JPH0560316B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a motional feedback (hereinafter abbreviated as MFB) type speaker suitable for use in, for example, a digital audio system.

BACKGROUND TECHNOLOGY AND PROBLEMS Conventional audio signal recording media such as LP records and cassette tapes have generally not been able to adequately reproduce low frequencies, especially low frequencies below 100Hz, but with the advent of so-called compact discs, etc. As a result, it has become possible to obtain a reproduced signal that has flat frequency characteristics from the low frequency range to the high frequency range, including the low frequency range of 100 Hz or less, and has a good S/N ratio.

For this reason, there is a growing demand for higher performance speakers, especially woofers, which are low-frequency speakers, for converting electrical signals reproduced from such recording media through amplifiers, etc. into acoustic signals. . However, since the amplitude of woofers used for bass sounds is large, usually about 10 mm or more, linearity deteriorates when signals have large amplitudes, and electric signals have a resonance frequency f ₀ within the operating band. There are accuracy problems in electrical-mechanical-acoustic conversion, such as errors in the vibration of the diaphragm and the vibration of the diaphragm, and as a result, the sound pressure frequency characteristics are not flat.
Further, there are disadvantages such as generation of non-linear distortion.

Therefore, in order to flatten the sound pressure frequency characteristics and reduce nonlinear distortion, an MFB method has been proposed. This MFB method detects the momentum of the speaker's vibration system, and feeds back the detected output to the speaker's drive system to control the movement of the speaker's vibration system. Various methods have been proposed to feed back the detection output in this MFB method.For example, the first method is to directly feed back a voltage proportional to the speed of the speaker's diaphragm to the input side, and the second method is to feed back a voltage proportional to the speed of the speaker's diaphragm. A third method is to feed back a voltage proportional to the acceleration of the diaphragm of the speaker, and a third method is to listen to a voltage proportional to the displacement of the diaphragm.

Among these methods, the first method is particularly commonly used, and specifically, this method detects the back electromotive force of the current flowing through the drive coil of the speaker and feeds it back to the input side. With this method, it is easy to detect the momentum of the speaker's vibration system, but the detected output does not correspond to the movement of the speaker's diaphragm, which generates sound waves at the interface with the air. Since the movement of the drive coil is essentially detected as a change in speed, there is a problem in that the signal detected by the coil does not necessarily correspond to the movement of the diaphragm.

Therefore, in order to obtain good sound, it is necessary that the movement of the speaker's diaphragm correspond faithfully to the electrical signal, and from this point of view, the third method is the most preferable among the three methods described above. It seems to be.

Furthermore, when considering the structure of a speaker, the vibration system of a conventional speaker is generally supported on the fixed side by a support member such as a damper or an edge, and FIG. 1 is an example of this. That is, the first
In the figure, for example, the end of a planar diaphragm 1 is fixed to a frame 3 via an edge 2, a voice coil 4 is wound around the lower part of the end, and a bobbin 5 connected to the diaphragm 1 is connected to the upper part of the end of the diaphragm 1. The intermediate portion is fixed to the plate 7 side via the damper 6. Further, 8 and 9 are a magnet and a yoke which together with the plate 7 form a magnetic circuit.

As described above, it has conventionally been considered essential to support the vibration system of a speaker with a support member such as a damper or an edge. However, due to this support member, especially the damper, the vibration system of the speaker has a resonance point, and the movement of the vibration system is restricted, resulting in nonlinearity during large-amplitude operation such as in the bass range, or nonlinearity in the high-frequency range. There are disadvantages such as complex resonance and phase rotation.

Purpose of the Invention In view of the above, the present invention detects the displacement of the vibration system of a speaker, returns this displacement information to the drive system of the speaker, and makes it possible to faithfully correspond the displacement of the vibration system of the speaker to an electric signal. It is an object of the present invention to provide a motional feedback type speaker which is capable of providing a vibration system and which can eliminate the above-mentioned inconveniences associated with the intervention of a support member for the vibration system of the speaker.

Summary of the Invention The present invention provides a speaker having a bearing mechanism that supports a diaphragm so as to be linearly displaceable with respect to a frame, in which light from a light source reflected by a reflecting means provided on the diaphragm is a light position detection means that detects the displacement of the diaphragm when the light is incident; a light amount control means that controls the amount of light incident on the light position detection means to be constant so that the displacement of the diaphragm is detected;
and a drive means for linearly displacing the diaphragm based on displacement information of the diaphragm from the optical position detection means, and the output of the optical position detection means is fed back to the drive system of the speaker. This is a motional feedback type speaker that flattens sound pressure frequency characteristics, reduces nonlinear distortion, and eliminates complex resonance and phase rotation in the high frequency range.

Embodiment Hereinafter, an embodiment of the present invention will be described in FIGS. 2 to 11.
This will be explained in detail based on the figures.

FIG. 2 shows an example of support means for the vibration system of a speaker according to the present invention.
Portions corresponding to those in the figures are given the same reference numerals, and detailed explanation thereof will be omitted.

In this embodiment, a bearing 10 is provided on the frame 7 side, and a shaft 11 is correspondingly provided on the diaphragm 1 side.
The shaft 11 is loosely fitted into the bearing 10 so that it can move back and forth according to the displacement of the diaphragm 1. It has a linear motor structure. As a result, the diaphragm 1 has basically linear displacement characteristics with respect to the driving force applied thereto, and vibrates obediently without having a resonance point. It is preferable that the bearing 10 and the shaft 11 are not in contact with each other, and for this purpose, a viscous fluid such as magnetic fluid may be filled between the bearing 10 and the shaft 11.

Now, in this way, by eliminating supporting members such as dampers and edges of the vibration system of the speaker and driving the diaphragm 1 by means of supporting means having a linear motor structure, there is no resonance point in the driving range. Although it is possible to generate vibrations with good linearity, when such a structure is used, the vibration system collides with one of the stoppers, etc., or the acceleration increases as described above. and speed.
Even if MFB is applied, the neutral point of vibration cannot be determined, so there is a problem that it cannot be driven.

Therefore, the present invention uses a method that detects the displacement of the vibration system of a speaker that actually moves air, feeds back this displacement information to the drive system, and drives the vibration system in faithful response to the electric signal.

FIG. 3 shows an example of an optical position detection means for detecting the displacement of the vibration system of the speaker used in this embodiment. In the figure, a mirror 12 is attached to the diaphragm 1 that moves in parallel. . Reference numeral 13 denotes a light source that causes a beam of light to enter the mirror 12, and uses, for example, a semiconductor laser light source. The light beam (laser beam) from this semiconductor laser light source 13 is made to enter the mirror 12 via the lens 14. N indicates the normal at the point of incidence of this ray on the mirror 12. Reference numeral 15 denotes a photodetector that receives the reflected light source from the mirror 12 at a position on the light receiving section corresponding to the amount of displacement of the diaphragm 1. This photodetector 15
is arranged such that the longitudinal direction (line direction) of its light-receiving surface is located within a plane containing the incident and reflected rays and the normal N.

θ is the angle of incidence and reflection of the incident light beam and the reflected light beam, respectively, on the mirror 12, and these angles are equal to each other.

Therefore, if the diaphragm 1 is translated in parallel as shown by the broken line, that is, it is displaced by l in the direction of the normal line N, then
The position of the reflected light beam from the mirror 12 that enters the light receiving surface of the photodetector 15 is in its longitudinal direction (line direction).
is displaced by x. This x is expressed as in the following equation.

x=2lsinθ...(1) In this equation, if θ is chosen to be 30°, sinθ becomes 1/2, so x=l.

Therefore, if the light beam reflected from the mirror 12 is captured by the photodetector 15, positional information of the diaphragm 1 can be obtained according to the displacement of the light receiving surface.

As an example of the above-mentioned photodetector 15, a semiconductor position detector using a PIN photodiode, the equivalent circuit of which is shown in FIG. 4, is used. In this semiconductor position detector, a photocurrent generated by an incident light beam flows through a uniform resistive layer to two output terminals t ₁ and t ₂ , and the resulting differential first and second position detection currents are By determining the difference, the position of the incident light beam can be detected. In Figure 4, t ₃
is an input terminal to which a bias voltage is supplied.
Also, R _s is a surface resistance with a constant resistivity, and depending on the position of the light incident on this resistor R _s , the resistance value between that position and each output terminal t ₁ and t ₂ changes depending on the length. It is determined proportionally, and the current obtained at the output terminals t ₁ and t ₂ changes depending on the ratio of the reciprocals of the resistances. The point where the currents obtained at the output terminals t ₁ and t ₂ are equal corresponds to the neutral point of the speaker's vibration system. In addition, P is a current source, D is an ideal diode, C _i is a junction capacitance,
R _SN is the parallel resistance.

FIG. 5 shows a servo system that feeds back the displacement information of the speaker's diaphragm detected by the above-mentioned optical position detection means to the speaker's drive system.
Parts corresponding to those in FIGS. 3 and 4 will be described with the same reference numerals.

As described above, the currents, which are the first and second position detection outputs obtained at the output terminals t ₁ and t ₂ of the photodetector 15, are supplied to the current-voltage conversion circuits 16 and 17, respectively, and converted into voltage signals. The converted signal is then supplied to an addition circuit 18 and a subtraction circuit 19.

Here, if the amount of light incident on the photodetector 15 changes, the detected position information will include a large error. This becomes clear from the graph in FIG. That is, FIG. 6 shows the state of change of the difference output and the sum output with respect to the amount of displacement of the diaphragm 1 as curves S _d and S _a, respectively. Note that the reason why the curves S _d and S _a are double curves is due to hysteresis. Therefore, the output of the adder circuit 18 is supplied to the light amount control circuit 20, and the output of this control circuit 20 controls the laser light source 13 to keep the amount of light incident on the photodetector 15 constant, so that the output of the adder circuit 18 is always The amount of displacement of the diaphragm 1, that is, the absolute position information can be accurately detected by keeping it constant. In order to accurately detect the amount of displacement of the diaphragm 1, instead of using the light amount control circuit 20 as described above, a division circuit (not shown) is provided on the output side of the addition circuit 18 and the subtraction circuit 19. By dividing the subtraction output by the addition output and using this division output, it is also possible to obtain a highly accurate displacement detection output of the diaphragm 1.

Further, the subtraction output from the subtraction circuit 19 is supplied to the inverting input terminal of the comparison circuit 21.

On the other hand, the audio signal supplied to the input terminal 22 is converted into a signal with a characteristic of 12 dB/oct by the range limiting filter 23, and then supplied to the non-inverting input terminal of the comparator circuit 21 as a reference voltage. The comparison circuit 21 compares this reference voltage with the output from the subtraction circuit 19 corresponding to the displacement information of the diaphragm 1, and the comparison error signal is sent to the drive coil (voice) via the phase compensation filter 24 and drive circuit 25 of the speaker. coil) 26.

As a result, the servo is applied so that the displacement of the diaphragm 1 faithfully corresponds to the audio signal supplied to the input terminal 22, that is, the servo is applied so that the output of the comparison circuit 21 becomes zero.

Here, when looking at the relationship between the reference voltage supplied to the non-inverting input terminal of the comparator circuit 21 and the displacement of the diaphragm 1, as shown in FIG. However, when the servo is applied, the relationship becomes linear as shown by the solid line. That is, the displacement of the diaphragm 1 faithfully corresponds to the audio signal, and nonlinear distortion is reduced.

Also, if we look at the relationship between the displacement of the diaphragm 1 and the sound pressure level, now the distance r of the observation point from the diaphragm 1 is
If the sound pressure P〓 of (m) is assumed, this sound pressure P〓 is generally expressed by the following formula.

P = -ω ² ρ ₀ S・X _n /2πre ^-jkr ...(2) However, in the above equation (2), ω is the angular frequency, ρ ₀ is the air density = 1.21Kg/m ³ , and S is The area of the diaphragm (m ² ), X _n is the displacement of the diaphragm (X _n = jωV _n if the speed of parallel movement of the diaphragm is V _n ), and k is the wave number 2π/λ
It is.

Here, the sound pressure P with the audible limit as 0 dB is
Using SPL as the dB display, this is (3)
Therefore, SPL=20log ₁₀ P/P ₀ ...(3) In this equation (3), if P ₀ = 2×10 ^-5 N (Newtons)/m ² , then SPL=20log ₁₀ P+94 ... (4) becomes. And instead of P in this equation (4),
By substituting the absolute value of P〓 |P〓|, the following equation is obtained.

SPL=20log ₁₀ (−ω ² ρ ₀ S・|X _n |/2πr+94) ...(5) From this equation (5), the sound pressure level SPL is a function of the frequency, the area of the diaphragm, and the displacement of the diaphragm. I understand that there is something. In other words, the displacement of the diaphragm X _n and the sound pressure level
The relationship between SPL is that the displacement of the diaphragm, that is, the amplitude
If it follows the 12dB/oct characteristic, the sound pressure level can be kept constant.

Therefore, if we look at the various characteristics of the diaphragm 1 when no displacement information is fed back to the diaphragm 1, the characteristics are as shown in FIG. That is, in FIG. 8, G represents the amplitude specificity of the diaphragm 1, and θ represents the phase characteristic of the diaphragm 1. From this amplitude characteristic G, the diaphragm 1
It can be seen that as the frequency becomes lower, it deviates from the characteristic of 12 dB/oct.

However, when the displacement information of the diaphragm 1 is fed back as described above and the servo is applied so that the output of the comparator circuit 21 becomes 0, the amplitude characteristic G of the diaphragm 1 becomes 12 dB/octave, as shown in FIG. It comes to match the characteristics of Therefore, looking at the sound pressure level characteristics at this time,
As shown in Fig. 10, when no displacement information is fed back, the frequency response decreases as shown by the broken line, and the sound pressure level decreases, especially below 100Hz.
As described above, when the displacement information is fed back, the sound pressure level extends to a low frequency range, so that the sound pressure frequency characteristic becomes flat.

In the above-described embodiment, if the movement of the diaphragm 1 deviates from the parallel movement, as shown in FIG. and 12b, a plurality of photodetectors 15a and 1
5b, the beam from the laser light source 13 is made incident on the mirrors 12a and 12b via the lens 14 and the diffraction grating 27, and each of the reflected beams is received by the photodetectors 15a and 15b, respectively, to obtain a correction signal. You can also do this.

Further, when the audio signal supplied to the input terminal 22 is a digital signal, a circuit capable of digital processing is used as the filter 23 and the comparison circuit 21, and an analog/
A digital/analog conversion circuit may be provided on the output side of the digital conversion circuit and comparison circuit 21.

Effects of the Invention As described above, according to the present invention, supporting members such as dampers and edges that were conventionally used as a linear motor structure for the part supporting the vibration system of the speaker are eliminated, and the displacement of the vibration system of the speaker is Since the displacement information is detected and fed back to the speaker drive system, non-linear distortion during large amplitude operation is reduced, complex resonance and phase rotation in the high frequency range are eliminated, and It is possible to flatten the sound pressure frequency characteristic, and it is particularly useful for use in bass speakers having a bearing mechanism that supports a diaphragm such as a linear motor structure so as to be linearly displaceable with respect to a frame.

[Brief explanation of the drawing]

Figure 1 is a structural diagram showing an example of a conventional speaker.
FIG. 2 is a structural diagram showing an example of a support means for a vibration system of a speaker according to the present invention, FIG. 3 is a layout diagram showing an example of a displacement detecting means for a diaphragm used in the present invention, and FIG. 3 is a circuit diagram showing an equivalent circuit of the photodetector of the displacement detecting means, FIG. 5 is a circuit diagram showing an embodiment of the present invention, and FIGS.
FIG. 11 is a characteristic diagram for explaining the operation of the diagram, and a layout diagram showing another example of the diaphragm displacement detecting means used in the present invention. 1 is a diaphragm, 4 is a voice coil, 10 is a bearing,
11 is a shaft, 12 is a mirror, 13 is a light source, 15 is a photodetector, 18 is an addition circuit, 19 is a subtraction circuit, 20
21 is a comparison circuit, and 26 is a drive coil.

Claims (1)

[Scope of Claims] 1. A guide shaft is provided on one of the diaphragm and the frame, and a bearing mechanism for slidably holding the guide shaft is provided on the other, and the diaphragm is supported movably with respect to the frame. A speaker comprising: an optical position detection means for detecting displacement of the diaphragm upon receiving light from a light source reflected by a reflection means provided on the diaphragm; and an optical position detection means for detecting displacement of the diaphragm. a light amount control means for controlling the amount of light incident on the diaphragm to be constant so that the displacement position of the diaphragm is detected; A motional feedback type speaker comprising driving means for linearly displacing the optical position detecting means, the output of the optical position detecting means being fed back to the driving system of the speaker.