An electroacoustic transducer converts an electric signal into sound by vibrating a diaphragm magnetically in response to the electric signal input thereto. A movement restricting means for restricting the movement of the diaphragm within an allowable movement range is provided on the inner wall of a resonant chamber disposed at one side of the diaphragm. The restricting means normally confronts at a magnetic member attached to the center of the diaphragm. The restriction means contacts the magnetic member when the diaphragm attempts a deflection beyond the allowable range.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an electroacoustic transducer for 
converting an electric signal input thereto into sound. 
2. Description of the Prior Art 
An electroacoustic transducer is a means for converting an electric signal 
input thereto into sound. The electroacoustic transducer will produce an 
acoustic output in response to an input electric signal. Accordingly, the 
electroacoustic transducer can be employed by electronic devices, etc. as 
a sounding means such as a buzzer. 
A prior art electromagnetic type electroacoustic transducer is shown in 
FIGS. 17-21 and includes a cylindrical outer casing 102 which is formed of 
synthetic resins and houses a magnetic driving portion 104 at the rear 
side thereof. Input terminals 106 and 108 are formed in the magnetic 
driving portion 104 for inputting an electric signal to the magnetic 
driving portion 104. The magnetic driving portion 104 has a columnar core 
110 at the center thereof and a coil 114 is wound around the core 110 by 
way of a bobbin 112. The input terminals 106 and 108 are connected to the 
ends of the coil 114 which is energized in response to the electric signal 
input thereto by way of the input terminals 106 and 108. A cylindrical 
magnet 116 is provided on the inner wall of the cylindrical outer casing 
102 and disposed about the coil 114. 
A diaphragm 118, which is driven by the magnetic driving portion 104, is 
provided on the peripheral edge of the cylindrical magnet 116 and it is 
formed of an elastic thin magnetic member. Accordingly, the diaphragm 118 
is attracted by the cylindrical magnet 116 and forms a closed magnetic 
circuit together with the core 110 and the cylindrical magnet 116. A 
magnetic piece 120 is attached to the center of the diaphragm 118 to 
establish a close magnetic relation with the core 110 and to add mass to 
the diaphragm 118. 
At the front side of the diaphragm 118, there are provided a resonant 
chamber 122 which is closed by the cylindrical outer casing 102 and serves 
as a resonant space and a sound emitting cylinder 124 which permits the 
resonant chamber 122 to be open to the atmosphere. A plurality of ribs 126 
for restricting the movement of the diaphragm 118 within an allowable 
moving range are provided on the wall surface of the resonant chamber 122 
at the edge of the diaphragm 118. 
FIG. 18 is an enlarged cross-sectional view of the diaphragm 118. The 
diaphragm 118 is formed of a very thin plate member and the disk-like 
magnetic piece 120 is attached to the center thereof as a rigid member to 
add mass to the diaphragm 118. The center of the magnetic piece 120 is 
attached to the center of the diaphragm 118 by spot welding. Denoted at 
127 shows the welded portion. 
It is necessary to sufficiently reduce the size of the welded portion 127 
without deterioration of the characteristics of the diaphragm 118 so as to 
stabilize and make uniform the electroacoustic conversion characteristics. 
Furthermore, it is necessary that the deformation or deterioration of the 
characteristics of the diaphragm 118 is lessened after the diaphragm 118 
and magnetic piece 120 are spot-welded and that they are brought into 
close contact with each other so as to have stable elasticity as a 
vibrating member. It is still necessary that the diaphragm 118 is very 
thin to assure a necessary sound pressure and sounding bandwidth. 
If the sound pressure or the sounding bandwidth is increased, bonding 
strength between the diaphragm 118 and the magnetic piece 120 is 
decreased, which results in deterioration of reliability and stability of 
the electroacoustic transducer. 
Meanwhile, such an electroacoustic transducer is provided in a variety of 
portable electronic devices and is subject to an external force such as a 
strong vibration, shock, etc. FIG. 19 shows the stationary state of the 
diaphragm 118 and FIG. 20 shows the vibrating state of the diaphragm 18 
when it is normally driven. In FIG. 20, (a) shows the movement of the 
diaphragm 118 toward the core 110 and (b) shows the movement of the 
diaphragm 118 toward the sound emitting cylinder 124. That is, the 
diaphragm 118 repeats a vibration to thereby emit a sound depending on the 
frequency of the input electric signal. Normally, rated input and limited 
input level corresponding to the allowable moving range are set so that 
the diaphragm 118 is prevented from moving beyond the allowable moving 
range. 
When an external force such as a shock, etc. is applied to the 
electroacoustic transducer, the diaphragm 118 is liable to be deformed 
beyond the allowable moving range as illustrated in FIG. 21. FIG. 21(a) 
shows the movement of the diaphragm 118 toward the core 110 wherein the 
diaphragm 118 contacts the head of the core 110. That is, the diaphragm 
118 is prevented from moving excessively due to the core 110 so that the 
diaphragm 118 is protected by the core 110. 
In the case as illustrated in FIG. 21(b) where an external force is applied 
to the electroacoustic transducer so as to push the diaphragm 118 upward 
toward the sound emitting cylinder 124, a stress is applied between the 
diaphragm 118 and the magnetic piece 120 in such a manner to tear the 
magnetic piece 120 from the diaphragm 118. As a result, there is a 
possibility that the diaphragm 118 is broken or deformed at the welded 
portion 127 or its peripheral portion. In case that the external force is 
strong, there is a possibility that the magnetic piece 120 falls out from 
the diaphragm 118. 
There are measures for protecting the diaphragm 118 from the external shock 
as disclosed in Japanese Utility Model Publication No. 57-28478 entitled 
Electromagnetic Type Electroacoustic Transducer for Wristwatch, in 
Japanese Utility Model Laid-Open Publication No. 59-159098 entitled 
Electromagnetic Type Electroacoustic Transducer and in Japanese Utility 
Model Laid-Open Publication No. 60-26099 entitled Electromagnetic Type 
Sounder, etc. However, there are the following problems. In Japanese 
Utility Model Publication No. 57-28478, it is difficult to assure a 
resonant effect since a space in front of the diaphragm is sacrificed so 
as to prevent an excessive vibration. In Japanese Utility Model Laid-Open 
Publication No. 59-159098 and also in Japanese Utility Model Laid-Open 
Publication No. 60-26099, there remains a possibility that the magnetic 
piece falls out by the shock because the magnetic piece is not restricted 
in vibration. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide an electroacoustic 
transducer that prevents a diaphragm from excessively moving at the center 
thereof beyond an allowable moving range and that protects the diaphragm 
from an external force such as a shock without sacrificing a resonant 
space. 
That is, the electroacoustic transducer of the invention converts an 
electric signal to sound by vibrating the diaphragm (18) magnetically in 
response to the input electric signal. As illustrated in FIGS. 1 to 16, 
movement restricting means include sound emitting cylinder 24, projections 
34, 36 and 38, are disposed at one side of the diaphragm (18) for 
restricting the movement of the diaphragm (18) within the allowable moving 
range of a magnetic piece (20) attached to the center of the diaphragm 
(18). 
Although a variety of shapes and positions of the movement restricting 
means are conceived, a single or a plurality of projections (34, 36 and 
38) for defining the allowable moving range are provided on the inner wall 
of the resonant chamber (22). 
A sound emitting cylinder (24) for permitting the resonant chamber (22) to 
be open to the atmosphere can also serve as the movement restricting 
means. 
According to the electroacoustic transducer of the invention, it is 
possible to prevent the generation of peeling force between the diaphragm 
and the magnetic piece and also prevent deformation, etc., of the 
diaphragm with assurance even if the shock is applied to the diaphragm 
(18) since the diaphragm (18) is restricted within the allowable moving 
range at the magnetic piece (20), i.e., at its central portion. If the 
movement restricting means is composed of a single projection or a 
plurality of projections, the resonant space can be less occupied by the 
movement restricting means, which enhances acoustic characteristics. If 
the sound emitting cylinder (24) serves also as the movement restricting 
means, it is not necessary to provide additional projections etc., to 
thereby simplify its structure. 
The electroacoustic transducer of the invention has the following features: 
(a) It is possible to protect the diaphragm against the damage or 
deformation caused by the shock etc. and to enhance the reliability of the 
electroacoustic transducer since the excessive movement of the diaphragm 
due to shock, etc., can be mechanically restricted at its central portion. 
(b) It is possible to surely prevent a welded portion from being broken and 
prevent the diaphragm and the magnetic piece from being peeled from each 
other by an external force. 
(c) It is possible to prevent deterioration of the characteristics of the 
diaphragm because the excessive movement of the diaphragm is restricted at 
the magnetic piece without directly contacting the diaphragm. 
(d) The electroacoustic transducer of the invention realizes miniaturized 
construction, comparatively low-frequency sound output, high quality and 
high reliability. Furthermore, the diaphragm can be thinner and the 
magnetic piece can be heavier due to the restriction of movement of the 
diaphragm while the reliability of the welded portion is enhanced. 
Other objects and features of the invention will be more apparent from 
embodiments as set forth hereinafter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An electroacoustic transducer according to first to eighth embodiments will 
be described with reference to FIGS. 1 to 16. 
First Embodiment (FIGS. 1 and 2) 
FIGS. 1 and 2 show the electroacoustic transducer according to the first 
embodiment of the invention. 
A cylindrical outer casing 2 formed of synthetic resins houses a diaphragm 
18, a magnetic driving portion 4 disposed at the rear side of the 
diaphragm 18 for vibrating the diaphragm 18 in response to an input 
electric signal and a resonant chamber 22 disposed at the front side 
(upper side in these figures) of the diaphragm 18 for serving as a 
resonant space. A sound emitting cylinder 24 is disposed in the resonant 
chamber 22 for permitting the resonant chamber to be open to the 
atmosphere. 
Input terminals for applying an electric signal, not shown, are formed in 
the magnetic driving portion 4 like the input terminals 106 and 108 as 
illustrated in FIG. 17. A columnar core 10 is disposed at the center of 
the magnetic driving portion 4 and a coil 14 is wound around the core 10 
by way of a bobbin 112, not shown, like the prior art electroacoustic 
transducer as illustrated in FIG. 17. The coil 14 is energized in response 
to the input electric signal through the input terminals like the prior 
art electroacoustic transducer as illustrated in FIG. 17. A cylindrical 
magnet 16 is disposed about the coil 14 and constitutes a closed magnetic 
circuit with the core 10, the diaphragm 18 and a magnetic piece 20. The 
magnetic piece 20 is fixed to the diaphragm 18 at a welded portion 27. The 
magnetic piece 20 has, like the prior art electroacoustic transducer, a 
close magnetic relation with the core 10 and adds mass to the diaphragm 
18. 
In this first embodiment, there is formed a movement restricting means for 
restricting the movement of the diaphragm 18 within an allowable moving 
range at the side of the magnetic piece 20 disposed at one side (upper 
side in FIG. 1) of the diaphragm 18, i.e. at the center of the diaphragm 
18. That is, the sound emitting cylinder 24 is designed so long that an 
interval D between the magnetic piece 20 and itself may be equal to or 
slightly greater than an ordinary allowable moving range. 
With such an arrangement, even if the diaphragm 18 moves away from the 
magnet 16 and moves upward when a strong shock is applied to the 
electroacoustic transducer, the magnetic piece 20 strikes against the end 
surface of the sound emitting cylinder 24 so that the diaphragm 18 is 
prevented from moving excessively as illustrated in FIG. 2. Furthermore, 
since the movement restriction is performed at the magnetic piece 20, 
namely, at the center of the diaphragm 18, the influence of the peeling 
force which was conventionally generated between the diaphragm 18 and the 
magnetic piece 20 can be removed completely, which enhances the 
reliability of the electroacoustic transducer. 
Second Embodiment (FIGS. 3 and 4): 
FIGS. 3 and 4 show the electroacoustic transducer according to the second 
embodiment of the invention. In this embodiment, the sound emitting 
cylinder 24 of the first embodiment has an incline surface 30 at its end. 
With such an arrangement, the average interval between the sound emitting 
cylinder 24 and the diaphragm 18 can be increased while the interval D for 
the free movement of the diaphragm 18 is maintained like the first 
embodiment. As a result, the front surface of the diaphragm 18 can be 
sufficiently open on the side thereof facing the resonant chamber 22 while 
the moving range of the diaphragm 18 is restricted, which enhances the 
acoustic characteristics. 
Third Embodiment (FIGS. 5 and 6) 
FIGS. 5 and 6 show the electroacoustic transducer according to the third 
embodiment of the invention. In this embodiment, a plurality of U-shaped 
notches 32 are formed at an end surface of the sound emitting cylinder 24. 
With such an arrangement, the average interval between the sound emitting 
cylinder 24 and the diaphragm 18 can be increased while the interval D for 
the free movement of the diaphragm 18 is maintained like the first 
embodiment. As a result, the front surface of the diaphragm 18 can be 
sufficiently open on the side thereof facing the resonant chamber 22 while 
the moving range of the diaphragm 18 is restricted, which advantages the 
acoustic characteristics. 
Fourth Embodiment (FIGS. 7 and 8) 
FIGS. 7 and 8 show the electroacoustic transducer according to the fourth 
embodiment of the invention. In this embodiment, the length of the sound 
emitting cylinder 24 of the first embodiment is the same as the prior art 
and a plurality of thin columnar projections 34 serving as a movement 
restricting means of the diaphragm 18 are formed on an end surface of the 
sound emitting cylinder 24. The projections 34 are disposed at equal 
angular intervals of 120.degree. to restrict the movement of the diaphragm 
18 on the average at the center of the diaphragm 18, i.e. at the magnetic 
piece 20 as illustrated in FIG. 8. 
With such an arrangement, the movement of the diaphragm 18 due to the 
application of an external force such as a shock can be restricted and the 
front surface of the diaphragm 18 can be sufficiently open on the side 
thereof facing the resonant chamber 22 and also the movement restricting 
means does not impede the acoustic characteristics. 
Fifth Embodiment (FIGS. 9 and 10) 
FIGS. 9 and 10 show the electroacoustic transducer according to the fifth 
embodiment of the invention. In this embodiment, a plurality of 
semicolumnar projections 34 are formed on the sound emitting cylinder 24 
by extending portions of the outer peripheral surface of the sound 
emitting cylinder 24 as illustrated in FIG. 9, or a plurality of 
projections 34 each having the shape of a square pillar are formed on the 
sound emitting cylinder 24 by extending portions of the wall of the sound 
emitting cylinder 24 as illustrated in FIG. 10. 
Sixth Embodiment (FIGS. 11 and 12) 
FIGS. 11 and 12 show the electroacoustic transducer according to the sixth 
embodiment of the invention. In this embodiment, a plurality of plate-like 
projections 36 which serve as the movement restricting means of the 
diaphragm 18 are radially extended in the resonant chamber 22 from the 
sound emitting cylinder 24. That is, each projection 36 constituting a 
wall plate for dividing the resonant chamber 22 are disposed at equal 
angular intervals of 120.degree. about the sound emitting cylinder 24 to 
restrict the movement of the diaphragm 18 on the average at the center of 
the diaphragm 18, i.e. at the magnetic piece 20. The upper surfaces of the 
projections 36 are higher than the end surface of the sound emitting 
cylinder 24. 
With such projections 36, the same functions and effects as the first to 
fifth embodiments can be expected too. 
Seventh Embodiment (FIGS. 13 and 14) 
FIGS. 13 and 14 show the electroacoustic transducer according to the 
seventh embodiment of the invention. In this embodiment, the sound 
emitting cylinder 24 in the outer casing 2 is displaced in its position 
and a plurality of projections 38 are formed on the ceiling surface of the 
resonant chamber 22 at the center thereof. 
Since the projections 38 serving as a protecting means of the diaphragm 18 
are disposed at the center of the resonant chamber 22 and the sound 
emitting cylinder 24 is displaced therefrom, a resonant sound in the 
resonant chamber 22 due to the vibration of the diaphragm 18 can be 
effectively emitted to the atmosphere. 
Eighth Embodiment (FIGS. 15 and 16) 
FIGS. 15 and 16 show the electroacoustic transducer according to the eighth 
embodiment of the invention. In this embodiment, the sound emitting 
cylinder 24 is formed on the side wall of the outer casing 2 and a 
plurality of plate-like projections 38 are formed on the ceiling surface 
of the resonant chamber 22 at the center thereof. Each projection 38 may 
be a columnar body. 
With such an arrangement, the diaphragm 18 can be protected at the magnetic 
piece 20 against the damage or injure caused by its excessive movement and 
a resonant sound in the resonant chamber 22 can be emitted from the side 
surface of the outer casing 2. 
Although the features of the invention have been described with reference 
to the first to eighth embodiments, the electroacoustic transducer of the 
invention is not limited to those embodiments, it is to be understood that 
the invention includes many variations and changes having the same effects 
as the first to eighth embodiments.