Patent Grant
9100755
The present invention relates to a sound reproducing apparatus with high directivity, capable of modulating a signal in an audible band and emitting a signal in an ultrasonic band as a carrier, thereby to reproduce a sound wave of the audible band in a specific space range.
A normal sound reproducing apparatus can directly emit a sound wave of an audible band into a medium such as air through a diaphragm, to propagate the sound wave of the audible band in a relatively broad range by a diffraction effect.
As opposed to this, a sound reproducing apparatus with high directivity has been put into practice for selectively propagating the sound wave of the audible band only to a specific space range. This sound reproducing apparatus is generally called a super directional loudspeaker or a parametric loudspeaker. This modulates a signal in the audible band with a signal in an ultrasonic band as a carrier, further amplifies the signal by a specific scaling factor, and thereafter inputs this modulated signal into a sound emitting unit made up of an ultrasonic transducer and the like, to emit the signal as a sound wave of the ultrasonic band into the medium such as air.
The sound wave emitted from the sound emitting unit propagates to the medium with high directivity due to a propagation characteristic of the ultrasonic wave as the carrier. Moreover, during propagation of the sound wave of the ultrasonic band in the medium, with the medium having elastic nonlinearity, an amplitude of the sound wave of the audible band accumulatively increases, while the sound wave of the ultrasonic band attenuates since being absorbed by the medium or diffused over a spherical surface. As a consequence, the sound wave of the audible band, having been modulated to the ultrasonic band, is self-demodulated to the sound wave of the audible band due to the elastic nonlinearity of the medium, thereby to allow reproduction of the sound wave of the audible band only in a restricted narrow space range.
That is, the super directional loudspeaker is one making use of the elastic nonlinearity of the medium where the sound wave propagates and the high directivity of the ultrasonic wave. For example, the use of the super directional loudspeaker as a loudspeaker for descriptions of exhibitions in an art museum or a museum allows transmission of a sound wave of an audible band only to a person present within a specific space range.
The foregoing sound reproducing apparatus uses, as a carrier frequency, a frequency in the vicinity of a resonance frequency for exciting a resonance mode of the ultrasonic transducer made up of a piezoelectric body and the like in order to increase sound pressure of the sound wave of the audible band which is reproduced by as small an input electric field as possible. In the vicinity of the resonance frequency, mechanical quality factor Qm (constant indicating sharpness of a mechanical vibration displacement in the vicinity of the resonance frequency at the time of the piezoelectric body or the like producing resonance vibration) is high, and a maximal vibration displacement can be obtained with respect to an alternating electric field that is applied.
However, there are variations in resonance frequency of the ultrasonic transducer between individuals, which is attributed to structural conditions such as shapes, dimensions and supporting and fixing methods of the piezoelectric body and the other constitutional elements, and is attributed to material characteristic conditions such as a piezoelectric constant and an elastic constant generated by such processes as polarization and sintering in the case of the piezoelectric body being ceramics. Further, mechanical quality factor Qm is also influenced by a temperature change of the ultrasonic transducer itself and load fluctuations due to the medium such as air, and there has thus been a problem in that, even when an electric fields with the same frequency and the same amplitude are applied to a plurality of ultrasonic transducers, respective vibration amplitudes of the ultrasonic transducers differ, and thereby at the time of demodulation and reproduction of the signal in the audible band, desired sound pressure cannot be obtained depending upon a frequency band of the signal in the audible band.
It is to be noted that Non-Patent Document 1 is known as prior art document information concerning the above sound reproducing apparatus.
[Non-Patent Document 1] “Regarding Practical Realization of Parametric Loudspeaker”, written by Tsuneo Tanaka , Mikiro Iwasa, and Youichi Kimura; The Acoustical Society of Japan Technical Report, US84-61, 1984 (pp. 1-2,
The present invention at least includes: an audible band signal source that produces a signal in an audible band; a carrier oscillator that produces a carrier; a modulator that modulates the signal in the audible band with the carrier; and a sound emitting unit that receives an input of a signal outputted from the modulator and outputs a reproduced sound by means of an ultrasonic transducer. The ultrasonic transducer of the sound emitting unit has a plurality of resonance modes in which vibration displacements are maximal at different frequencies, and excites vibration mode-coupled between frequencies for exciting the plurality of resonance modes. Part of a frequency band where the mode-coupled vibration can be excited is regarded as a carrier frequency.
Accordingly, even in the case of variations or fluctuations in resonance frequency of the ultrasonic transducer due to load variations or the like in the manufacturing process of the ultrasonic transducer or during the operation thereof, a vibration amplitude of the ultrasonic transducer fluctuates in a small scale and is stable within the range of frequencies where the mode-coupled vibration can be excited. This can result in realization of stable sound pressure in a broad band at the time of self-demodulation of the sound wave of the audible band.
(Embodiment 1)
Hereinafter, a configuration of a sound reproducing apparatus in present Embodiment 1 is described with reference to the drawings.
A signal (as a frequency of about 20 Hz to 20 kHz) in an audible band produced in audible band signal source 2 and a carrier (ultrasonic wave of about 20 kHz or larger) produced in carrier oscillator 3 are inputted into modulator 4, and the signal in the audible band is modulated with the carrier. The modulated signal is amplified in power amplifier 5, and inputted into sound emitting unit 6. The signal inputted from modulator 4 into sound emitting unit 6 is emitted as an ultrasonic wave to a medium such as air and propagates a certain distance, whereafter a sound wave of the ultrasonic band as the carrier attenuates, while a sound wave of the audible band is self-demodulated due to elastic nonlinearity of the medium.
As thus described, sound reproducing apparatus 1 in present Embodiment 1 is configured so as to allow reproduction of the sound wave of the audible band only in a very narrow space range by making use of the ultrasonic wave with high directivity as the carrier.
Next, ultrasonic transducer 7 constituting sound emitting unit 6 is described with reference to
Ultrasonic transducer 7 is a portion that vibrates piezoelectric body 8 upon input of the signal from modulator 4, and emits a sound wave to the medium such as air. Piezoelectric body 8 is cylindrical piezoelectric ceramics made of a complex perovskite-based piezoelectric material (e.g., three component-based piezoelectric ceramic material such as PbTiO3—ZrTiO3—Pb (Mg1/2Nb1/2)TiO3), and is disposed in almost the central part of one top surface of acoustic matching layer 9 in the thickness direction, as shown in
In the vicinity of the periphery of acoustic matching layer 9, tubular case 10 is fixed so as to surround piezoelectric body 8, thereby protecting piezoelectric body 8 from the outside. In present Embodiment 1, case 10 is made of aluminum.
Further, terminal block 11 is provided at an opening of case 10 (on the inner surface in the vicinity of the opposite end of the case to the portion connected with acoustic matching layer 9). There is a certain clearance provided between this terminal block 11 and piezoelectric body 8 so as to prevent mutual contact therebetween due to a shock from the outside, vibration of piezoelectric body 8, or the like. Moreover, two rod-like terminals 12 are provided on terminal block 11, and these terminals 12 are respectively electrically connected to electrodes of piezoelectric body 8 through leads 13. That is, an alternating electric field can be applied to piezoelectric body 8 through terminals 12.
When an alternating electric field with a specific frequency is applied to the electrodes provided on both principal surfaces of piezoelectric body 8 in ultrasonic transducer 7 configured as thus described, elastic vibration can be excited which is decided based upon a material coefficient, shape, dimensions, and the like. A sound wave generated by this elastic vibration is emitted to the medium such as air through acoustic matching layer 9, and propagated in a specific direction (upward direction in
Here, acoustic matching layer 9 serves to match acoustic impedances of piezoelectric body 8 and the medium such as air, to reduce attenuation of the sound wave caused by reflection or the like on a boundary plane due to a difference in acoustic impedance between the piezoelectric body and the medium.
It is to be noted that in present Embodiment 1, only one set each of audible band signal source 2, carrier oscillator 3, modulator 4 and power amplifier 5 described above is configured.
Next, a method for deciding a carrier frequency as a point of the present invention is described in detail.
When the frequency of the alternating electric field that is applied to the piezoelectric body is changed from the low frequency side to the high frequency side, as shown in
Further, as the frequency is made higher, a second resonance mode occurs in which a vibration displacement in a radial direction is maximal in the vicinity of frequency fD1 at which admittance Y is maximal. The resonance mode at this frequency fD1 is one called expansion vibration in the radial direction. It is to be noted that a vibration displacement in the radial direction of this expansion vibration in the radial direction is not shown in
As shown in
At frequencies other than the vicinities of frequency fL1 and frequency fD1, the vibration displacement in the thickness direction of the piezoelectric body rapidly decreases, to be hardly obtained. Similarly, at the frequencies other than the vicinities of frequency fL1 and frequency fD1, the vibration displacement in the radial direction also decreases, to be hardly obtained. That is, at the frequencies other than the vicinities of frequency fL1 and frequency fD1, the piezoelectric body hardly vibrates both in the thickness direction and in the radial direction. This means that the two resonance modes, namely the longitudinal vibration in the thickness direction and the expansion vibration in the radial direction, independently vibrate in the vicinities of the respective resonance frequencies without having an effect upon each other.
As thus described, in the cylindrical piezoelectric body, either thickness L or diameter D is made larger (generally, a cylindrical shape with thickness L made more than 2.5 times as large as diameter D, or a disk shape with diameter D made more than 15 times as large as thickness L), whereby the respective resonance modes independently vibrate without having an effect upon each other, while mechanical quality factors Qm of the respective resonance modes become high.
As opposed to this, in ultrasonic transducer 7 of sound reproducing apparatus 1 in present Embodiment 1, cylindrical piezoelectric body 8 with dimensional ratio L/D of thickness L to diameter D made about 0.7 is used. The use of piezoelectric body 8 with such a dimensional ratio allows excitation of mode-coupled vibration at a frequency between resonance frequencies for exciting two resonance modes of the longitudinal vibration in the thickness direction and the expansion vibration in the radial direction, so as to obtain vibration displacement ξL not smaller than a certain value in the thickness direction. Further, it becomes possible to make piezoelectric body 8 vibrate vibration displacement ξL that makes a small change with respect to frequency fluctuations. In present Embodiment 1, part of a frequency band where the mode-coupled vibration can be excited is regarded as a frequency band of a carrier.
As shown in
The foregoing mode-coupled vibration is excited, and a frequency area with frequency fLm, at which vibration displacement ξL in the thickness direction is minimal, regarded as a reference is used as the carrier frequency. Even in the case of respective fluctuations in resonance frequencies of the longitudinal vibration in the thickness direction and the expansion vibration in the radial direction of piezoelectric body 8 due to variations in material or shape, or the like, a vibration amplitude of the ultrasonic transducer 7 fluctuates in a small scale and is stable within the range of frequencies where the mode-coupled vibration can be excited. This can result in realization of stable sound pressure in a broad band at the time of self-demodulation of the signal in the audible band.
In terms of the fact that stable sound pressure can be obtained at the time of self-demodulation of the signal in the audible band, details are described below.
As thus described, when ultrasonic transducer 7 is excited by a signal obtained by modulating a signal in the audible band being a broad band with resonance frequency fm1 regarded as the carrier frequency, since an amount of change in vibration displacement of ultrasonic transducer 7 within the range of the frequency of the electric field to be applied is large, fluctuations in sound pressure become large with respect to a frequency of the sound wave emitted from the ultrasonic transducer, and the demodulated sound wave of the audible band has a large amplitude fluctuations due to the frequency, thereby making it difficult to obtain stable sound pressure.
Thereat, as in sound reproducing apparatus 1 in present Embodiment 1, part of a frequency band, where mode-coupled vibration can be excited with an amount of change in vibration displacement ξL with respect to frequency fluctuations being relatively small, is regarded as the carrier frequency, thereby allowing reproduction of the signal in the audible band with stable sound pressure in a broad band.
Herein, a result of considering conditions for making piezoelectric body 8 excite mode-coupled vibration from the relation between two resonance frequencies, frequency fm1 and frequency fm2, are hereinafter described.
In
As shown in
On the other hand, in frequency characteristic c and frequency characteristic d where resonance frequency fm2 is brought near resonance frequency fm1 as compared with frequency characteristic a and frequency characteristic b and respective resonance frequencies fm2 are made fm2c (=2.44) and fm2d (=2.25), minimal values ξLmc and ξLmd of vibration displacements ξL are large as compared with minimal values ξLma and ξLmb. That is, by bringing resonance frequency fm2 near resonance frequency fm1, vibration displacement ξL in the thickness direction comes to show a value not smaller than a certain value, and it is possible to make piezoelectric body 8 on such a condition excite mode-coupled vibration between frequencies for exciting the resonance mode.
From the numerical calculation, there is obtained a result that, when a normalized value of resonance frequency fm2 of piezoelectric body 8 is about 2.5 or smaller, a waveform of the frequency characteristic is shown as those of frequency c and frequency d, to cause occurrence of mode coupling in piezoelectric body 8.
It is therefore found that mode coupling occurs in piezoelectric body 8 when a frequency showing a first resonance mode of piezoelectric body 8 is referred to as fm1 and a frequency showing a second resonance mode thereof as fm2, fm1/fm2 as a ratio of the frequency showing the first resonance mode and the frequency showing the second resonance mode is at least not smaller than 0.4 (=1/2.5). It should be noted that, for making fm1/fm2 be not smaller than 0.4, dimensional ratio L/D of piezoelectric body 8 may, for example, be adjusted as appropriate. Adjusting dimensional ratio L/D can adjust frequency fm1 showing the first resonance mode and frequency fm2 showing the second resonance mode.
In addition, although
Further, as obvious from the frequency characteristic of admittance Y shown in
Further, the use of piezoelectric body 8 of present Embodiment 1 can give sound reproducing apparatus 1 capable of exerting stable performance on stress applied from the surroundings due to disturbance such as a temperature change or vibration. This is specifically described below.
It is found that in the vicinities of the respective resonances frequencies, frequency fm1 and frequency fm2, for exciting the first and second resonance modes, mechanical quality factor Qm of the resonance mode fluctuates depending upon the presence or absence of stress, while vibration displacement ξL significantly changes.
For example, in the case of the first resonance mode (longitudinal vibration in the thickness direction: resonance frequency fm1), mechanical quality factor Qm becomes lower when stress is applied due to disturbance or the like, and vibration displacement ξL decreases down to about one fifth of that in the case of application of no load. On the other hand, in the vicinity of frequency fLm as the carrier frequency used in present Embodiment 1, vibration displacement ξL hardly changes even when similar stress is applied.
That is,
Therefore, in present Embodiment 1, the use of part of the frequency band where mode-coupled vibration can be excited as the carrier frequency leads to a small change in vibration displacement ξL even in the case of stress being applied to piezoelectric body 8 due to disturbance such as a temperature change, vibration, or support and fixation conditions. As a consequence, it is possible to obtain sound reproducing apparatus 1 capable of reproducing a sound wave of an audible band with stable sound pressure in a broad band.
Further, the ultrasonic transducer 7 may also be susceptible to heat generated at the time of driving sound reproducing apparatus 1 of present Embodiment 1. That is, a sound velocity of piezoelectric body 8 changes with a change in temperature of ultrasonic transducer 7, and this change thereby causes a change in resonance frequency of ultrasonic transducer 7. Especially, as in present Embodiment 1, in piezoelectric ceramics used as piezoelectric body 8, the temperature dependence of the resonance frequency is high, and the stability of the resonance frequency with respect to the temperature change is low. Therefore, in the case of using a frequency in the vicinity of the resonance frequency as the carrier frequency, it is considered that desired sound pressure cannot be obtained when the resonance frequency changes due to the temperature change.
On the other hand, in present Embodiment 1, part of the frequency band, where mode-coupled vibration insusceptible to a temperature change can be excited, is used as the carrier frequency, and even if a temperature of ultrasonic transducer 7 changes due to heat generated at the time of driving sound reproducing apparatus 1, it is possible to reproduce a sound wave of an audible band with stable sound pressure.
In addition, it is desirable to select the carrier frequency in the frequency band where the mode-coupled vibration can be excited especially with a frequency, at which vibration displacement ξL of ultrasonic transducer 7 is minimal, regarded as a reference.
This is because, as apparent from
Next described is a method for designing dimensional ratio L/D of thickness L to diameter D of cylindrical piezoelectric body 8.
A horizontal axis is one representing normalized dimensional ratio L/D of piezoelectric body 8. A left-hand axis of vertical axes represents a frequency normalized based upon frequency fLm in the case of dimensional ratio L/D being made 1. Similarly, a right-hand axis of the vertical axes represents a vibration displacement normalized based upon vibration displacement ξLm in the thickness direction at the time of dimensional ratio L/D being made 1. It should be noted that frequency fm1 is indicated by a solid line, frequency fm2 by an alternate long and short dash line, and vibration displacement ξLm by a broken line.
It is found from
It is to be noted that dimensional ratio L/D of piezoelectric body 8 is not restricted to 0.7, but may be in the range of ±0.3 with 0.7 at the center, with which vibration displacement ξLm takes the maximal value, namely, dimensional ratio L/D may be a value not smaller than 0.4 and not larger than 1.0. When dimensional ratio L/D is a value not smaller than 0.4 and not larger than 1.0, piezoelectric body 8 efficiently vibrates with respect to the alternating electric field to be applied, to allow emission of a sound wave from ultrasonic transducer 7, so as to efficiently output a sound wave of the audible band as the sound reproducing apparatus.
As opposed to this, when dimensional ratio L/D of piezoelectric body 8 is made a value below 0.4 or exceeding 1.0, a vibration loss of piezoelectric body 8 becomes large, thereby making the vibration amplitude small with respect to the alternating electric field to be applied. With decrease in sound wave emitted from ultrasonic transducer 7, heat generation due to the vibration loss has an adverse effect upon the material characteristic of piezoelectric body 8, to make the operation reliability of ultrasonic transducer 7 more likely to deteriorate, which is not preferred.
In addition, although the above description is an example of forming piezoelectric body 8 by use of the complex perovskite-based piezoelectric material, even in the case of using a different material such as a piezoelectric monocrystal or piezoelectric ceramics like PZT-based ceramics, optimal dimensional ratio L/D of cylindrical piezoelectric body 8 can be decided by performing similar numerical calculation and prototype review.
(Embodiment 2)
In Embodiment 1, sound emitting unit 6 is configured by one ultrasonic transducer, but in Embodiment 2, an example of constituting the sound emitting unit by a plurality of ultrasonic transducers 7 is described below.
As shown in
Thereat, in present Embodiment 2, as in Embodiment 1, not the resonance frequency for exciting the resonance mode, but part of the frequency band, where mode-coupled vibration to be excited between the resonance modes can be excited, is used as the carrier frequency.
As piezoelectric body 8 in present Embodiment 2, there is used one similar to piezoelectric body 8 in Embodiment 1, as well as a cylindrical piezoelectric body with dimensional ratio L/D of thickness L to diameter D made 0.7. With such a dimensional ratio being set, when the plurality of piezoelectric bodies 8 constitute sound emitting unit 14 as shown in
Although sound emitting unit 14 is the example of the case of individual differences existing in resonance frequencies of piezoelectric bodies 8 constituting ultrasonic transducers 7, it is also effective in the case of constituting sound emitting unit 14 by piezoelectric bodies 8 having the same resonance frequency. That is, a change in temperature of ultrasonic transducer 7 during the operation or application of stress to piezoelectric body 8 at the time of assembly of ultrasonic transducer 7 may lead to a change in frequency characteristic of a vibration amplitude of ultrasonic transducer 7, and also in such a case, the configuration of present Embodiment 2 is applicable.
Further, although sound reproducing apparatus 1 according to present Embodiment 2 in
(Embodiment 3)
Hereinafter, a configuration of ultrasonic transducer 15 in Embodiment 3 is described with reference to
It is to be noted that present Embodiment 3 is one obtained by making part of the configuration of ultrasonic transducer 7 shown in Embodiment 1 different. Since the configuration other than this is similar to in Embodiment 1, the same portions are provided with the same numerals, and a detailed description thereof is omitted while only different portions are described.
As shown in
Conical resonator 17 is fixed with an adhesive to the central part of the top surface of piezoelectric body 8. A material for this resonator 17 is desirably one with light weight and a sound velocity of the degree of 3000 m/s to 10000 m/s. For example, with the use of metal such as aluminum or SUS (Stainless Used Steel), resonator 17 capable of following an amplitude of piezoelectric body 8 can be configured so that the amplitude can be amplified on a vibration mode as it is without changing the shape of the vibration mode. That is, resonator 17 in present Embodiment 3 is one showing a resonant characteristic corresponding to vibration of piezoelectric body 8, and capable of emitting a stable ultrasonic wave to the medium such as air with respect to the amplitude of piezoelectric body 8.
It is to be noted that resonator 17 is also configured to be surrounded by case 16 as shown in
In ultrasonic transducer 15 as thus configured, resonator 17 is provided to extend a diameter of a sound source, so as to allow improvement in output of the sound pressure.
Further, since sound reproducing apparatus 1 in Embodiment 1 outputs an ultrasonic wave with high directivity as described above, a sound wave of the audible band can be reproduced only in a very narrow space range. Herein, in the case of wishing to widen to some degree the space range where the sound wave of the audible band is reproduced, or in some other case, such widening can be achieved by providing resonator 17, as in ultrasonic transducer 15 of present Embodiment 3, so as to expand the directivity of sound reproducing apparatus 1.
Further, in the case of parallely arranging a plurality of ultrasonic transducers 15 of present Embodiment 3 to constitute the sound emitting unit as in above Embodiment 2, the ultrasonic transducer 15 has a characteristic of a directivity spread to some degree by resonator 17, as described above. For this reason, the emission range of the ultrasonic wave outputted from each ultrasonic transducer 15 tends to overlap an emission range of the ultrasonic wave of ultrasonic transducer 15 arranged in the vicinity thereof. That is, in a position where the emission ranges overlap each other as thus described, the ultrasonic wave outputted from each ultrasonic transducer 15 is added up, thereby to allow hearing of the reproduced sound wave of the audible band at further larger sound pressure.
Moreover, the directivity by resonator 17 is adjustable by appropriately changing an angle of the conical portion of resonator 17. Furthermore, a circular portion of the cone is not restricted to a perfect circle, but may be an ellipse.
It is to be noted that in each embodiment in the present invention, the case has been described where piezoelectric body 8 constituting ultrasonic transducer 7,15 is formed into a cylindrical shape, and as vibration to be excited by piezoelectric body 8, there is used vibration obtained by mode-coupling the resonance vibration of the longitudinal vibration in the thickness direction and the resonance vibration of the expansion vibration in the radial direction. However, in the present invention, the shape of the piezoelectric body and the vibration mode for excitation in the piezoelectric body are not restricted to a specific shape or a specific resonance mode. For example, a similar effect can also be obtained in the case of forming piezoelectric body 8 into a prismatic shape and using vibration obtained by mode-coupling longitudinal vibration in the thickness direction and expansion vibration in a diagonal direction or a side direction.
A sound reproducing apparatus of the present invention regards part of a frequency band where mode-coupled vibration can be excited, as a carrier frequency, thereby to allow sound pressure of a reproduced sound wave of an audible band to be stabilized in a broad band. By making use of high directivity of the ultrasonic wave, the sound reproducing apparatus is useful as one for reproducing the sound wave of the audible band only in a restricted space range.
1 sound reproducing apparatus
2 audible band signal source
3 carrier oscillator
4 modulator
5 power amplifier
6 sound emitting unit
7 ultrasonic transducer
8 piezoelectric body
9 acoustic matching layer
10 case
11 terminal block
12 terminal
13 lead
14 sound emitting unit
15 ultrasonic transducer
16 case
17 resonator
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-239129 | Sep 2008 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2009/004668 | 9/17/2009 | WO | 00 | 3/2/2011 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2010/032463 | 3/25/2010 | WO | A |
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| Entry |
|---|
| Manabu Aoyagi et al.; Ultrasonic Motors Using Longitudinal and Bending Multimode Vibrators with Mode Coupling by Externally Additional Asymmetry or Internal Nonlinearity; Japanese Journal of Applied Physics; The Japan Society of Applied Physics; Japan Society of Applied Physics; Tokyo, JP; vol. 31, No. 9B; Sep. 1, 1992; pp. 3077-3080. |
| Supplementary European Search Report for EP 09 81 4308, May 6, 2013. |
| International Search Report for Application No. PCT/JP2009/004668, Nov. 24, 2009, Panasonic Corporation. |
| Tanaka et al., “Regarding Practical Realization of Parametric Loudspeaker” (“Consideration on the Practical Use of the Parametric Loudspeaker”), The Acoustical Society of Japan Technical Report, US84-61, 1984 (pp. 1-2, FIGS. 1 and 2) (with English abstract). |
| Number | Date | Country | |
|---|---|---|---|
| 20110170712 A1 | Jul 2011 | US |
