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DESCRIPTION JPH11234794

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DESCRIPTION JPH11234794
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
method for attenuating mechanical resonance of a loudspeaker attached to a loudspeaker
structure and having a tuned mechanical resonance, and an apparatus for attenuating the
mechanical resonance of the loudspeaker.
[0002]
BACKGROUND OF THE INVENTION For about 50 years, mechanical resonances of speakers have
been known to reduce the quality of the emitted sound, and various methods have been
developed to overcome this problem. In some of these methods, the speaker elements are
resiliently attached to the speaker enclosure (speaker box, speaker cabinet) to attenuate the
vibrations transmitted to the enclosure. However, such a structure proposed in many different
embodiments is difficult to construct, expensive to produce, and requires non-standard design
and manufacturing techniques for speaker attachment.
[0003]
U.S. Patents by Thomasen (Refs. 1 and 5) disclose a suitable vibration damper for attachment to
the wall of the enclosure to dampen vibrations in the wall of the enclosure. This method does not
reduce the vibrations excited at the source, but for the secondary effects of wall motion and does
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not provide an effective way to control the vibrations. Furthermore, it has been shown that
controlling vibration over a wide bandwidth and obtaining both broadband operation and high
efficiency, within practical design and material constraints, is a general technical goal.
[0004]
The European specification of Akroyd (Ref. 2) proposes a structure having a tube made of an
elastic material which mechanically couples the elements of the dynamic loudspeaker to the
enclosure wall. This device is for supporting the frame of the loudspeaker element to reduce
vibrations. This tube simultaneously functions as an acoustic resonant structure. This structure
provides mechanical support but does not function as an effective attenuator of enclosure
resonances. Arcroyd's invention contemplates that this mechanical connection achieves vibration
cancellation, whereby damping is achieved. Since the stiffness of the material is increased, for
example by additional support, the natural frequency of the characteristic resonance is increased,
but the resonance does not move unless the friction losses of the structure causing the damping
of the mechanical resonance are simultaneously increased. In fact, if the drive of the dynamic
speaker element is mechanically coupled to the rear wall of the enclosure, the amount of
mechanical vibration at the outer wall of the speaker enclosure may not decrease but may even
increase. That is because this structure actually emphasizes the mechanical coupling of
vibrations at many frequencies instead of reducing coupling. However, such additional
mechanical support has the effect of moving the natural frequency of the mechanical resonance
to a higher direction.
[0005]
In the Tanaka patent (reference 3), the drive of the dynamic speaker element is fixed not to the
front wall but to the outside of the speaker enclosure. Furthermore, elastic means are provided
for securing the frame of the loudspeaker element to the loudspeaker enclosure. It is also
described that this invention is equally applicable to reference 2 where the fixed point is not the
front of the enclosure but the loudspeaker drive is mechanically fixed to the enclosure using
means with low mechanical losses Structure is shown. Since the vibrations coupled to the
enclosure wall usually occur on all walls of the enclosure, this construction does not give good
end results. It does not matter which wall the speaker drive is attached to, the mechanical
vibration appears on all walls of the enclosure, the transfer of mechanical energy to the
enclosure is due to the mechanical elasticity of the attachment means and the mass of the
loudspeaker element The efficiency is particularly high at the resonant frequency. Therefore, this
Tanaka invention can significantly reduce the vibration coupled to the front wall of the enclosure,
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but generally can not reduce the coupling of the vibration to the enclosure.
[0006]
In the French patent specification (Ref. 4) of Favali (Ref. 4), a loudspeaker coupled to an
enclosure wall and having a structure for damping mechanical vibrations using an elastic plate
with loudspeaker elements attached to the enclosure The enclosure is described. The present
invention contemplates creating shear forces in an elastic body that functions to convert
mechanical energy into heat by internal friction in the material. The present invention does not
reduce the tendency of the speaker element to cause mechanical vibration. The structure is not
effective at resonant frequencies where no maximum displacement occurs at the elastic joints.
That is because there is no acoustic energy loss at these frequencies in the elastic.
[0007]
The methods described in these references 1 to 5 are not intended to control mechanical
vibration at the source, ie the speaker element, but secondary vibration in the speaker enclosure
Investigate the impact on The present invention differs from the prior art as described above, and
the object of the present invention is to damp mechanical vibrations of the loudspeaker element
drive, thereby eliminating the need for damping of vibrations in the enclosure structure. . Thus,
the present invention is an excellent one that is fundamentally different from the prior art.
[0008]
SUMMARY OF THE INVENTION The object of the present invention is to elastically attach at least
one additional mass to a loudspeaker magnet circuit according to the invention, the natural
frequency of the system typically matching the mechanical resonance of the loudspeaker This is
achieved by choosing this additional mass. Such a configuration allows mechanical vibrational
energy generated in the magnet circuit to be transferred to this additional mass as additional
mass oscillations, so that the elastic coupling element is not affected by the friction losses of the
material. It will be possible to absorb Typically the overall additional mass is chosen to be
comparable to the mass of the magnet circuit. The mass may also be selected to differ, for
example, by an order of magnitude from the mass of the magnet circuit.
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[0009]
More particularly, the present invention provides a method of damping mechanical resonances of
a loudspeaker attached to the loudspeaker structure and having a tuned mechanical resonance
structure, in the loudspeaker magnet structure or in the part of the frame close to the magnet
structure At least one additional mass is elastically mounted, this additional mass being at least
one resonance point or effective resonance range of the drive, the frame, the drive or the
loudspeaker enclosure mechanically coupled to the frame It is characterized by having an
internal resonance frequency. How to attenuate the mechanical resonance of the speaker.
[0010]
Furthermore, the present invention provides an acoustic radiation cone, a voice coil attached to
the acoustic radiation cone, a frame of the speaker element, a magnet circuit coupled to the
frame of the speaker element, and a structure of the speaker element In a device for attenuating
mechanical resonances of a loudspeaker system comprising one or more additional masses and a
loudspeaker enclosure associated with the loudspeaker element, wherein the magnet structure of
the loudspeaker or the part of the frame close to the magnet structure is elastic. Comprising at
least one additional mass mounted in a manner such that the additional mass is at least one
resonance point or effective of the drive, the frame, the drive or the frame mechanically coupled
to the frame It has a resonance frequency within the following resonance range.
[0011]
According to the present invention, the following various effects can be obtained.
The control of the damping of the resonance according to the method of the invention is very
simple, easy and inexpensive to implement compared to using the prior art. That is because it is
not necessary to change the structure of a good and reliable current loudspeaker to eliminate
unwanted resonances. This is not possible if the loudspeaker element is attached to the enclosure
by elastic means, if the magnet circuit is attached to the frame of the loudspeaker element by
elastic means, or if an elastic structure is used in the loudspeaker enclosure . Furthermore, it is
possible to adjust the peak Q value of the resonance, the effective frequency range of control,
and the amount of vibration reduction by appropriate choice of the additional mass and the
elasticity and loss of the mounting part. Exemplary embodiments will be described in detail with
reference to the following accompanying drawings.
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[0012]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The dynamic loudspeaker element
shown in FIG. 1 comprises a drive 6 which displaces an acoustic radiating element which is
typically a diaphragm cone 5 when energized by an electromagnetic force. The normal drive 6
comprises a magnet circuit 7 and a voice coil (not shown) which moves in the gap in the magnet
circuit 7. Usually, the voice coil is adhesively bonded to a diaphragm cone 5 which drives
(displaces) air. Thus, the loudspeaker element comprises the mass of the air displacement
mechanism 8 (i.e. the cone and the voice coil), the mass of the stationary part (i.e. the magnet
arrangement circuit 7) and the mass of the frame structure 4 of the loudspeaker element .
[0013]
The magnet circuit 7 and the drive for displacing the cone including the voice coil moving in the
gap in the magnet circuit 7 are mounted around the frame 4 of the loudspeaker element to an
external structure, typically a loudspeaker enclosure. The frame 4 is typically made of steel plate,
plastic or metal die cast and has a certain degree of elasticity in the axial direction of
displacement of the voice coil. Also, the front wall of the speaker enclosure has a certain degree
of elasticity, which can generally be considered in addition to the elasticity of the frame 4 of the
loudspeaker element.
[0014]
When the speaker operates, the electromagnetic force acts on the magnet circuit 7 in the
opposite direction to the force acting on the voice coil, thus one or more of the elasticity of the
frame 4 of the speaker element and the elasticity at the mechanical attachment to the front wall
of the enclosure. The resonance is generated and the mass is mechanically coupled to any of
these. Vibrational energy has to be delivered from the magnet circuit to the enclosure wall,
causing them to vibrate. This is undesirable and this transmission of mechanical vibrational
energy produces acoustic radiation from the enclosure wall, which is added to the acoustic
radiation emitted from the speaker element. The sound output is thus no longer determined by
the loudspeaker elements alone as originally thought, degrading the quality of the sound output.
[0015]
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For a typical loudspeaker, first, the mass of the magnet circuit displaces the voice coil and finds
the angular frequency .omega.0 at which the rigidly coupled part of the frame resonates due to
the elasticity of the frame 4. FIG. This mechanical resonance can be modeled as a lossy massspring system.
[0016]
The present invention discloses a method of attaching an additional mass to the magnet circuit 7.
The additional mass 1 resonates with the magnet circuit 7 at a frequency selected to coincide, for
example, with the resonant frequency .omega.0 of the magnet circuit / frame system.
Furthermore, these frequencies can be selected to any other frequency that requires the transfer
of vibrational energy to the enclosure wall to be reduced. By appropriate choice of the amount of
additional mass and the elasticity at their attachment parts, it is possible to control multiple
resonances at multiple frequencies or overlapping frequency bands. In this way it is possible to
adjust and control the efficiency and the effective frequency range of the reduction of mechanical
vibrations.
[0017]
The theoretical background of the present invention is described below. Referring to FIG. 5a, one
elastically mounted mass (mass m2) is shown, which mass m2 together with the stiffness k1 of
the magnet circuit m1 and the frame 4 of the loudspeaker element and its loss c1 It forms a
system. The displacement amplitude has a maximum at the resonant frequency of this system.
FIG. 5a shows a state in which a mass m2 having an elasticity k2 and a loss factor c2 is elastically
attached to this system.
[0018]
The resonant frequency of this system consisting of two coupled masses thus formed is properly
adjusted by changing the elasticity k2 and the loss factor c2, and the displacement of the mass
m2 of the magnet circuit at the mechanical resonant frequency The amplitude x1 can be
minimized. The second law of motion of Newton is as the following equation 1. FF = ma Equation
1 This equation shows that the system is kept at rest if the sum of all the acting forces is zero.
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[0019]
The equations of motion for conventional mass systems are given by equations 2 and 3 below.
Using the electromechanical analogy, the mechanical force F (t) is represented as a voltage V (t)
and the kinetic velocity dx / dt is represented as a current i (t), the equivalent circuit shown in
FIG. 5b is obtained .
[0020]
The properties of a two-mass mechanical system can be analyzed using the differential equations
of Equations 2 and 3 or using electrical analogy. The properties of this system are examined
using electro-mechanical analogy as follows. In the absence of the additional mass m 2, the massspring system m 1 formed by the loudspeaker magnet circuit vibrates at a velocity v given by the
following equation 4 according to the angular velocity [6]. When the additional mass m2 is not
used, the maximum velocity occurs at the resonance frequency, at which frequency the
imaginary part of the denominator becomes zero, whereby the mechanical energy transfer is the
most efficient. The angular frequency of this resonance is given by
[0021]
Now consider the change in situation due to the use of the additional mass m2. The analysis of
the two-mass system of FIG. 5 using an electromechanical analogy makes it possible to reduce
the displacement amplitude x1 of the magnet circuit by adjusting the resonance frequency of the
additional mass. This resonant frequency is determined by the mass m2 and the elasticity k2 of
its attachment and is adjusted to be the same as the resonant frequency of the magnet circuit.
[0022]
The ability of the additional mass to reduce the rate of movement depends on the loss of the
elastic attachment part (component R2 in the electromechanical analogy). After setting the
resonant frequency to be correct by using an appropriate material, adjust the losses to the
correct level and correct the mechanical dimensions to the elastic attachment to get mechanical
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vibration to any level It is possible to reduce and obtain the desired level of vibration damping.
[0023]
The ability of the additional mass generated resonator to absorb the kinetic energy of the drive is
characterized by the Q value of the resonant system. This Q value is given by the following
equation 6. Q = m 2 ω 0 / c 2 Equation 6 This equation 6 shows that at the resonant frequency
ω 0 the Q-factor of the resonance and hence the ability to damp mechanical vibrations depend
on the additional mass and elasticity of the attachment to the magnet system ing. If the loss
factor of the elastic mounting portion is kept constant, the desired Q value is obtained by
choosing the correct amount of additional mass and the correct elasticity of the mounting spring.
If the additional mass is kept constant, the amount of loss in the mounting section must be
reduced as the frequency decreases.
[0024]
An example of the determination of the parameters for an actual embodiment of the invention
will now be described. The value of the additional mass according to the invention is selected, for
example, by measuring the resonant frequency of the mass-spring system formed by the magnet
circuit and the loudspeaker element frame mounted in the loudspeaker enclosure with the aid of
an acceleration transducer. Can. After the resonant frequency is known, an additional mass
having a weight approximately equal to the mass of the magnet circuit is attached to the magnet
circuit 7 and the measurement is repeated. By using the physical principle as described above,
the correction value (represented by the correct loss factor and elasticity) and the mass for the
spring constant are selected.
[0025]
As an example of a system encountered in the actual technology, the speaker element has a
measured resonant frequency ω 0 of 3300 rad / s and a magnet circuit 7 of 1.80 kg. In this case,
the additional mass I was mounted using a spring made of nitrile rubber in a sheet of 4 mm
thickness and 4.5 cm 2 area. The elasticity of the material is 4.3 MN / m. In this case the
additional mass was chosen to be 0.4 kg. Vibrations could be effectively reduced by these
choices. This example shows how the nature of the mounting spring influences the amount of
additional mass required, and the optimum result is not exactly the same as the mass producing
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the mass resonant system in the loudspeaker, but the mass Indicates that the order is the same.
Furthermore, in some cases, it is advantageous to divide the additional mass and its mounting
spring into several parts. Mass and elasticity can be varied according to the principles described
above, and the effects of mechanical resonance can be reduced to the desired low level.
[0026]
It should be understood that many variations and modifications to the above embodiments are
possible. It should be understood that the foregoing description is only illustrative of specific
embodiments of the present invention, and that various other configurations are readily possible
without departing from the scope of the present invention. .
[0027]
Reference 1. Thomasen US 5 583 324 (1996) 2. Akroyd EP 0 459 682 (1990) 3. タナカ US 4
797 935 (1989)4. Favali FR 2 417 229 (1978) 5. Thomasen US 5 240 221 (1996) 6. Alonso
M., Finn E .: Physics Addison-Wesley, 19927. William W. Seto: Theory and Problems of Acoustics,
McGraw-Hill, Inc, 1971
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