Dynamic transducer device

A dynamic speaker device having a small-sized vibrating plate for reproducing a high frequency sound is further provided with an additional coil in the vicinity of the magnet assembly of the speaker. The additional coil is mounted on a comparatively heavy vibrating element which is supported by a spring plate. The additional coil and the vibrating element vibrates to reproduce lower frequency sound and vibration in response to audio signal supplied to the additional coil.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to dynamic transducer devices and, in 
particular, to a dynamic speaker device which is a small type but can 
reproduce a vibration of a frequency lower than about 250 Hz, more 
particularly about 100 Hz or less, as well as a higher frequency band. 
(2) Description of the Prior Art 
A known dynamic speaker comprises a magnetic assembly having a permanent 
magnet and a magnetic yoke with a magnetic gap. A voice coil is disposed 
in the magnetic gap. A vibrating plate of, usually, a cone shape is 
mechanically connected to the voice coil. The vibrating plate is 
elastically supported by spring means. When an audio signal is fed to the 
voice coil, the coil axially reciprocates according to the amplitude and 
frequency of the audio signal. Accordingly, the vibrating plate vibrates 
and reproduces the sound. 
Generally speaking, a speaker having a vibrating plate of a small diameter 
cannot reproduce a low frequency sound or vibration because the vibrating 
amplitude is limited at the lower frequency. 
In a known audio system, plural speakers of small and large diameter are 
often used together to cover the frequency range. 
In another system which reproduces from an electric audio signal not only 
sound felt by ear but also vibration of, preferably, undertones lower than 
about 150 Hz to be directly transmitted to a body, an electromechanical 
vibrator is used for reproducing the mechnical vibration in addition to 
sound speakers, as disclosed in U.S. Pat. No. 4,064,376. 
The use of two speakers of different sizes or the vibrator in addition to 
the speaker results in an increased size of an apparatus, device, or 
instrument to which they are assembled. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a dynamic transducer 
device which can reproduce vibration of a frequency band lower than about 
250 Hz, especially 150 Hz or less, as well as sound of the higher 
frequency. 
It is another object of the present invention to provide such a dynamic 
transducer device which is small in size and simple in construction and 
assembly. 
As described above, a dynamic transducer device comprises a magnetic 
assembly having a permanent magnet and a magnetic yoke with a magnetic 
gap, and a voice coil disposed in the magnetic gap. According to the 
present invention, the dynamic transducer device further comprises an 
additional coil disposed in the vicinity of the magnetic assembly, a 
vibrating body mechanically connected to, and supporting, the additional 
coil, and support means elastically supporting the vibrating body. 
According to one aspect of the present invention, the vibrating body is an 
annular magnetic coil housing, in which the additional coil is mounted. 
According to another aspect of the present invention, a magnetic yoke is 
provided with an additional magnetic gap in which the additional coil is 
disposed. The vibrating body comprises an annular plate and a cylinder 
fixed thereon. The additional coil is mounted on the cylinder. 
According to still another aspect, the support means comprises a support 
rod and a spring plate fixedly mounted thereon. The annular coil housing 
or the annular plate is joined to the spring plate at equiangularly-spaced 
positions. 
In the present invention, since the additional coil is also disposed in the 
magnetic field generated by the permanent magnet, supply of the audio 
signal to the additional coil results in vibration of the additional coil. 
Accordingly, the vibrating body vibrates together with the additional 
coil. 
In this construction, the lower frequency vibration can be generated from 
the vibrating body by the fact that the total amount of weight of the 
vibrating body, the spring plate, and the additional coil is designed to 
be comparatively large, and/or that the mechanical resistance and/or 
stiffness of the spring plate is selected to be comparatively large. 
Further objects, features and other aspects will be understood from the 
following detailed description of preferred embodiments of the present 
invention with reference to the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to FIG. 1, a speaker device according to an embodiment of the 
present invention comprises a known speaker assembly 10 and an additional 
vibrating assembly 20. 
Speaker assembly 10 includes a magnetic assembly comprising an annular 
permanent magnet 101 and a magnetic yoke 102. The magnetic yoke comprises 
a circular plate portion 102a, a center pole portion 102b mounted at the 
center of circular plate portion 102a, and an annular plate portion 102c 
having a center hole. Permanent magnet 101 is mounted on circular plate 
portion 102a, and annular plate portion 102c is mounted on permanent 
magnet 101. Center pole 102b extends through the center aperture in 
annular permanent magnet 101. The extended end of pole 102b is in the 
center hole of annular plate portion 102c with a small annular magnetic 
gap 103 remaining between the radially outer surface of the extended end 
and the radially inner surface of the central hole. 
A voice coil 104 is disposed in magnetic gap 103, and is mounted on a 
cylindrical bobbin 105. Bobbin 105 is supported by a centering device or a 
spider 106 which is connected to a frame 107. Numeral 108 represents a 
baffle plate. 
A vibrating plate, which is usually a cone 109, is supported at its outer 
periphery by frame 107, and is connected at its central portion to bobbin 
105. 
Voice coil 104 is connected to electric lead wires 110 which are connected 
to an amplifier (not shown) for supplying an audio signal. 
The above-described arrangement is well known as a dynamic speaker. When an 
audio signal is supplied to voice coil 104 through lead wires 110, voice 
coil 104 reciprocates and drives vibrating plate 109 to reproduce sound as 
well known in the prior art. 
In this connection, there is a relationship between the diameter of the 
vibrating plate 109 and the reproduceable frequency. The smaller the 
diameter is, the higher the reproduceable frequency generally is. 
Therefore, a speaker of small size generally cannot reproduce low 
frequency sound and vibration. 
The present invention attempts to add an additional vibrating assembly to 
the known speaker assembly of small size so as to enable reproduction of 
low frequency sound and vibration. 
According to the present invention, the embodiment of FIG. 1 is provided 
with the additional vibrating assembly 20. The vibrating assembly 
comprises an additional coil 201 having comparatively many turns. The 
additional coil is mounted in an annular magnetic coil housing 202. The 
coil housing has a "U" shape in cross-section and is made of iron to have 
comparatively great weight. 
The additional coil 201 and coil housing 202 are disposed opposite to, and 
adjacent to, the back surface of circular plate portion 102a of speaker 
assembly 10. 
A spring plate 203 of, for example, phosphor bronze is joined to the bottom 
of annular housing 202 at equiangular-spaced positions. The spring plate 
comprises a center circular plate 203a, a concentric outer annular plate 
203b, and a plurality of radial beams 203c, as shown in FIG. 2. The radial 
beams bridge between the equiangularly-spaced positions of the outer 
margin of center circular plate 203a and the inner margin of outer annular 
plate 203b to connect the center plate 203a and outer annular plate 203b 
together. The outer annular plate 203b is joined to the bottom of the 
annular housing 202 at equiangularly-spaced positions by, for example, 
rivets 204. 
The spring plate 203 is fixedly mounted on one end of a support rod 205. 
That is, central circular plate 203a has a central hole in which an end 
screw portion 205a of support rod 205 is inserted, with a nut 206 being 
fastened to screw portion 205a. 
Support rod 205 extends through a hollow of annular coil housing 202, and 
the extended end is fixedly mounted to a center of the circular plate 
portion 102a of the magnetic yoke, so that the coil 201 is disposed 
adjacent the circular plate portion 102a. 
Additional coil 201 is connected to lead wires 110 by its leads 207 so as 
to be in parallel with the voice coil 104. 
According to the arrangement, additional coil 201 is within a magnetic 
field of magnetic fluxes .phi..sub.1 leaking from magnetic assembly 
101-102 of speaker assembly 10, as shown in FIG. 3. In the figure, 
.phi..sub.2 represents magnetic fluxes produced in magnetic gap 103. 
When the audio signal is applied to additional coil 201, the additional 
coil receives electromagnetic force to vibrate together with coil housing 
202 and spring plate 203. .phi..sub.3 is magnetic fluxes generated by 
electric current flowing through the additional coil. 
Since the number of winding turns of the additional coil 201, the combined 
weight of additional coil 201, coil housing 202 and spring plate 203, and 
the stiffness of spring plate 203 are great in comparison with that of 
voice coil 104, cone 109 and spider 106 in speaker assembly 10, the 
additional coil and the coil housing vibrate at the lower frequency. 
Therefore, the lower frequency sound and vibration are reproduced by the 
vibrating assembly. 
It is not necessary that the diameters of the coil 201 and coil housing 202 
be made larger than the cone 109 of the speaker assembly. 
Thus, the speaker device of small size of the present invention can 
reproduce low frequency sound and vibration as well as the higher 
frequency sound. 
It is not necessary that the vibrating assembly 20 be directly mounted on 
the speaker assembly 10, but the former should be fixedly disposed 
adjacent to the latter so that the additional coil 201 is 
electromagnetically coupled to the magnetic assembly 101-102 of the 
speaker assembly. 
Referring to FIG. 4, the vibrating assembly 20 is fixedly mounted within a 
speaker box 30 in which a speaker 10 is mounted. Thus, the vibrating 
assembly 20 is electromagnetically coupled to the speaker 10, and 
therefore, the lower frequency sound can be reproduced by the vibrating 
assembly. 
Referring to FIG. 5, a permanent magnet 208 can be additionally disposed 
adjacent to additional coil 201, so that the additional coil 201 is 
advantageously placed within a static magnetic field of an increased 
magnetic strength. As seen in the drawing, the spring plate 203, for 
supporting the coil housing 202 along with coil 201, is mounted on the 
support rod 205. The spring plate 203 is so mounted by joining it to the 
rod 205 about the radially outer surface of the rod. 
The additional permanent magnet 208 is also mounted on support rod 205. 
Referring to FIG. 6, the additional permanent magnet 208 can be fixedly 
mounted in speaker box 30 in which the speaker device with the vibrating 
assembly is mounted, as shown in the figure. 
Referring to FIG. 7, another embodiment shown therein is generally similar 
to the embodiment of FIG. 1 but with a different arrangement for 
electromagnetically coupling the additional coil with the magnetic 
assembly in the speaker assembly. 
Similar parts are represented by the same reference numerals as in FIG. 1 
and detailed description thereof is omitted for the purpose of 
simplification of the description. 
In this embodiment of FIG. 7, annular plate portion 102c of the magnetic 
yoke extends radially outwardly from its point of attachment to magnet 
101, and the extended portion turns rearwardly towards plate portion 102a, 
as shown at 102d, and then radially inwardly to provide portion 102e which 
extends towards the radially outer margin of circular plate portion 102a 
of magnetic yoke 102 so as to form an additional annular magnetic gap 111 
between the inner margin of the portion 102e and the outer margin of the 
circular plate portion 102a. Gap 111 is in addition to the previously 
described gap 103. 
An additional coil 201' is formed in a shape similar to voice coil 104 i.e. 
annular. The additional coil 201' is fixedly mounted on a cylindrical 
bobbin or member 209 and is disposed in the additional magnetic gap 111. 
Bobbin 209 is mounted coaxially on an annular vibrating plate element 210 
which is made of, for example, iron and has a comparatively great weight. 
Vibrating plate element 210 is joined to spring plate 203 by rivets 204 
similar to FIGS. 1 and 2. The spring plate 203 is similarly supported on 
support rod 205 which is fixedly mounted on circular plate portion 102a of 
the magnetic yoke. Thus, in the embodiment of FIG. 7, the additional 
vibrating assembly 20 includes the coil 201', the bobbin 209, the annular 
plate 210, rivets 204 and the spring plate 203. 
In the arrangement, magnetic fluxes flow through annular plate portion 
102c, rearwardly directed portion 102d, inwardly directed portion 102e, 
additional magnetic gap 111, and circular plate portion 102a. Therefore, 
the additional coil 201' is exposed to a magnetic field of an increased 
strength. Accordingly, the vibrating amplitude of the coil 201' and 
vibrating plate element 210 is larger than that of the coil 201 and the 
coil housing 202 in FIG. 1 under the condition that equal current signals 
are applied to the respective coils 201' and 201. 
FIG. 9 shows another spring plate 203' which is used in place of spring 
plate 203 as shown in FIG. 2. 
Referring to FIG. 9, the spring plate 203' comprises a central circular 
plate portion 203'a which is fixedly secured at the center to support rod 
205 by nut 206. A plurality of fingers 203'b (four fingers are shown) 
extend outwardly from equiangularly-spaced positions on the outer 
periphery of the central circular plate portion 203'a. Fingers 203'b are 
further curved to extend concentrically as shown at 203'c in the figure, 
around central circular plate portion 203'a. The concentrically-extended 
ends are joined to vibrating plate element 210 by rivets 204 at 
equiangularly-spaced positions. 
In this arrangement, the distance from the central circular plate portion 
203a to rivet portion 204 along each finger is greater than the length of 
each beam 203c in FIG. 2. Therefore, the vibrating plate element 210 can 
smoothly vibrate at an increased amplitude, and the vibrating frequency 
band is enlarged. 
An input signal of 1 W was applied to additional coil 201' in the device of 
FIG. 7 where the spring plate 203 of FIG. 2 is used. Frequency 
characteristic of the output vibration was measured as shown in FIG. 10a. 
It will be understood from FIG. 10a, that the output vibration is 
especially strong at a frequency of about 80 Hz. 
The spring plate 203' of FIG. 9 was used in place of spring plate 203 and a 
frequency characteristic was measured. The measured data is shown in FIG. 
10b. In this case, there are four peaks at about 45 Hz, 60 Hz, 170 Hz and 
200 Hz, which have generally equal levels. 
For an increased input power of 5 W, the characteristic of FIG. 11a was 
obtained with use of the spring plate of FIG. 2, while the characteristic 
of FIG. 11b was observed with use of the spring plate of FIG. 9. 
Comparing FIG. 11a and FIG. 11b, it will be understood that, although 
vibration at a frequency of 80 Hz is quite stronger than at other 
frequencies with use of the spring plate of FIG. 2, the use of the spring 
plate of FIG. 9 unifies the vibrating levels at various frequencies from 
about 30 Hz to about 250 Hz. 
Referring to FIG. 12, a modification of the spring plate of FIG. 9 is 
shown. In FIG. 9, each finger 203'b extends radially from central circular 
plate 203a along a diameter of the circular plate. 
In comparison with this, each finger 203'b in FIG. 12 extends from the 
central circular plate 203'a along a chord of the circular plate offset 
from the diameter thereof. In the arrangement, the vibrating energy of 
fingers 203'b is distributed over the entire central circular plate 203'a 
without concentrating at the center thereof. As a result, the output 
vibration is more uniform over a wide frequency band. 
It will be noted that the spring plates of FIGS. 9 and 12 can be used in 
the device of not only FIG. 7 but also FIG. 1. 
Each spring plate of FIGS. 2, 9 and 12 is made of phosphor bronze, but it 
can be formed of a synthetic resin material reinforced by carbon fibers. 
For example, a molded cloth plate can be used wherein a carbon-fiber cloth 
is molded with synthetic resin such as epoxy resin. Alternatively, carbon 
fibers are mixed into plastic resin materials and the spring plate can be 
formed by injection molding.