JP4331659B2 - Alarm acoustic structure - Google Patents

Description

The present invention relates to an acoustic structure of an alarm device for detecting and alarming an abnormality such as a fire, and more particularly to an acoustic structure of an alarm device for alarming by a buzzer using a piezoelectric diaphragm.

  Conventionally, in a home alarm device, when a fire is detected, a message by voice synthesis or an alarm sound is output from a speaker to notify the fire. In order for such an alarm device to be widely used for home use, it is important to reduce costs, and there is a strong demand for downsizing.

  In order to reduce the cost and size of the alarm device, it is desirable to use a piezoelectric buzzer using a piezoelectric diaphragm rather than an alarm output by a speaker. Piezoelectric buzzer is a layer of piezoelectric layer laminated on one side of a diaphragm made of metal, supports the outer part of the piezoelectric diaphragm against the rib formed at the mounting position, and supports the inner part of the diaphragm in a floating state. Acoustic vibration is generated by switching the polarity of the voltage applied between them.

In addition, a resonance space in which an acoustic hole is formed on one side of the piezoelectric diaphragm is formed so as to produce a sufficient volume by producing a resonance vibration by the piezoelectric diaphragm. The resonance space is usually formed by arranging a cylindrical body with one end closed on one side of the piezoelectric diaphragm, and the acoustic hole opens at the center of the closed end face of the cylinder in accordance with the center position of the piezoelectric diaphragm. ing.
JP 2004-54356 A JP-A-10-124769

  By the way, when a piezoelectric buzzer composed of a resonance space with a piezoelectric diaphragm and an acoustic hole is installed inside the alarm device, it must be installed with a circuit board equipped with a smoke detector equipped with a scattered light smoke detection structure. become.

  In this case, in order to reduce the installation space, a piezoelectric buzzer may be placed behind the circuit board. However, if a circuit board with a smoke chamber fixed is positioned in front of the acoustic hole of the electronic buzzer, the sound outlet is blocked and the sound volume is reduced. For this reason, the piezoelectric buzzer cannot be arranged on the circuit board, and the alarm device is increased in size accordingly because it is arranged side by side.

  Further, the diameter of the piezoelectric diaphragm of the piezoelectric buzzer becomes smaller as the vibration frequency is higher. For this reason, the vibration frequency of the electronic buzzer can be increased to reduce the size, and an increase in size when arranged side by side with the circuit board can be suppressed. However, it tends to be difficult for elderly people to hear high-frequency sounds, and it is not preferable to downsize the piezoelectric buzzer and increase the vibration frequency.

  Further, if the circuit boards are partially overlapped so as not to be positioned in front of the acoustic holes of the piezoelectric buzzer, the installation space can be reduced as compared with the case where they are arranged side by side. However, the acoustic hole of the piezoelectric buzzer is opened relative to the center position of the piezoelectric diaphragm, and the portion of the electronic buzzer that can be placed on the circuit board is very small, and the contribution to miniaturization is small.

  Furthermore, in consideration of the convenience that a resident can easily install a home alarm device, the alarm device has a structure that is attached not only to the ceiling surface but also to the wall surface.

  However, when the alarm is attached to the wall, the position of the acoustic hole is in the height direction, so the sound directivity differs depending on the height. In order to adjust the sound directivity for the first time after sounding the piezoelectric buzzer, it is necessary to remove the alarm and change the mounting position, which makes it difficult to adjust the sound directivity when mounted on the wall. is there.

SUMMARY OF THE INVENTION An object of the present invention is to provide an alarm structure for an alarm device that can be arranged in a piezoelectric buzzer on a circuit board on which a smoke detector is mounted, thereby reducing the size and thickness of the circuit and reducing the cost.

  In order to achieve this object, the present invention is configured as follows. The present invention relates to an acoustic structure of an alarm device for detecting and alarming an abnormality, a circular piezoelectric diaphragm placed on a rib formed on the inner surface of a device housing, and one side of the piezoelectric diaphragm The cylindrical resonant space forming member having one end opened to form the resonant space and closed at the other end, and the other end face of the resonant space forming member displaced from the center of the piezoelectric diaphragm are opened to the end face position. And an acoustic hole.

  Furthermore, in the acoustic structure of the alarm device of the present invention, the circuit board and the cover are disposed outside the resonance space forming member, and the circuit board and the cover are opposed to the acoustic hole at the other end of the resonance space forming member. Provide.

  Further, the present invention is characterized in that the resonance space forming member is rotatably attached to a support member on which the piezoelectric diaphragm is fixedly arranged, and the position of the acoustic hole of the resonance space forming member can be changed.

In another embodiment of the present invention, in the acoustic structure of an alarm device for detecting and alarming an abnormality, a circular piezoelectric diaphragm placed on a rib formed on the inner surface of the equipment housing; A cylindrical resonance space forming member which is disposed on one side of the piezoelectric diaphragm and forms both ends of the resonance space, a circuit board disposed opposite to the outer opening of the resonance space member, and a piezoelectric diaphragm An acoustic hole opened at a position of the circuit board eccentric from the center position, and a cover provided with the acoustic hole at a position opposite to the acoustic hole of the circuit board are provided.

  According to the acoustic structure of the alarm device of the present invention, an acoustic hole is formed at a position displaced from the center of the piezoelectric diaphragm in the acoustic space forming member that is disposed on one side of the piezoelectric diaphragm to form an acoustic space. The acoustic hole can be located at a peripheral position outside the installation position of the smoke chamber mounted on the circuit board, which makes it possible to place a piezoelectric buzzer on the circuit board and reduce the size of the alarm Can be planned.

  In addition, by providing the part where the acoustic hole is formed in the acoustic space forming member so as to be rotatable with respect to the fixed side, the sound directivity can be changed by changing the position of the acoustic hole in the height direction when the alarm is attached to the wall surface. Can be adjusted easily.

Furthermore, the circuit board is used for forming the acoustic space, and the acoustic hole is formed in the circuit board at a position deviated from the center of the piezoelectric diaphragm, thereby reducing the dimension in the overlapping direction of the acoustic space forming member. The cost can be reduced by downsizing and sharing components.

  FIG. 1 is an explanatory view of an alarm device according to the present invention. FIG. 1 (A) shows a front view and FIG. 1 (B) shows a rear view. FIG. 2 is a perspective view of the alarm device according to the present invention, and FIG. 2 (A) shows the front side and FIG. 2 (B) shows the back side.

  1 and 2, the alarm device 1 of the present invention includes a main body 2 and a cover 3. The cover 3 protrudes in a dish shape at the center, and a smoke inlet 4 is opened around the cover 3. A smoke chamber, which will be clarified later, is incorporated inside the cover provided with the smoke inlet 4, and smoke from a fire flows into the chamber through the smoke inlet 4 and includes a light emitting element and a light receiving element. The smoke from the fire is detected by the scattered light smoke detection mechanism.

  A notification indicator lamp 5 is provided at the lower right side of the cover 3, and an acoustic hole 6 is provided at the lower left side. A piezoelectric buzzer is accommodated in the interior corresponding to the acoustic hole 6. A push button 7 instructed to be cantilevered by a U-shaped notch is formed between the alarm indicator 5 and the acoustic hole 6, and a switch used for testing and alarm stop is incorporated inside the push button 7. ing.

  An attachment portion 8 having an attachment hole 8a is formed on the back surface of the main body 2 at a position that becomes an upper portion when attached to the wall surface. A battery housing portion 9 is formed on the back surface of the main body so as to open to the back side, and a 9-volt, for example, 6F22Y type battery 40 having a square laminated structure is housed therein. The battery stored in the battery storage unit 9 is held by a pair of battery holding pieces.

  Further, a commercially available battery snap is connected to the positive terminal and the negative terminal of the battery 40, and a lead wire drawn from the battery snap is drawn into the main body and connected to an internal circuit board.

  Furthermore, a transfer connector 10 is provided on the back side of the main body 2, and an alarm buzzer installed in another room or a receiver for dwelling units is connected to output a transfer signal so that it can be operated. A pull string 17 is pulled out from the back surface of the main body 2, and the pull string 17 turns on a switch provided on a circuit board in the main body. That is, the switch provided inside can be operated by either the push button 7 on the cover 3 side or the main body 2 side.

  FIG. 3 is an explanatory diagram of the internal configuration of the alarm device of the present invention with the cover removed. The circuit board 11 is incorporated in the main body with the cover removed, and a smoke chamber 12 is fixed to the center of the circuit board 11.

  The smoke chamber 12 has an insect net 13 disposed at a position opposite to the smoke inlet 4 formed in the cover 3 of FIGS. 1 and 2, and has a smoke inflow portion having a labyrinth structure inside the insect net 13. A scattered light type smoke detection structure in which the light-emitting element and the light-receiving element are arranged at a position where the optical axis is shifted by a predetermined angle is provided in the smoke detection space inside.

  Further, an alarm indicator lamp 5 is mounted on the circuit board 11, and a switch 15 is further provided. The switch 15 is turned on by pulling the switch 15 with the pull string 17 and contacting the lead contact 16 provided on the circuit board 11. The switch 15 is also switched on by pressing a push button 7 provided on the cover 3 side in FIGS.

  A piezoelectric buzzer 18 is placed on the lower left corner of the circuit board 11 in an overlapping manner. The piezoelectric buzzer 18 has an acoustic hole 22 (see FIG. 4) in the upper part, and an acoustic opening 30 (see FIG. 4) is formed at a position of the circuit board 11 opposite to the acoustic hole 22.

FIG. 4A shows a plan view of the internal structure of the sensor with the cover of FIG. 3 removed. When the circuit board 11 is removed in FIG. 4A, the piezoelectric buzzer 18 disposed on the lower side can be seen as shown in FIG. 4B. The piezoelectric buzzer 18 has an acoustic hole 22 at a position displaced from the center of the buzzer.

FIG. 5 is an explanatory view illustrating the acoustic box as the acoustic space forming member used in the piezoelectric buzzer 18 of FIG. In FIG. 5, the acoustic box 20 has a thin cylindrical shape that opens to the lower side where the piezoelectric diaphragm of the piezoelectric buzzer is disposed, and is closed at the top, and is provided with a pair of fixed arms 21 on the left and right. An acoustic hole 22 is opened at a position deviated from the center of the upper end surface of the acoustic box 20. In addition, the recessed part formed in the acoustic box 20 is a hollow which accommodates the circuit components mounted in the circuit board 11. FIG.

  FIG. 6 is a cross-sectional view showing an embodiment of the acoustic structure in the alarm device of the present invention, and FIG. 6 (A) is a cross-section in the direction of the fixed arm 21 in the acoustic box 20 of FIG. Is a cross section in a direction passing through the acoustic hole 22 orthogonal to the fixed arm 21.

  6A, ribs 27a and 27b are formed inside the main body 2, and a piezoelectric diaphragm 24 is placed on the ribs 27a and 27b. The piezoelectric diaphragm 24 has a piezoelectric layer 26 bonded to one side of a diaphragm 25 using a thin metal plate. The piezoelectric layer 26 contracts when a voltage in the same direction as the polarization direction is applied, and expands when a voltage in the opposite direction is applied. Thus, the diaphragm 25 can be vibrated.

  As the piezoelectric diaphragm 24, a self-excited oscillation type piezoelectric diaphragm having a piezoelectric layer 26 provided with a pair of drive electrodes and feedback electrodes is used. The piezoelectric diaphragm 24 is further supported by ribs 27c and 27d as is apparent from the cross section in the direction of FIG. 6B orthogonal to FIG. 6A, thereby supporting the piezoelectric diaphragm in a circular contact at four points. is doing.

  Since the circular contact support of the piezoelectric diaphragm 24 is in a state close to free vibration without controlling vibration, the impedance characteristic of the piezoelectric diaphragm 24 can be faithfully reproduced. In addition, strong and stable characteristics can be obtained against mechanical stress, and a large sound pressure can be obtained with increased efficiency. As the piezoelectric diaphragm 24 used in the present invention, the one having a resonance frequency of about 2.8 kilohertz is used in consideration of the point that it is difficult for elderly people to hear high-frequency sounds.

  An acoustic box 20 is disposed on the piezoelectric diaphragm 24 to form an acoustic space 23. The acoustic box 20 has fixed arms 21 projecting on both sides, and a cylindrical tip is formed as a tapered edge 29 at the bottom, and a ring-shaped packing 28 is interposed between the tapered edges 29 and the fixed arm 21 is screwed on the main body 2 side. It is fixed to.

As is apparent from FIG. 6B, the acoustic box 20 has an acoustic hole 22 opened at a position displaced outward from the center of the piezoelectric diaphragm 24. An acoustic opening 30 is formed at the position of the circuit board 11 facing the acoustic hole 22, and a plurality of acoustic holes 6 are formed at the position of the cover 3 facing the acoustic opening 30.

  As described above, in the acoustic structure of the alarm device according to the present invention, the acoustic hole 22 provided in the acoustic box 20 arranged on one side of the piezoelectric diaphragm 24 is displaced to the outside from the center of the piezoelectric diaphragm 24. As is clear from the internal structure of FIG. 3 and FIG. 4 (A), the opening is secured at a position away from the smoke chamber 12 fixed to the circuit board 11 while sufficiently securing a portion overlapping the circuit board 11. The acoustic hole 22 can be positioned, whereby the alarm device 1 can be reduced in size by the amount of the piezoelectric buzzer 18 superimposed on the circuit board 11.

  If the piezoelectric buzzer 18 cannot be superimposed on the circuit board 11, the piezoelectric buzzer 18 must be arranged side by side in the lateral direction outside the circuit board 11. In this case, the piezoelectric buzzer is arranged horizontally. Accordingly, the main body and the cover 3 are enlarged, and it can be seen that the alarm device is effectively reduced in size by overlapping the piezoelectric buzzer 18 and the circuit board 11.

  FIG. 7 is a cross-sectional view showing another embodiment of the acoustic structure in the alarm device of the present invention in which a part of the acoustic structure is also used for the circuit board. In this embodiment, FIG. 7A is a cross section in the direction of the fixed arm 21 of the acoustic box 20 as in FIG. 6A, and FIG. 7B is similar to FIG. It is the cross section of the direction which passed perpendicularly.

  In FIG. 7, in this embodiment, the acoustic box 20 has a cylindrical shape opened at the top. The acoustic box 20 is open at the upper end, and the circuit board 11 is disposed on the upper end, and the upper end face side of the acoustic box 20 is partitioned by the circuit board 11.

  For this reason, as shown in FIG. 7B, the circuit board 11 located at the upper part of the acoustic space 23 surrounded by the acoustic box 20 is displaced outward from the center of the piezoelectric diaphragm 24 and acoustic holes 32. Is forming. As described above, in the embodiment of FIG. 7, the acoustic space 23 is formed by the combination of the cylindrical acoustic box 20 opened at the upper end and the circuit board 11, and the acoustic box 20 is as in the embodiment of FIG. 6. Therefore, the dimension in the height direction can be reduced and the alarm device can be downsized. In addition, since the acoustic box 20 needs to be closed at the upper end, the part structure is simplified and the cost can be reduced accordingly.

  The circuit board 11 that regulates the cross-section side with respect to the acoustic box 20 opened at the top is not particularly required to be in close contact with the cross section of the acoustic box 20, and even if there is a slight gap as shown, It can function effectively as the space 23.

  FIG. 8 is a cross-sectional view showing another embodiment of the alarm device of the present invention in which the acoustic hole portion is rotatable. FIG. 8A shows the acoustic box 33 used in this embodiment, and FIG. 8B is a cross-sectional view of the opposite side of the acoustic structure using the acoustic box 33.

  The acoustic box 33 in FIG. 8A allows a rotating box 35 having a cylindrical shape that is open on the lower side and closed on the upper side to be rotatable with respect to the fixed box 34 as a fixing member obtained by taking out the fixing arm 37 in the lateral direction. Wearing. An acoustic hole 36 is opened at the upper end surface of the rotation box 35 so as to be displaced from the center of the piezoelectric diaphragm 24.

  8B, the piezoelectric diaphragm 24 is in contact with the ribs 27a and 27b (not shown) protruding to the inside of the main body 2 and at four points of the ribs 27c and 27d in an orthogonal direction (not shown). It is supported by a circle and fixed to the main body 2 by screwing screws 31 into fixed arms 37 formed on both sides of a fixed box 34 on the upper side of the piezoelectric diaphragm 24, and a packing 28 is interposed at the tapered edge portion of the fixed box 34. . The fixed box 34 has a flange structure, and the lower end portion of the opening of the rotary box 35 is rotatably fitted therein.

  For this reason, when the alarm device 1 having the acoustic structure of FIG. 8 is attached to the wall surface, the position of the acoustic hole 36 in the height direction can be easily changed by rotating the rotary box 35, thereby improving the sound directivity. Can be adjusted easily.

When an acoustic structure capable of rotating the acoustic hole 36 of FIG. 8 is used, an arc-shaped acoustic opening is formed in the circuit board 11 in a range in which the acoustic hole 36 can rotate, or the acoustic hole 36 It is necessary to cut out the circuit board 11 in a rotatable range so that the acoustic hole 36 can be directly opposed to the acoustic hole 6 of the cover 3.

  FIG. 9 is a block diagram of the circuit configuration of the alarm device according to the present invention. 9, the alarm device 1 of the present invention includes a battery 40, a noise absorbing circuit 41, a control circuit 42, an oscillation circuit 43, a light emitting circuit 44, a light receiving circuit 45, a buzzer driving circuit 46, an indicator lamp circuit 47, a switch circuit 48, A voltage drop detection circuit 49, an acoustic stop circuit 50, and a transfer circuit 51 are included.

  The battery 40 supplies power to the alarm 1, and the battery 16 is a square stacked 9-volt 6F22Y type dry battery. The noise absorbing circuit 41 stabilizes the circuit voltage supplied from the battery 40.

  As the control circuit 42, a dedicated IC for a fire alarm according to the UL standard is used. The control circuit 42 has a built-in timing circuit. The light emission circuit 44 emits light, the light receiving circuit 45 is sampled, the fire is determined based on the amount of received light, the buzzer drive circuit 46 is driven, and the voltage drop detection circuit 49 is set at a predetermined timing. The power supply voltage is monitored by the switch circuit, the test operation by the switch circuit 48 and the alarm stop, and the light emitting element of the light emitting circuit 44 and the light receiving element of the light receiving circuit 45 are automatically tested.

  The smoke detection cycle by the control circuit 42 is performed at a cycle of about 10.5 seconds, and when a fire is judged twice in succession, the buzzer drive circuit 46 is driven to issue an alarm, and simultaneously the transfer circuit 51 is operated, A transfer signal is sent to the external connection device 52 connected to the transfer terminals 10a and 10b to operate. The alarm stop after the fire determination stops the buzzer drive circuit 46 and the transfer circuit 51 when the non-fire is determined twice consecutively.

  The automatic test of the light emitting element and the light receiving element and the monitoring of the power supply voltage detected by the voltage drop detection circuit 49 are each performed at a cycle of about 40 seconds. When an abnormality is detected, the buzzer driving circuit 46 is driven to Sound an alarm momentarily for milliseconds.

  The light emitting circuit 44 causes a current to flow through the infrared light emitting diode in accordance with a pulse signal from the control circuit 42 to emit light. There are two types of pulse signals from the control circuit 42: smoke detection pulses with a period of about 10.5 seconds during normal monitoring and pulses for automatic tests with a period of about 40 seconds.

  The light receiving circuit 45 enters scattered light from the smoke of light emitted by driving the infrared light emitting diode by the light emitting circuit 44, converts it into a photocurrent, further converts it into a voltage signal, and outputs it to the control circuit 42. .

  As shown in FIGS. 1 to 4, the switch circuit 48 is turned on by operating the push button 7 or the drawstring 17, and performs a manual fire test and sound stop. When the switch provided in the switch circuit 48 is turned on, the power supply voltage Vc is applied to the test terminal of the control circuit 42, the control circuit 42 enters the test mode, and the light emitting circuit 44 is driven at a cycle of about 336 seconds. Only when the output for monitoring the zero point is normal, the buzzer driving circuit 46 sounds the buzzer instantaneously to notify that it is normal.

  When the switch of the switch circuit 48 is turned on, the power supply voltage Vc is also applied to the acoustic stop circuit 50, and while the switch is on, the operation of the acoustic stop circuit 50 is temporarily canceled, and the switch circuit 48 When the switch is turned off, the sound is stopped by the buzzer driving circuit 46 for about 6 minutes from the time when the switch is turned off.

  The buzzer driving circuit 46 includes a piezoelectric buzzer. As the piezoelectric buzzer, a self-excited piezoelectric diaphragm including a feedback terminal is used in addition to a pair of electrode terminals. The piezoelectric buzzer of the buzzer drive circuit 46 is supplied with a drive voltage at the time of fire judgment and failure occurrence in the control circuit 42, and reverses the voltage by a feedback signal to sound the buzzer.

  In order to stop the piezoelectric buzzer sounded by the control circuit 42, the buzzer sounding is stopped by fixing the feedback signal of the buzzer driving circuit 46 to the ground by the sound stop signal from the sound stop circuit 50.

  The resonance frequency of the piezoelectric diaphragm used in the buzzer drive circuit 46 is about 2.8 kHz. When a fire is detected, the cycle is about 330 milliseconds, and the ringing is intermittent for about 250 milliseconds. Will ring for about 10.5 milliseconds.

  The oscillation circuit 43 is a circuit for setting the basic clock and the light emission pulse width for generating the operation timing of the control circuit 42, and the basic clock and the light emission pulse width are set by external connection of a resistor and a capacitor. In the embodiment of the present invention, the basic clock is about 10.5 milliseconds and the light emission pulse width is 105 microseconds.

  The indicator lamp circuit 47 receives the output when the fire determination is made by the control circuit 42 and drives the LED as the alarm indicator lamp 5 shown in FIGS. 1 to 4 to emit light. When the power supply voltage Vc drops below 7.4 volts, for example, the voltage drop detection circuit 49 sounds the buzzer for about 10.5 milliseconds with a period of about 40 seconds.

  The transfer circuit 51 receives the fire detection signal when the fire determination is performed by the control circuit 42, switches the transistor of the open collector configuration, flows the notification current from the external connection device 52, and flows it out. Send a signal.

The present invention includes appropriate modifications that do not impair the objects and advantages thereof, and the present invention is not limited by the numerical values of the above-described embodiments.

Illustration of the alarm device according to the present invention The perspective view of the alarm device by this invention Explanatory drawing of the internal structure of the alarm device of the present invention with the cover removed Plan view of the internal structure of the alarm device of the present invention with the cover removed and the internal structure with the circuit board removed Explanatory drawing which took out and showed the acoustic box which forms the acoustic space by this invention Sectional drawing which showed embodiment of the acoustic structure in the alarm device of this invention Sectional drawing which showed other embodiment of the acoustic structure in the alarm device of this invention which used a part of acoustic structure for the circuit board. Sectional drawing which showed other embodiment of the acoustic structure in the alarm device of this invention which made the part of the acoustic hole freely rotatable The block diagram of the circuit structure of the alarm device by this invention

Explanation of symbols

1: Alarm device 2: Main body 3: Cover 4: Smoke inlet 5: Alarm indicator lamp 6: Sound hole 7: Push button 8: Mounting part 9: Battery storage part 10: Transfer connector 11: Circuit board 12: Smoke Chamber 13: Insect net 15: Switch 16: Lead contact 17: Pull string 18: Piezoelectric buzzer 20: Acoustic box 21, 37: Fixed arms 22, 32, 36: Acoustic hole 23: Acoustic space 24: Piezoelectric diaphragm 25: Vibration Plate 26: Piezoelectric layers 27a to 27d: Rib 28: Packing 29: Tapered edge 30: Acoustic opening 31: Screw 33: Acoustic box 34: Fixed box 35: Rotating box 40: Battery 41: Noise absorption circuit 42: Control circuit 43: Oscillation Circuit 44: Light emitting circuit 45: Light receiving circuit 46: Buzzer drive circuit 47: Indicator lamp circuit 48: Switch circuit 49: Voltage drop detection circuit 50: Sound stop circuit 51: Transfer circuit 5 : External connection device

Claims (2)

In the acoustic structure of an alarm device that detects and alerts for abnormalities,
A circular piezoelectric diaphragm placed on a rib formed on the inner surface of the device housing;
A cylindrical resonance space forming member that is disposed on one side of the piezoelectric diaphragm and that opens one end and closes the other end that forms a resonance space;
An acoustic hole opened at the end face position of the other end of the resonance space forming member displaced from the center of the piezoelectric diaphragm;
The resonance space forming member is rotatably attached to a support member on which the piezoelectric diaphragm is fixedly arranged, and the position of the acoustic hole of the resonance space forming member can be changed . Acoustic structure.
  The acoustic structure of the alarm device according to claim 1, further comprising a circuit board and a cover arranged outside the resonance space forming member, the circuit board facing the acoustic hole at the other end of the resonance space forming member, and An acoustic structure of an alarm device, wherein an acoustic hole is provided in each cover.

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