Everyone may have heard the sound of air defense alarm, which is very penetrating and can clearly hear the alarm signal even in a far place. However, what kind of equipment is an air defense alarm? How does the alarm sound? What makes the sound of air defense alarm spread to this day?

(1) How does the air defense alarm sound?

In fact, air defense warning is not a new product. As early as World War II, this kind of alarm that can transmit sound far away already existed. In some old movies, we often see some air defense personnel controlling the mechanical air defense alarm. Air defense alarm, similar to the low hum of bees, originally originated from mechanical work.

Early air defense alarms relied on people shaking to make the machine run and then make a sound. At that time, the air defense alarm made such a loud sound without electricity because of its special internal structure.

The sound principle of the alarm is similar to that of ordinary diabolo. When it rotates, the internal gear will change from large to small, gradually changing the slow rotation into high-speed rotation, and then driving the internal buzzer. When it rotates at high speed, the surrounding air will be compressed, and then the compressed air will be squeezed into the dense sound window, thus making a low sound like a bee.

(2) How to distinguish different alarm sounds?

There are three kinds of air defense alarm signals: early warning, air raid alarm and disarming alarm. Identifying air defense warning is the basis of correctly implementing protective actions. If the three alarm signals are confused, the consequences can be said to be unimaginable.

The early warning sound signal is defined as: sounding for 36 seconds, stopping for 24 seconds, repeating for three times as a cycle, and lasting for 3 minutes. Its characteristic is "long interval", which is an alarm signal when the enemy is about to attack the city. After hearing the warning, citizens should immediately turn off the power switch, turn off the gas, put out the fire, take personal protective equipment and necessary daily necessities, take good care of the elderly and children, and quickly and orderly enter the civil air defense project or designate hidden areas.

The sound signal of air raid alarm is defined as: sounding for 6 seconds, stopping for 6 seconds, repeating 15 times, and the time is 3 minutes. Its characteristic is "short interval", which is an alarm signal when the enemy air attack weapon is near the city and has started or is about to attack the city target.

After the citizens hear the air raid alarm, all personnel should immediately go to the nearest civil air defense project to hide, except the staff who must stick to their posts in various industries (units). The sound signal for canceling the alarm is defined as: continuous playing for 3 minutes. Its characteristic is "no interval", which is an alarm signal when the enemy's air raid danger has been lifted.

After hearing that the alarm has been lifted, citizens should return to their homes and jobs in an organized and orderly manner. When the enemy's air strikes cause houses to collapse, fires and casualties, etc. Under the unified organization of units or communities, we should actively carry out self-help, mutual rescue, fire fighting and emergency repair, eliminate the consequences of air strikes, restore production and living order as soon as possible, and minimize the losses caused by air strikes. What we heard on the anniversary of the September 18th Incident was an early warning.

(3) What does the air defense alarm look like?

For air defense alarms, there is often only sound without shape. So, what is the alarm like? Where is it installed, so that it is so penetrating that it can be heard from all angles of the city?

Modern air defense warning has developed into many types and models and has become an important part of urban air defense system. As an important combat readiness facility, air defense warning is usually used for disaster prediction and emergency notification in emergency rescue and disaster relief and sudden accidents, and is used for civil air defense in wartime, which is the basic means for people's governments at all levels to carry out civil air defense command and organize personnel evacuation.

Air defense alarm usually sends out air defense alarm signals by combining wireless and wired control. Its control equipment is divided into wired control and wireless control, and the power supply system is divided into commercial power and self-power. The main alarm facilities are:

Electric alarm

The electric alarm takes the motor as the main alarm device and converts mechanical energy into sound. The alarm drives the chime wheel to run at high speed through the motor, and the general speed is about 2880 rpm. When the motor drives the chime wheel to rotate, air enters the chime wheel from the air inlet on the side, and the sucked air is squeezed out of the window of the fixed wheel at high speed and high pressure, resulting in resonance. When the air defense alarm stops, it is actually not silent, but because of the inertia of the motor rotor, it still emits a low-frequency hum.

For example, the warning sounded for 36 seconds and stopped for 24 seconds, but what was actually heard was that the sound fluctuated up and down for 36 seconds and up and down for 24 seconds. Motor-type air defense alarm is composed of clock wheel, fixed wheel, positioning seat, positioner, stainless steel cover, motor, base and other main components. There are two kinds of electric alarms: separated alarm and horizontal alarm.

1, honking wheel: it is the most important part of the alarm, and it generates sound source when working at high speed.

2. Fixed wheel: the part matching the height of the clock wheel, usually about 10 pronunciation windows; The pronunciation window is equal to the pronunciation blade of the Ming wheel.

3. Positioning seat: the bracket for fixing the positioner.

4. Positioning device: When the buzzer does not work or stops working, the sound window can be automatically closed to prevent mice, birds, spiders and sundries blown by the air from entering the alarm and protect the alarm.

5. Stainless steel cover: on the one hand, it can amplify the refracted sound, on the other hand, it can protect the whole alarm and prevent rainwater from entering the alarm;

6. Motor: the power of the alarm drives the chime wheel to rotate, which plays the role of sounding.

7. Base: the support of the whole alarm.

Electroacoustic alarm

The electroacoustic alarm consists of a host and a speaker. Through the terminal of electroacoustic alarm, language transmission and shouting can be carried out. Electric alarm and electroacoustic alarm mainly cover the main urban area and important target areas, and are the main parts of air defense alarm network.

Multimedia alarm terminal

That is, LED multimedia alarm terminal, focusing on residential areas. It can not only issue air defense warning signals, but also issue emergency announcements and convenience information through words, sounds and images, which enriches its functions.

Mobile alarm vehicle

The mobile alarm car mainly covers the dead angle blind area of the whole city. In addition, researchers are actively developing new air defense warning facilities that can be used in buildings and underground spaces, and gradually establishing and perfecting multi-means and full-coverage air defense warning networks. In general, an air defense alarm can cover the range of 1 square kilometer, and the air defense department will set the alarm according to this distance.

But in the process of sound propagation, it will also be blocked by high-rise buildings and other buildings. Generally, in urban areas with dense buildings, the installation of alarms is relatively dense; In the clearing, not so much. In addition, places where people gather, such as squares or schools, are the key places to install alarms.

The reason why the air defense alarm has a strong sense of existence and can be clearly heard in every corner of the city is the result of the synergy of various alarms.