaining after the carbon nanofiber powder 112 is uniformly dispersed in the amorphous carbon 110 and the PMMA particles disappear. An acoustic diaphragm having the layer 116 and the high-density layer 118 made of amorphous carbon covering both surfaces thereof was obtained.

The porosity of the low density layer 116 of the acoustic diaphragm thus obtained was 70%, and the number average pore diameter was 60 μm. The entire diaphragm had excellent physical properties such as a thickness of about 350 μm, a bending strength of 25 MPa, a Young's modulus of 8 GPa, a sound velocity of 4200 m / sec, a density of 0.45 g / cm ³ , and a hygroscopicity of 1% by mass or less.

  The sound speed was obtained by calculation from measured values of density and Young's modulus (the same applies hereinafter). The hygroscopicity is a mass increase rate (%) when dried at 100 ° C. for 30 minutes and then left in an environment at a temperature of 25 ° C. and a humidity of 60%. FIG. 8 shows the relationship between elapsed time and mass change rate. As Comparative Example 1, the result when the final firing (carbonization) temperature is 1000 ° C. is also shown. As can be seen from FIG. 8, by setting the carbonization temperature to 1200 ° C. or higher, a diaphragm with low hygroscopicity in which the increase in mass after 250 hours is 5% or less can be obtained.

(Example 2) Example in which filler (graphite) is put in high-density layer 35% by mass of vinyl chloride resin as an amorphous carbon source, 1.4% by mass of carbon nanofibers having an average particle size of 0.1 μm and a length of 5 μm Then, after adding diallyl phthalate monomer as a plasticizer to a composition in which PMMA is combined as a hole forming material for pore formation and dispersing it using a Henschel mixer, it is sufficiently kneaded using a pressure kneader. The composition was repeatedly obtained and pelletized by a pelletizer to obtain a molding composition. The molding composition pellets are formed into a sheet-like molded product having a thickness of 400 μm by extrusion molding. Further, 5% by mass of graphite (SP270 made from Nippon Graphite) having an average particle size of about 4 μm is dispersed in a furan resin, and a curing agent is added. The solution was coated on both sides and cured to obtain a multilayer sheet. This multilayer sheet was treated in an air oven at 200 ° C. for 5 hours to obtain a precursor (carbon precursor). Thereafter, the temperature was increased in nitrogen gas at a temperature increase rate of 20 ° C./h, and the temperature was maintained at 1000 ° C. for 3 hours. After natural cooling, after holding in a vacuum at 1500 ° C. for 3 hours, natural cooling was completed to complete firing, and a composite carbon diaphragm was obtained.

The porosity of the low density layer of the acoustic diaphragm thus obtained was 70%, and the number average pore diameter was 60 μm. The entire diaphragm had excellent physical properties such as a thickness of about 350 μm, a bending strength of 23 MPa, a Young's modulus of 5 GPa, a sound velocity of 3333 m / sec, and a density of 0.45 g / cm ³ .

(Example 3) Example of porous body only 50% porosity Single layer molded body As a carbon source, 54 mass% of vinyl chloride resin, carbon nanofibers having an average particle diameter of 0.1 µm and a length of 5 µm, 1.4 mass %, After adding a diallyl phthalate monomer as a plasticizer to a composition in which PMMA is combined as a hole forming material for pore formation and dispersing it using a Henschel mixer, it is sufficiently kneaded using a pressure kneader Was repeated to obtain a composition, which was pelletized with a pelletizer to obtain a molding composition. Using this pellet, a film-like extrusion molding having a thickness of 400 μm was performed. This film was treated in an air oven heated to 200 ° C. for 5 hours to obtain a precursor (carbon precursor). Thereafter, the temperature was increased in nitrogen gas at a temperature increase rate of 20 ° C./hour or less, and the temperature was maintained at 1000 ° C. for 3 hours. After natural cooling, after holding at 1500 ° C. for 3 hours in a vacuum atmosphere, natural cooling was performed to complete the firing, and a composite carbon diaphragm was obtained.

The porous acoustic diaphragm thus obtained has a porosity of 50%, a pore diameter of 60 μm, a thickness of about 350 μm, a bending strength of 29 MPa, a Young's modulus of 7 GPa, a sound velocity of 3055 m / sec, a density of 0.75 g / cm ³ , And had excellent physical properties.

  Next, frequency characteristics of the speaker when the diaphragm created in the first embodiment is used for the above-described digital speaker unit will be described. The voice coil 24 provided in the digital speaker unit is composed of six voice coils, and the delta-sigma modulator 11 converts a 16-bit digital audio signal into 4 bits and outputs a thermometer code output from the thermometer code converter 12. Has a 6-bit configuration.

  FIG. 11 shows frequency characteristics when the diaphragm obtained in Example 1 is used. As shown in the figure, in the case of only the carbonaceous diaphragm, a very flat characteristic can be realized from about 700 Hz to 20 kHz which is said to be the upper limit of the audible frequency range. With the frequency characteristics shown in FIG. 11, it is possible to reproduce extremely good quality sound quality. In addition, a peak sound pressure of 85 dBspl or more can be realized.

  As described above, according to the digital speaker unit according to the embodiment of the present invention, the carbonaceous acoustic diaphragm 25 having low rigidity and light weight but sufficient rigidity is directly driven by the digital audio signal, Good acoustic characteristics can be realized.

DESCRIPTION OF SYMBOLS 10 Digital sound source 11 Delta-sigma modulator 12 Thermometer code conversion part 13 Driver circuit 14 Speaker main body 21 Centerpiece 22 Yoke 23 Magnet 24 Voice coil 25 Carbonaceous acoustic diaphragm 26 Edge 27 Frame

Claims (5)

Speaker main body equipped with carbonaceous acoustic diaphragm, conversion circuit for converting digital audio signal of multi-value bit supplied from digital sound source to digital signal of required bit, and number of bits of digital signal output from said conversion circuit And a plurality of voice coils that respectively vibrate the carbonaceous acoustic diaphragm, and a drive circuit that individually drives the voice coils based on digital signals output from the conversion circuit. and,
The carbonaceous acoustic diaphragm includes amorphous carbon and a carbon powder uniformly dispersed in the amorphous carbon, and includes a low density layer made of a porous body having a porosity of 40% or more, amorphous carbon, and the low density layer. A digital speaker unit comprising a high-density layer that is thinner than the low-density layer and has a higher density than the low-density layer .   2. The digital speaker unit according to claim 1, wherein the conversion circuit includes a delta-sigma modulator that delta-sigma-modulates a digital audio signal of multi-level bits supplied from the digital sound source.   The conversion circuit includes a thermometer code conversion unit that converts a digital signal of a predetermined bit output from the delta-sigma modulator into a thermometer code having a number of bits corresponding to the number of voice coils. Item 3. The digital speaker unit according to Item 2. The digital speaker unit according to any one of claims 1 to 3 , wherein the speaker body vibrates by contacting the voice coil with the carbonaceous acoustic diaphragm. The speaker main body, the carbonaceous acoustic diaphragm held by a flexible film body, from claim 1, wherein the vibrating by contacting the voice coil with respect to the film body according to claim 3 The digital speaker unit according to any one of the above.

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