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JP2010213062

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DESCRIPTION JP2010213062
The present invention is to realize a speaker unit which directly drives a diaphragm having low
density and light weight while having sufficient rigidity with a digital audio signal. A speaker
main body (14) provided with a carbonaceous acoustic diaphragm (25), a delta sigma modulator
(11) for converting a multi-bit digital audio signal supplied from a digital sound source (10) into
a digital signal of required bits, and a thermometer code A plurality of conversion units 12 and a
plurality of voice coils 24 provided corresponding to the number of bits of digital signals for
respectively vibrating the carbonaceous acoustic diaphragm 25 and a driver for driving each
voice coil 24 individually based on digital signals It is a digital speaker unit provided with the
circuit 13. [Selected figure] Figure 1
スピーカユニット
[0001]
The present invention relates to a speaker unit for sound reproduction, and more particularly to
a speaker unit directly driven by a digital sound signal.
[0002]
2. Description of the Related Art Conventionally, digital speakers have been developed which
directly reproduce digital audio signals by supplying them to speakers without converting them
into analog signals (see, for example, Patent Document 1).
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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 such that a driving force corresponding to each bit of the
digital signal is generated, and is applied constant to each voice coil By switching the polarity of
the voltage according to the binary value of each 2 bits of the digital signal, the direction of the
current flowing through the voice coil is set according to the binary value. According to this
configuration, it is possible to generate the driving force at a ratio corresponding to the
quantization of the digital signal.
[0003]
In addition, a speaker unit has been proposed in which a digital-to-analog converter that
generates high-quality analog signals from digital signals is applied to a drive unit for digital
speakers to improve the reproduction voice quality and reduce the 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 the formatter, performs mismatch
shaping processing by the post filter, inputs the output to the buffer circuit, and It is described
that the coil is controlled by the output digital signal to add the magnetic field (see paragraphs
0063 and 0078).
[0004]
On the other hand, the diaphragm of a speaker used in various audio devices, video devices,
mobile devices such as mobile phones, etc. is required to have the property of being able to
faithfully reproduce clear sound in a wide frequency band, especially high tone range. .
Therefore, the material of the diaphragm is required to have seemingly contradictory properties
such as a high elastic modulus to give sufficient rigidity to the diaphragm and a low density to
reduce the weight of the diaphragm. In particular, in the diaphragm for digital speakers that has
been attracting attention in recent years, these properties are strongly demanded from the
demand for vibration response.
[0005]
Patent Document 1: Japanese Patent Application Laid-Open No. 4-326291 Patent Publication
WO 2007/135928
[0006]
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2
Therefore, an object of the present invention is to provide a speaker unit which realizes low noise
and light weight, and a diaphragm having sufficient rigidity directly with digital audio signals to
realize good acoustic characteristics.
[0007]
A digital speaker unit according to the present invention comprises: a speaker main body
provided with a carbonaceous acoustic diaphragm; a conversion circuit for converting a multi-bit
digital audio signal supplied from a digital sound source into a digital signal of a required bit; A
plurality of voice coils are provided corresponding to the number of bits of the digital signal to be
output, each voice coil for vibrating the carbonaceous acoustic diaphragm and the voice coils
individually based on the digital signal output from the conversion circuit And a driving circuit
for driving.
[0008]
According to this configuration, since the speaker main body provided with the carbonaceous
acoustic diaphragm is directly driven by the digital signal, it is good using the characteristics of
the carbonaceous acoustic diaphragm having a low density and light weight yet having sufficient
rigidity. Acoustic characteristics can be realized.
[0009]
In the digital speaker unit according to the present invention, the conversion circuit includes a
delta sigma modulator that delta sigma modulates a digital audio signal of multi-bit data supplied
from the digital sound source.
[0010]
With this configuration, by providing the delta sigma modulator, it is possible to eliminate
quantization noise generated in the process of converting a multi-bit digital audio signal supplied
from a digital sound source into a digital signal of required bits by the noise shaping effect. It is
also possible to suppress the quantization error by the oversampling method.
[0011]
Further, according to the present invention, in the digital speaker unit, the conversion circuit
converts a digital signal of a predetermined bit output from the delta sigma modulator into a
thermometer code of the number of bits corresponding to the number of voice coils. A code
conversion unit is provided.
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[0012]
With this configuration, since the binary number output from the delta sigma modulator is a
signal with a weight for each bit, direct digital driving is difficult if using the signal as it is, but
the temperature does not have a weight for each bit By converting into a meter code, the speaker
body can be directly driven by a digital signal.
[0013]
In the digital speaker unit, the carbonaceous acoustic diaphragm may include an amorphous
carbon and a carbon powder uniformly dispersed in the amorphous carbon, and may be formed
of a porous body having a porosity of 40% or more.
[0014]
In the digital speaker unit, the carbonaceous acoustic diaphragm may include amorphous carbon
and carbon powder uniformly dispersed in the amorphous carbon, and a low density layer made
of a porous material having a porosity of 40% or more; And a high density layer thinner than the
low density layer and higher in density than the low density layer.
[0015]
Further, in the digital speaker unit, the speaker body may be configured to cause the voice coil to
be in contact with and vibrate 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.
[0016]
According to the present invention, it is possible to provide a speaker unit that achieves excellent
acoustic characteristics by directly driving a low density, lightweight yet sufficiently rigid
diaphragm with a digital audio signal.
[0017]
A schematic overall view of a digital speaker unit according to an embodiment of the present
invention A schematic cross sectional view showing a structure of a speaker main body in the
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above embodiment A schematic view showing a plurality of voice coil arrangements in the above
embodiment A schematic diagram showing the relationship between a voice coil, a carbonaceous
acoustic diaphragm and a driver circuit. A circuit diagram showing the relationship between a
voice coil and a driver circuit. A circuit diagram of a delta sigma modulator according to one
embodiment. Low density layer and high Characteristic diagram of carbonaceous acoustic
diaphragm with density layer Characteristic diagram of carbonaceous acoustic diaphragm
showing relationship between elapsed time and mass change rate (a) Overall waveform diagram
of digital signal that directly drives a speaker, (b) digital The wave form which expanded a part of
signal (a) Cross section of the speaker main body which supports a carbonaceous acoustic
diaphragm with a flexible film, (b) The top view carbonaceous acoustic vibration of the figure (a)
Frequency characteristic diagram of a digital speaker when only
[0018]
Hereinafter, embodiments of the present invention will be described in detail with reference to
the attached drawings.
One embodiment of the present invention is a digital speaker unit including a carbonaceous
acoustic diaphragm as a diaphragm of a speaker body, and directly driving a voice coil by a
digital signal supplied from a digital sound source to vibrate the carbonaceous acoustic
diaphragm. It is.
[0019]
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 configured by a CD player, a DVD player, or another
digital type of audio reproduction device, and outputs a digital audio signal to a digital speaker
unit.
[0020]
The digital speaker unit according to the present embodiment includes a multi-bit delta sigma
modulator 11, a thermometer code converter 12 for converting a digital signal output from the
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5
delta sigma modulator 11 into an N-bit thermometer code without weight. The driver circuit 13
that performs drive control based on a thermometer code, and the speaker main body 14
provided with the carbonaceous acoustic diaphragm are main components.
[0021]
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 having a plate-like center
pole 21 at a central portion, and a magnet 23 disposed at the base end of the center pole 21.
A magnetic circuit is configured by the magnet 23, the yoke 22, and the center pole 21.
Further, the speaker body 14 is attached to a plurality of voice coils 24 and a tip end portion of
the voice coil 24 via a coil bobbin (not shown) surrounding the center pole 21 with a gap in a
magnetic circuit. And a carbonaceous acoustic diaphragm 25.
The outer peripheral edge of the carbonaceous acoustic diaphragm 25 is vibratably supported by
the frame 27 via the edge 26.
The number N of coils of the plurality of voice coils 24 corresponds to the number N of output
bits of the thermometer code converter 12.
[0022]
The conceptual diagram of a speaker drive system is shown in FIGS. 3-5.
N voice coils (24-1 to 24-N) are disposed independently (FIG. 3), and wound around a coil
holding portion 28 whose one end is connected to the carbonaceous acoustic diaphragm 25
(Figure 4).
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The end of the voice coil (24-1 to 24 -N) may be directly connected to one surface of the
carbonaceous acoustic diaphragm 25 without using the coil holding portion 28.
Further, as shown in FIG. 5, each voice coil (24-1 to 24-N) is configured to be controllable
independently of the driver circuits (1) to (N).
[0023]
In the speaker main body 14, a current is supplied to the voice coil 24 placed in the magnetic
circuit formed by the magnet 23, the yoke 22 and the center pole 21, and the force generated in
the direction orthogonal to the magnetic lines of force is used for the voice coil 24. Then, the
carbonaceous acoustic diaphragm 25 is vibrated to generate a sound wave.
A current is supplied to the voice coil 24 in accordance with each bit value of the digital signal
output from the thermometer code converter 12.
[0024]
FIG. 6 is a circuit diagram of the delta sigma modulator 11. As shown in FIG.
The circuit configuration shown in the figure is an example, and a higher order delta sigma
modulator can also be used. Here, it is assumed that the digital audio signal represented by multilevel input bits is 16 bits, and the n bit output from the delta sigma modulator 11 is 4 bits.
[0025]
The delta sigma modulator 11 basically comprises an integrator 31, a quantizer 32, a delay 33,
and a feedback loop. τ is a feedback gain. The 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 during the
quantization is returned to the input end in a feedback loop passing through the delay unit 33,
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and only the quantization error is integrated. Assuming that the input is X, the output is Y, and
the quantization error is Q, the relational expression is represented by Y = X + (1-Z <-1>) Q. The
transfer function (1-Z <-1>) multiplied by the quantization error Q has a frequency characteristic
and becomes smaller near a direct current, so this characteristic is the noise shaping effect
described later.
[0026]
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 generated by the
quantizer 32 can be eliminated by applying the oversampling method. Oversampling is one of the
techniques for sampling at a frequency sufficiently higher than the signal band. Also, in the case
of delta sigma modulation, the noise shaping effect can improve the accuracy of the original
signal. That is, when quantization is performed using a quantizer, quantization noise is
distributed uniformly over all frequencies, but by delta sigma modulation, unnecessary noise
components are shifted to the oversampled high frequency region, Noise in the vicinity of the
original signal can be suppressed, and the accuracy of the original signal can be improved.
[0027]
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, in the
case of converting into an 8-bit thermometer code, delta sigma modulator outputs (0010),
(0101), (1000) are respectively converted into thermometer codes (00000011), (00011111),
(11111111). Convert. Since the binary number output from the delta sigma modulator 11 is a
signal with a weight for each bit, direct digital driving is difficult if using the signal as it is, but a
thermometer code without weight for each bit By converting, the speaker body 14 can be directly
driven by a digital signal.
[0028]
The driver circuit 13 drives each voice coil 24-1 to 24 -N independently based on the
thermometer code output from the thermometer code converter 12. Specifically, each voice coil
24-1 to 24-N corresponds to each bit value of the thermometer code on a one-to-one basis. For
each bit of the thermometer code from the thermometer code conversion unit 12, a diagram of
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FIG. A 1-bit signal (ON / OFF) as shown in 9 (a) (b) is output. The current is supplied to the voice
coil 24 of the thermometer code "1", and the voice coil 24 of the thermometer code "0" is driven
so that the current does not flow. 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.
[0029]
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 having a porous body containing amorphous carbon and carbon powder
uniformly dispersed in the amorphous carbon and 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, includes amorphous carbon, is thinner than the low
density layer, and has a high density layer higher in density than the low density layer. It is
preferable to further include.
[0030]
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 covered with the
high density layer, and conversely, both sides of the high density layer are Various configurations
are possible, such as a three-layer structure, a single-layer structure with only a high density
layer, or the like.
[0031]
It is desirable that the shape of the pores of the porous body is spherical, 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 uniformly dispersed in the amorphous carbon. It is desirable that this carbonaceous
acoustic diaphragm has a weight increase of 5% or less when it is left in an environment of
temperature 25 ° C. and humidity 60% for 250 hours after drying.
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[0032]
In addition, a carbonaceous acoustic diaphragm is prepared by uniformly mixing carbon powder
with a carbon-containing resin, shaping the mixture into a film, heating the mixture to form a
carbon precursor, and carbonizing the carbon precursor in an inert atmosphere. Can be
manufactured. In the method of manufacturing such a carbonaceous acoustic diaphragm, the
particles of a puncturing material which is solid or liquid at the carbon precursor forming
temperature and disappears at the carbonizing temperature to leave pores are mixed in the
mixture in advance. Thus, after the carbonization, a porous body containing amorphous carbon
and carbon powder is obtained.
[0033]
By forming a layer of a carbon-containing resin on at least one surface of the plate of the carbon
precursor before the carbonization, the low density layer and the low density layer made of the
porous body after the carbonization can be obtained. It is preferable to further include a
carbonaceous acoustic diaphragm including a high density layer with high density. In the
structure in which both sides of the high density layer are sandwiched by the low density layers,
for example, a carbon precursor layer containing a hole forming material is bonded and
integrated with a resin to both sides of the carbon precursor not containing a hole forming
material. Can be obtained by
[0034]
It is desirable that the particles of the punching material be spherical. The carbon powder
preferably contains carbon nanofibers. The layer of carbon-containing resin may include graphite
uniformly dispersed therein. The carbonization is preferably performed at a temperature of 1200
° C. or higher.
[0035]
As described above, a drilling material that is solid or liquid at the temperature when carbon
precursor is formed into a mixture of carbon-containing resin and carbon powder, and
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disappears at the carbonization temperature to leave pores, such as polymethyl By mixing
particles of methacrylate (PMMA), in the process of carbonization, the drilling material
disappears leaving pores of a three-dimensional shape corresponding to its 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 facilitated
by selecting the three-dimensional shape and size of the particles of the drilling material. It can
be controlled, and a porous body having a porosity of 40% or more can be realized.
[0036]
The porosity is a percentage of the volume of pores to the volume of the whole porous body
including pores, and the density of carbon is 1.5 g / cm <3>, and the porosity calculated from the
volume and mass of the whole porous body Define as
[0037]
If the multi-layered structure of the low density layer and the high density layer made of the
porous body is used, the porosity can be made 60% or more while maintaining the necessary
rigidity, and the density of the whole diaphragm is 0.5 g / It can be less than cm <3>.
The high-density layer exerts an effect at about 1 to 30% of the total thickness, and plays a role
of high-range reproduction with rigidity of about 100 GPa of Young's modulus.
[0038]
The Young's modulus of the low density layer is about 2 to 3 GPa, and the entire diaphragm is
made lightweight to maintain the overall sound quality and improve the vibration response.
[0039]
Since these are integrated and baked to carbonize to form a plurality of layers of carbonaceous
materials, it becomes possible to control characteristics, in particular, a multilayer flat speaker
diaphragm capable of outputting sound in the audible range up to the high range. .
[0040]
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A flat diaphragm with a high reproduction limit frequency can be obtained by the balance
between the dense, high-rigidity high-density layer and the low-density low-density layer, which
is the core, instead of the dome shape to give rigidity.
The design of the porosity also changes the reproduction range, but the pore size does not
greatly affect it.
Handleability is improved and impact resistance is also improved. Also, by covering one side or
both sides of the low density layer of the porous body with the high density layer, it is possible to
prevent the suction of the adhesive at the time of incorporation into the unit.
[0041]
A further property required of the acoustic diaphragm is that it has low hygroscopicity so that it
does not change in acoustic characteristics by absorbing moisture in the air and making it heavy.
By setting the temperature of carbonization to 1200 ° C. or higher, it is possible to obtain one
having an increase in mass of 5% or less when left in an environment of 25 ° C. and 60%
humidity after drying for 250 hours.
[0042]
Although the above description exemplifies the structure in which the carbonaceous acoustic
diaphragm is held by the frame via the edge, it is also possible to support the carbonaceous
acoustic diaphragm with a flexible film.
[0043]
FIG. 10 (a) is a cross-sectional view of a speaker main body supporting a carbonaceous acoustic
diaphragm with a flexible film, and FIG. 10 (b) 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 that of 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 dome-shaped bulging shape at its central portion, and is fixed to the upper
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surface of the plate-like film base 43. The end of the voice coil 24 abuts on the lower surface
outer peripheral edge of the film base 43 to transmit the vibration. The flexible film 42 is
provided with a concavo-convex process for securing its strength.
[0044]
A digital drive system as shown in FIG. 1 is connected to the speaker main body configured as
described above to constitute a digital speaker unit. The method of driving the speaker main
body by the digital audio signal supplied from the digital sound source is as described above.
[0045]
As described above, by holding the carbonaceous acoustic diaphragm 41 with the flexible film 42
having the required rigidity and flexibility, a high sound pressure can be obtained compared to a
structure in which the carbonaceous acoustic diaphragm is held by the frame. realizable. In the
verification experiment by the present inventor, 90 dBspl as a peak sound pressure could be
realized by combining the film with the carbonaceous diaphragm. Therefore, in an application
requiring high sound pressure, a configuration in which the carbonaceous acoustic diaphragm 41
is held by the flexible film 42 as shown in FIG. 10 is desirable.
[0046]
(Example 1) Example of Three Layers Covering Both Surfaces of Low-Density Layer with HighDensity Layer 35% by mass of vinyl chloride resin as an amorphous carbon source and carbon
nano-fiber 1.4 having a mean particle diameter of 0.1 μm and a length of 5 μm A diallyl
phthalate monomer as a plasticizer is added to a composition in which a mass% and PMMA as a
pore forming material are combined and added as a plasticizer and dispersed using a Henschel
mixer, and then sufficiently using a pressure kneader The mixture was repeatedly kneaded to
obtain a composition, which was pelletized by a pelletizer to obtain a molding composition.
Pellets of this molding composition were extruded into a sheet-like molded product with a
thickness of 400 μm, and furan resin was coated on both sides and cured to form a multilayer
sheet. The multilayer sheet was treated in an air oven at 200 ° C. for 5 hours to form a
precursor (carbon precursor). Thereafter, the temperature was raised in nitrogen gas at a
temperature rising rate of 20 ° C./h, and maintained at 1000 ° C. for 3 hours. After natural
cooling, it was maintained at 1400 ° C. in vacuum for 3 hours, and then naturally cooled to
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complete firing. As a result, as schematically shown in FIG. 7, the low density of the porous body
having the spherical pores 114 remaining after the powder 112 of carbon nanofibers is
uniformly dispersed in the amorphous carbon 110 and the particles of PMMA disappear. An
acoustic diaphragm was obtained having a layer 116 and a dense layer 118 of amorphous
carbon covering both sides thereof.
[0047]
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 whole 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, an acoustic velocity of 4200 m / sec, a density of 0.45 g / cm <3> and a
hygroscopicity of 1% by mass or less.
[0048]
The sound velocity was calculated from the measured values of density and Young's modulus
(the same applies hereinafter). Hygroscopicity is a mass increase rate (%) when left in an
environment of temperature 25 ° C. and humidity 60% after drying at 100 ° C. for 30 minutes.
The relationship between elapsed time and mass change rate is shown in FIG. The result when
setting the temperature of the last baking (carbonization) to 1000 ° C. is also shown as
Comparative Example 1. As can be seen from FIG. 8, by setting the carbonization temperature to
1200 ° C. or higher, a diaphragm with low hygroscopicity whose increase in mass after 250
hours is 5% or less can be obtained.
[0049]
(Example 2) Example in which filler (graphite) is added to the high density layer 35% by mass of
vinyl chloride resin as an amorphous carbon source and 1.4% by mass of carbon nanofibers
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 is combined as a holeforming material for pore formation and dispersing using a Henschel mixer, sufficient kneading
is performed using a pressure kneader The composition was repeatedly obtained, and pelletized
by a pelletizer to obtain a molding composition. Pellets of this molding composition are extruded
into a sheet-like molded product with a thickness of 400 μm, and 5 mass% of graphite (SP 270
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manufactured by Nippon Graphite Co., Ltd.) is dispersed in furan resin, and a curing agent is
added. The solution was coated on both sides and cured to form a multilayer sheet. The
multilayer sheet was treated in an air oven at 200 ° C. for 5 hours to form a precursor (carbon
precursor). Thereafter, the temperature was raised in nitrogen gas at a temperature rising rate of
20 ° C./h, and maintained at 1000 ° C. for 3 hours. After natural cooling, it was held at 1500
° C. for 3 hours in vacuum, and then naturally cooled to complete firing, to obtain a composite
carbon diaphragm.
[0050]
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, an acoustic velocity of 3333 m / sec, and a density of 0.45 g / cm <3>.
[0051]
(Example 3) Example of porous body only 50% porosity 50% monolayer molded body
Amorphous carbon source, 54% by mass of vinyl chloride resin and carbon nanofiber with an
average particle diameter of 0.1 μm and a length of 5 μm 1.4 mass %, Diallyl phthalate
monomer as a plasticizer is added to a composition in which PMMA is compounded as a poreforming material, and dispersed using a Henschel mixer, and then sufficiently kneaded using a
pressure kneader Were repeated to obtain a composition, which was pelletized by a pelletizer to
obtain a molding composition. Using this pellet, a film-like extrusion with a thickness of 400 μm
was performed. The film was treated in an air oven heated to 200 ° C. for 5 hours to form a
precursor (carbon precursor). Thereafter, the temperature was raised in nitrogen gas at a
temperature rising rate of 20 ° C./hour or less, and held at 1000 ° C. for 3 hours. After natural
cooling, it was held at 1500 ° C. for 3 hours in a vacuum atmosphere, and then natural cooling
was carried out to complete firing to obtain a composite carbon diaphragm.
[0052]
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, an
acoustic velocity of 3055 m / sec, a density of 0.75 g / cm <3. >, And had excellent physical
properties.
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[0053]
Next, the frequency characteristic of the speaker in the case of using the diaphragm made in the
above-mentioned Example 1 for the above-mentioned digital speaker unit is explained.
The voice coil 24 included 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 the thermometer code
output from the thermometer code converter 12 Has a 6-bit configuration.
[0054]
FIG. 11 shows frequency characteristics when the diaphragm obtained in Example 1 is used. As
shown in the figure, in the case of the carbonaceous diaphragm alone, very flat characteristics
were realized from around 700 Hz to 20 kHz which is said to be the upper limit of the audio
frequency range. With the frequency characteristics shown in FIG. 11, it is possible to reproduce
very good quality sound. Moreover, 85 dBspl or more can be realized as peak sound pressure.
[0055]
As described above, according to the digital speaker unit according to the embodiment of the
present invention, the carbonaceous acoustic diaphragm 25 having low density and light weight
but having sufficient rigidity is directly driven by the digital audio signal, Good acoustic
characteristics can be realized.
[0056]
DESCRIPTION OF SYMBOLS 10 digital sound source 11 delta sigma modulator 12 thermometer
code conversion part 13 driver circuit 14 speaker main body 21 center piece 22 yoke 23 magnet
24 voice coil 25 carbonaceous acoustic diaphragm 26 edge 27 frame
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