JP7521918B2 - Alarm bell - Google Patents

Description

This invention relates to an alarm bell that produces a bell sound by striking a gong with a striking rod driven by a motor.

An alarm bell is equipped with a gong that produces a bell sound when struck, a striking rod that strikes the gong, a motor that drives the striking rod, and a housing that houses the motor, and is used to ring and sound an alarm in the event of an emergency such as a fire (see, for example, Patent Document 1).

JP 2000-235394 A

The main uses of this type of alarm bell are as the local sounding devices of automatic fire alarm systems and the emergency bells of emergency alarm systems (hereafter simply referred to as "emergency bells"). Standards for emergency bells are set based on the Fire Service Act. One of these standards is sound pressure. Alarm bells used as emergency bells must meet the sound pressure standard.

If the size of the alarm bell gong could be reduced, manufacturing costs could be reduced. However, simply reducing the size of the gong reduces the sound pressure, making it difficult to obtain the sound pressure required for use as an emergency bell.

In view of the above, the objective of this invention is to provide an alarm bell that can increase the sound pressure of the chime produced by a gong.

This invention is an alarm bell that includes a gong that produces a bell sound when struck, a striking rod that strikes the gong, a motor that drives the striking rod, and a housing that houses the motor, and is characterized in that the housing has a plurality of curved wall sections that form part of its peripheral wall and are bent in a V-shape in plan view in the circumferential direction.

This invention is an alarm bell comprising a gong that produces a bell sound when struck, a striking rod for striking the gong, a motor for driving the striking rod, and a housing for accommodating the motor, wherein the housing has a plurality of curved wall sections which form part of its peripheral wall and which are bent in a continuous, straight V-shape in a planar view , the plurality of curved wall sections having substantially the same shape and being disposed substantially evenly in the circumferential direction, and wherein the housing is Y-shaped in a planar view .

The invention also provides an alarm bell that includes a gong that produces a bell sound when struck, a striking rod that strikes the gong, a motor that drives the striking rod, and a housing that contains the motor, the gong having a slit formed in the rim and a vibration antinode located within an angular interval of 35 to 55 degrees circumferentially from the slit, and the striking rod striking a position within that angular interval.

Here, the striking rod can strike a position at an angle of approximately 45 degrees circumferentially from the slit, where the antinode of the vibration of the gong is located.

The gong may further include a mounting plate for mounting the alarm bell at an installation position, and the gong may have an outer diameter such that, when mounted, an insertion hole for a fixing member provided at the end of the mounting plate is exposed to the outside. The housing may also have an opening on the back side and a lid for closing the opening, and an insulating sheet for sealing the opening may be provided between the opening and the lid.

According to this invention, the peripheral wall of the housing has multiple curved wall sections that are bent in a V shape when viewed from above, and the space on the inner surface of the gong is approximately uniform, which improves the resonance effect in the space on the inner surface of the gong and increases the sound pressure of the chime produced by the gong.

In addition, according to this invention, the striking rod strikes a position within an angular interval of 35 to 55 degrees circumferentially from the slit, where the antinode of the gong's vibration is located, thereby improving the efficiency of the gong's vibration caused by the strike and increasing the sound pressure of the chime produced by the gong.

FIG. 1 shows an example of an embodiment of the alarm bell of the present invention, and is a rear view (with the rear cover of the housing open) showing the inner side (rear side) of the alarm bell. 2 is a cross-sectional view taken along line AA in FIG. 1. FIG. This shows an example of an installation form when the above alarm bell is installed as a rainproof type, and is a surface view showing the alarm bell, the mounting plate, and the surface of the wall on which it is to be installed. 6 is a cross-sectional view of the alarm bell, the mounting plate, and the wall surface side of the installation target (wall surface, switch box, etc.), taken along line E-E in FIG. 5. FIG. 2 is an exploded oblique view of the alarm bell, the mounting plate, and the wall surface side of the object to be installed (the wall surface, the switch box, etc.).

An example of an embodiment of the alarm bell of this invention will be described with reference to the drawings. The alarm bell of this invention can be used, for example, as a district sounding device in an automatic fire alarm system or an emergency bell in an audio device in an emergency alarm system.

<Basic configuration>
The alarm bell 1 comprises a gong 2 that produces a bell sound when struck, a striking rod 3 that strikes the gong 2, a motor 4 that drives the striking rod 3, a cam 5 that is fixed to the drive shaft 4a of the motor 4 and rotates together with the drive shaft 4a, a power transmission section 6 that is connected to the striking rod 3 and the cam 5 and transmits the power of the motor 4 to the striking rod 3, and a housing 7 that contains the motor 4 and the like (see Figures 1 to 3).

<Gong>
The gong 2 has a bottom 2b and a peripheral portion 2c, and is shaped like a shallow circular bowl in plan view. A space 2a is formed on the inner side, surrounded by the peripheral portion 2c. The housing 7 is disposed in the space 2a so that the striking rod 3 strikes the inner surface of the peripheral portion 2c from the inside. Specifically, a fixing portion 2ba for fixing the housing is provided on the inner surface of the bottom portion 2b (see Figures 2 and 3), and the housing 7 is fixed to the fixing portion 2ba in the above-mentioned arrangement. A slit 2e is formed in the rim portion 2d of the peripheral portion 2c. In the illustrated example, two slits 2e are formed, one on each side at 180° symmetrical positions in the circumferential direction of the peripheral portion 2c.

＜Batting＞
The striking rod 3 is positioned so that when it moves longitudinally forward (towards the gong 2 ), the striking portion 3 a at the tip side strikes the inner surface of the peripheral portion 2 c of the gong 2 .

<Motor>
The motor 4 is disposed such that the drive shaft 4a is oriented at a substantially right angle to the longitudinal direction of the hitting rod 3. A cam 5 that rotates together with the drive shaft 4a is provided around the drive shaft 4a.

<Power transmission section>
The power transmission unit 6 consists of a linear member whose tip end is connected to the striking rod 3 via connection part 6a and whose rear end is connected to the cam 5 via connection part 6b, and is arranged in such a way that the power of the rotational motion of the motor 4 is converted into reciprocating motion power via the cam 5 and transmitted to the striking rod 3, moving the striking rod 3 back and forth in the longitudinal direction.

<Housing>
Shape and arrangement The housing 7 has a plurality of curved wall portions 7aa that are bent in a V-shape in a plan view (opening toward the circumferential side portion 2c of the gong 2) in the circumferential direction as a part of the peripheral wall 7a. In the illustrated example, the number of curved wall portions 7aa is three, and the housing 7 has a Y-shape in a plan view. The curved wall portions 7aa are substantially the same shape and are arranged substantially evenly in the circumferential direction. The housing 7 having such a shape is arranged to be located substantially in the center in a plan view in the space portion 2a on the inner surface side of the gong 2. Here, since the curved wall portions 7aa are bent at an angle rather than in a curved shape, the space portion 2a on the inner surface side of the gong 2 can be enlarged, and the direction in which the bell sound is reflected can be expanded.

The housing 7 further has a plurality of end walls 7ab that form part of the peripheral wall 7a and connect the curved wall portions 7aa. In the illustrated example, the number of end walls 7ab is three, corresponding to the number of curved wall portions 7aa. These end walls 7ab are also of approximately the same shape and are arranged approximately evenly in the circumferential direction. The housing 7 further has a bottom wall 7b. The bottom wall 7b has a protrusion 7ba (with a recess on the inner surface) on its outer surface that is fixed to the fixing portion 2ba on the gong 2 side. The housing 7 is arranged in the above-mentioned manner within the space 2a on the inner surface of the gong 2 by fixing the protrusion 7ba to the fixing portion 2ba on the gong 2 side with a screw 7bb.

The housing 7 further has a back cover 7g that closes the opening 7f on the back side, and in the illustrated example, an insulating sheet 7h is provided between the opening 7f and the back cover 7g (see FIG. 3). The insulating sheet 7h seals the opening 7f to prevent corrosive gases and the like from entering the interior 7c of the housing 7. This protects the motor 4 and other components housed in the housing 7. The portion marked with a symbol on the outer surface of the back cover 7g is a hook 7i used to hang the alarm bell 1 on an object to which it is attached.

- Equalization of surrounding space: Here, the peripheral wall 7a of the housing 7 has a plurality of curved wall portions 7aa that are V-shaped when viewed in a plane, and these curved wall portions 7aa are of approximately the same shape and are arranged in an approximately even manner in the circumferential direction.As a result, the spaces formed between these curved wall portions 7aa and the peripheral side portion 2c of the gong 2 in the space portion 2d on the inner surface of the gong 2 can also be formed in an approximately uniform shape and arranged in an approximately even manner in the circumferential direction.

・Improved sound pressure (due to the shape of the housing, the surrounding space, etc.)
In the alarm bell 1 of the present invention, the housing 7 has a plurality of curved wall portions 7aa bent in a V-shape in plan view as a part of the peripheral wall 7a, so that the curved wall portions 7aa can function as acoustic walls, improving the resonance effect in the space portion 2a on the inner side of the gong 2. Furthermore, by providing the curved wall portions 7aa with substantially the same shape and substantially evenly spaced in the circumferential direction, the spaces formed between the curved wall portions 7aa and the peripheral side portion 2c of the gong 2 in the space portion 2d on the inner side of the gong 2 can also be formed with substantially the same shape and substantially evenly spaced in the circumferential direction, as described above. This also improves the resonance effect in the space portion 2a on the inner side of the gong 2. And, for example, by using a housing 7 with a Y-shape in plan view as shown in the example shown in the figure, the sound pressure of the bell sound generated by the gong 2 can be increased, as shown in the results of the sound pressure measurement experiment described later.

・Sound pressure measurement experiment (compared to a case using an L-shaped housing in plan view)
The inventors conducted an experiment to measure the sound pressure of the alarm bell using a housing with a Y-shape in plan view as in the present invention, and an alarm bell using a housing with an L-shape in plan view as a housing for accommodating a motor and a power transmission unit. The results of the experiment showed that the sound pressure was higher in the alarm bell using a housing with a Y-shape in plan view than in the alarm bell using a housing with an L-shape in plan view. This result is believed to be due to the fact that, in the alarm bell using a housing with a Y-shape in plan view as in the present invention, the multiple curved wall portions function as acoustic walls as described above, the multiple curved wall portions are approximately the same shape and are arranged approximately evenly, and the spaces formed between the curved wall portions and the peripheral side portions of the gong are also approximately the same shape and are formed approximately evenly in the circumferential direction, which improves the resonance effect in the space on the inner surface side of the gong.

The inventors also analyzed the sound pressure distribution of the alarm bells for which the above-mentioned measurement experiments were performed, using a simulation program. The results of this analysis showed that in the space on the inner side of the gong, when a housing that is L-shaped in plan view is used, the high sound pressure areas are unevenly distributed, whereas when a housing that is Y-shaped in plan view is used, the high sound pressure areas are distributed approximately evenly radially. In addition, on the outside of the gong, when a housing that is L-shaped in plan view is used, the high sound pressure areas are distributed around the sides of the gong, and the high sound pressure areas are also distributed around the front of the gong, while when a housing that is Y-shaped in plan view is used, the high sound pressure areas are distributed in a continuous ring shape around the sides of the gong, and the high sound pressure areas are distributed in a concentrated front direction in front of the gong. From these results, it can be understood that by using a housing that is Y-shaped in plan view as in this invention, the resonance effect in the space on the inner side of the gong can be improved, and the sound pressure of the bell sound generated by the gong can be increased.

Branch Spaces A plurality of branch spaces 7cb-7cd are formed in the interior 7c of the housing 7 in accordance with the case shape. In the illustrated example, three branch spaces 7cb-7cd are formed branching off from the central space 7ca in accordance with the Y-shaped case shape in plan view. The shape of each of the branch spaces 7cb-7cd towards the tip end (the side facing the circumferential portion 2c of the gong 2) is generally rectangular in plan view.

Of the branch spaces 7cb-7cd, the striking rod 3 penetrates the end wall 7ab of the branch space 7cb, with the connection part 3b with the power transmission part 6 at the rear end located inside the branch space 7cb and the striking part 3a at the front end located outside the branch space 7cb. The branch space 7cb in which the striking rod 3 is located is connected to the adjacent branch space 7cc via the central space 7ca, and together with these spaces, forms a space that is continuous in a V-shape when viewed from above. The motor 4 and power transmission part 6 are housed in the space that is continuous in a V-shape. Specifically, the motor 4 is housed in such an arrangement that the main body side is located in the branch space 7cc and the drive shaft 4a side is located in the central space 7ca, and the power transmission unit 6 is housed in such an arrangement that the connection part 6b with the cam 5 is located in the central space 7ca and the connection part 6a with the batting rod 3 is located in the branch space 7cb while extending in a direction approximately perpendicular to the drive shaft 4a. In addition, a support member such as a collar 7d that supports the batting rod 3 so that it can move back and forth in the length direction is provided at the part of the end wall 7ab of the branch space 7cb where the batting rod 3 passes through.

The number and shape of the branch spaces in the housing 7 can be changed by changing the angle, number, and arrangement of the bends in the curved wall portion 7aa, which is V-shaped in plan view, and by changing the shape of the housing 7.

Bending angle of the curved wall portion having a V-shape in plan view In the illustrated example, the bending angle of the curved wall portion 7aa is approximately 120 degrees. The reason for setting the bending angle at approximately 120 degrees is that the shape toward the tip side of the branch spaces 7cb to 7cd can be made approximately square in plan view. Note that this bending angle can be changed by changing the number of the curved wall portions 7aa and the shape of the housing 7.

Shape of the part through which the batting bar penetrates Although the multiple curved wall portions 7aa and the end walls 7ab have been described as having substantially the same shape, in the illustrated example, there are some parts with slightly different shapes. That is, only the end wall 7ab through which the batting bar 3 penetrates and the curved wall portion 7aa continuing to the end wall 7ab are slightly different in shape from the others. Specifically, in the end wall 7ab through which the batting bar 3 penetrates, only the part through which the batting bar 3 penetrates is slightly bent so as to be in a direction substantially perpendicular to the length direction of the batting bar 3 (the direction of movement of the batting bar 3). In addition, in the curved wall portion 7aa continuing to the end wall 7ab, the end wall 7ab is bent inwardly, so that only the part continuing to the end wall 7ab is slightly shorter in length. The part of the end wall 7ab through which the striking rod 3 passes is oriented approximately perpendicular to the longitudinal direction of the striking rod 3 because the amplitude direction of the vibration of the gong 2 is perpendicular to the striking position, making it easier for the striking force to be transmitted as vibration.

<Gong striking position>
Vibration antinodes, angular interval from the slits The gong 2 used may have vibration antinodes located at angular intervals of 35 to 55 degrees circumferentially from the slit 2e in the circumferential side portion 2c (angular intervals from the circumferential midpoint of the slit; the same applies below). The striking rod 3 may be disposed so as to strike positions within this angular interval. In the example shown in the figure, the gong 2 used has vibration antinodes located at angular intervals θ of approximately 45 degrees circumferentially from the slit 2e in the circumferential side portion 2c, and the striking rod 3 is disposed so as to strike positions at angular intervals θ of approximately 45 degrees.

-Improved sound pressure (due to the gong striking position)
In the alarm bell 1 of this invention, the efficiency of vibration of the gong 2 caused by striking can be improved by striking positions within an angular interval of 35 to 55 degrees in the circumferential direction from the slit 2e in the peripheral side portion 2c, which is the antinode of vibration of the gong 2. By striking positions within such an angular interval, for example positions spaced at angular intervals of approximately 45 degrees as in the illustrated example, the sound pressure of the sound produced by the gong 2 can be increased, as will be shown in the results of a measurement experiment of the sound to be described later.

- Sound pressure measurement experiment (compared to striking positions spaced approximately 90 degrees apart)
The inventors conducted an experiment to measure the sound pressure of an alarm bell struck at a position on the gong that is the antinode of the gong's vibration and that is approximately 45 degrees apart from the slit on the periphery of the gong, as in the present invention, and at a position approximately 90 degrees apart in the same direction.The results of the experiment showed that the sound pressure of the bell struck at a position approximately 45 degrees apart from the slit was higher than that struck at a position approximately 90 degrees apart from the slit.This result is believed to be due to the fact that striking a position that is the antinode of the gong's vibration and that is closer to the slit improves the vibration efficiency of the gong when struck, as mentioned above.

The inventors also conducted experiments to measure sound pressure by striking a position approximately 8 degrees circumferentially away from the slit (a position approximately 53 degrees from the slit) from a position approximately 45 degrees apart from the slit (the position of the antinode of vibration), and obtained sound pressures of the same order as when striking a position approximately 45 degrees apart. From this, it is most preferable to strike the gong 2 with the striking rod 3 at the position of the antinode of vibration of the gong 2, but even if the striking position is shifted from that position, it is understood that the same degree of improvement in sound pressure can be expected as long as the shift is within an angular range of about ±10 degrees in the circumferential direction. Of course, it is understood that the same degree of improvement in sound pressure can be expected as long as the shift is within an angular range of about ±5 degrees in the circumferential direction. That is, for example, if the position of the antinode of vibration of the gong 2 is approximately 45 degrees circumferentially from the slit 2e as in the illustrated example, even if the striking position is shifted from that position, it can be expected that the same degree of improvement in sound pressure can be expected as long as the striking position is within an angular range of 35 to 55 degrees circumferentially from the slit 2e. Of course, it can be expected that the same degree of improvement in sound pressure can be expected as long as the striking position is within an angular range of 40 to 50 degrees circumferentially from the slit 2e.

<Examples of changes to housing shape>
The shape of the housing 7 can be appropriately changed as long as it has portions corresponding to the plurality of V-shaped curved wall portions 7aa in plan view. For example, the bending angles of the V-shaped curved wall portions 7aa in plan view may be approximately right angles, the number of the V-shaped curved wall portions 7aa may be four, and the housing 7 may have a cross shape in plan view.

<Other parts that contribute to improving sound pressure>
- Thinning of the bottom of the gong in the vicinity of the center In the illustrated example, the gong 2 has a continuous groove 2bb formed in a ring shape around the fixing part 2ba, which is provided on the inner surface of the bottom 2b at approximately the center in a plan view. In other words, the groove 2bb is provided to the gong 2 so that the thickness of the part of the bottom 2b near the approximately center in a plan view is thin. When the sound pressure of the bell sound was measured in both cases where the thickness of the part was thinned and where it was not, the sound pressure was higher in the case where the thickness was thinned than in the case where it was not. From this, it can be understood that the sound pressure of the bell sound can also be increased by providing the groove 2bb as in the illustrated example and thinning the thickness of the bottom 2b of the gong 2 near the approximately center in a plan view.

<Benefits of improved sound pressure>
In the alarm bell 1 of this invention, as described above, the sound pressure of the bell can be increased. As a result, even if the gong 2 is made smaller, it can still satisfy the required sound pressure standard under the Fire Service Act when used as an emergency bell.

- Sound pressure measurement experiment (sound pressure when the gong is made smaller)
The outer diameter of the gong in a conventional emergency bell is usually 150 mm. The inventors prepared a prototype alarm bell with a gong having an outer diameter of 130 mm, which was made smaller than the alarm bell 1 of the present invention, and conducted an experiment to measure the sound pressure of the bell sound. As a result of the experiment, it was found that the prototype was able to obtain a sound pressure of 90 dB or more, which is the standard under the Fire Service Act. It is believed that this result was obtained because the prototype's housing is Y-shaped in plan view, as in the housing 7 of the illustrated example, and the striking position of the striking rod is a position at the antinode of the vibration of the gong, and is spaced at an angle of approximately 45 degrees from the slit on the circumferential side of the gong, as in the striking position of the striking rod 3 of the illustrated example.

In other words, in the alarm bell 1 of this invention, as shown in the example, the housing 7 has a Y-shape in plan view, and the striking position of the gong 2 with the striking rod 3 is the position of the antinode of the vibration of the gong 2, and is a position spaced at an angle of approximately 45 degrees in the circumferential direction from the slit 2e in the peripheral side portion 2c of the gong 2. Therefore, even if the outer diameter of the gong 2 is reduced to, for example, 130 mm to make it compact, it will still meet the required sound pressure standards under the Fire Service Act when used as an emergency bell.

In addition, in Figures 1 to 3, as well as in Figures 4 to 6, which will be referred to later in the explanation of the case where the alarm bell 1 is a weatherproof type, the dimensional ratios of each part are shown assuming that the outer diameter of the gong 2 is 130 mm.

<Advantages of a smaller gong>
Reduction in manufacturing costs As described above, in the alarm bell 1 of the present invention, the gong 2 can be made smaller, which reduces manufacturing costs.

・Improved workability (when used as a rainproof type)
4 and 5 show a manner in which the alarm bell 1 is of a rainproof type and is attached to a recessed switch box 50 embedded in a wall W as an installation position. The switch box 50 is embedded in the wall W together with a paint cover 40 on the front side, and a rainproof mounting bracket 30 (a three-layer structure having waterproof packing on the front and back sides of the bracket) is exposed on the front side of the wall W and attached to the paint cover 40 by screws. The rainproof alarm bell 1 has a rainproof mounting plate 7j (which has a space inside and may be called a case or box, etc.) on the back side of the housing 7 in place of the back cover 7g (it may be provided in addition to the back cover 7g and the insulating sheet 7h), and when it is installed at the position of the switch box 50 embedded in the wall W, it is fixed to the mounting bracket 30 on the switch box 50 side via the mounting plate 7j by screws 7jb (an example of a fixing member).

Here, the mounting bracket 30 on the switch box 50 side is a general-purpose product, and the screw hole 30a into which the alarm bell 1 is screwed is formed at a position determined by the standard. The screw hole 7ja (an example of a through hole for a fixing member) of the mounting plate 7j on the alarm bell 1 side is formed at a position corresponding to the screw hole 30a of the mounting bracket 30 on the switch box 50 side. In the illustrated example, the screw hole 7ja is provided at each corner (an example of an end) of the mounting plate 7j. If the gong 2 is a large one with an outer diameter of 150 mm as in the conventional product, the screw hole 7ja will be hidden behind the gong 2 in a plan view. In that case, when the mounting plate 7j is attached to the gong 2 together with the housing 7, it is not possible to insert the screw 7jb into the screw hole 7ja and screw it into the screw hole 30a of the mounting bracket 30 on the switch box 50 side. Therefore, once the gong 2 and housing 7 are assembled, they must be removed, the mounting plate 7j must be screwed to the mounting bracket 30 on the switch box side, and then the gong 2 and housing 7 must be reassembled. In contrast, if the gong 2 is small, for example with an outer diameter of 130 mm, as in the present invention, even when the housing 7 is attached to the gong 2, the screw hole 7ja of the mounting plate 7j is located on the outside of the gong 2 and is exposed to the outside, so that the screw 7jb can be used to screw the mounting bracket 30 on the switch box 50 side.

In other words, in the alarm bell 1 of this invention, the gong 2 can be made smaller, which improves ease of installation when making it a rainproof type.

<Example of configuration change>
In the alarm bell 1 of the present invention, the configuration can be changed, for example, as follows.

For example, the outer diameter of the gong 2 is not limited to 130 mm, and may be larger or smaller. Even if the outer diameter is increased or decreased in this way, it is possible to improve the sound pressure of the bell sound by configuring the shape of the housing 7 and the striking position of the striking rod 3 as described above.

1: Alarm bell 2: Gong 2a: Space 2b: Bottom 2ba: Fixing part 2bb: Groove 2c: Circumferential side 2d: Mouth rim 2e: Slit 3: Striker 3a: Striking part 3b: Connection part 4: Motor 4a: Drive shaft 5: Cam 6: Power transmission part 6a: Connection part (striker side) 6b: Connection part (cam side)
7: Housing 7a: Peripheral wall 7aa: Curved wall portion 7ab: End wall 7b: Bottom wall 7ba: Convex portion 7bb: Screw 7c: Inside 7ca: Central space portion 7cb to 7cd: Branch space portion 7d: Collar 7f: Opening 7g: Back cover 7h: Insulating sheet 7i: Hook 7j: Mounting plate 7ja: Screw hole 7jb: Screw 30: Mounting bracket 40: Paint margin cover 50: Switch box W: Wall surface

Claims (6)

An alarm bell comprising : a gong that produces a bell sound when struck; a striking rod that strikes the gong; a motor that drives the striking rod; and a housing that houses the motor,
The housing has a plurality of curved wall portions that are bent in a V-shape and are continuous linearly in a circumferential direction in a plan view as part of a peripheral wall thereof ,
The curved wall portions have substantially the same shape and are arranged substantially uniformly in the circumferential direction,
The alarm bell is characterized in that the housing is Y-shaped when viewed from above . The alarm bell according to claim 1, characterized in that the housing has multiple branch spaces formed therein. The gong has a slit formed in its rim and has vibration antinodes at angular intervals of 35 to 55 degrees around the rim of the gong.
3. The alarm bell according to claim 1, wherein the striking rod strikes a position within the range of the angular interval. 4. The alarm bell according to claim 3 , wherein the striking rod strikes positions at angular intervals of approximately 45 degrees in a circumferential direction from the slit, where the antinodes of vibration of the gong are located. The alarm bell further includes a mounting plate for mounting the alarm bell at a mounting position.
5. An alarm bell as claimed in claim 1, wherein said gong has an outer diameter such that, in an attached state, an insertion hole for a fixing member provided at an end of said mounting plate is exposed to the outside. the housing has an opening on a back side and a back cover for closing the opening,
6. The alarm bell according to claim 1, further comprising an insulating sheet provided between said opening and said back cover to seal said opening.

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