It turns out that air raid sirens don’t use loudspeakers!
Perhaps everyone has heard the sound of air raid sirens. This sound is extremely penetrating, and the alarm signal from a distance can be clearly heard even if it is far away. But what exactly is an air raid siren? How do the sirens sound? What allows the sound of an air raid siren to travel so far?
(1) How do air defense sirens make sounds?
Actually, air defense sirens are not a new product. As early as World War II, sirens that can spread sound far have already existed. In some old movies, we often see some Air defense personnel control a mechanical air raid siren. The bee-like drone of an air raid siren originally originated from mechanical work.
Early air raid sirens were manually shaken to make the machine run and then make a sound. At that time, the reason why air raid sirens could make such a loud sound without using electricity was because air raid sirens had a special internal structure.
The sounding principle of the siren is similar to that of a common diabolo. When it rotates, the internal gears will gradually change from large to small from slow rotation to high speed rotation, and then drive the internal sounding wheel. When rotating at high speed, the surrounding air will be compressed, and then the compressed air will be continuously squeezed into the dense sound window, thus emitting a low hum similar to that of a bee.
(2) How to distinguish different alarm sounds?
Air defense warning signals are divided into three types: advance warning, air raid warning and disarming warning. Identifying air defense warnings is the basis for correct protective actions. If the three warning signals are confused, the consequences can be said to be disastrous.
The pre-warning sound signal is stipulated as follows: sound for 36 seconds, pause for 24 seconds, and repeat three times as a cycle, lasting 3 minutes. It is characterized by "long interval", which is used when learning that the enemy is about to attack the city. Warning signal issued in case of air raid. After hearing the advance warning, citizens should immediately turn off the power switch, turn off the gas, extinguish the fire, carry personal protective equipment and necessary daily necessities, take care of the elderly and children, and quickly and orderly enter civil air defense projects or designated hidden areas.
The air raid alarm sound signal is stipulated as follows: sound for 6 seconds, pause for 6 seconds, repeat 15 times as a cycle, and last 3 minutes. Its characteristic is "short interval", which is used when the enemy's air raid weapons are close to the city. In the sky, a warning signal is sounded when an attack on urban targets has begun or is about to begin.
After citizens hear the air raid siren, except for staff in various industries (units) who must stick to their posts, all personnel should immediately take cover in the nearest civil air defense project. The audible signal for clearing the alarm is as follows: one long blast continuously for 3 minutes. It is characterized by "no interval". It is a warning signal issued when the danger of enemy air attack has been eliminated.
After hearing the warning, citizens should return to their homes and work in an organized and orderly manner. When enemy air strikes cause house collapse, fires, casualties, etc., under the unified organization of the unit or community, we must actively carry out self-rescue, mutual rescue, fire-fighting, emergency repairs and other actions to eliminate the consequences of the air strikes and restore production and living order as soon as possible. Minimize the damage caused by air strikes. What we heard on the anniversary of the September 18th Incident was one of the early warnings.
(3) What does an air defense siren look like?
For air defense sirens, the sound is often heard but not the form. So, what exactly does a siren look like? Where exactly is it installed so that it is so penetrating that it can be heard from every angle in the city?
Modern air defense sirens have developed into many types and models and have become an important part of the urban air defense system. As an important combat readiness facility, air defense sirens are used for disaster relief and disaster forecasting and emergency notification in the event of emergencies. They are used for civil air defense in wartime. They are the basic basis for people's governments at all levels to implement civil air defense command and organize the evacuation of personnel. means.
Air defense alarms usually use a combination of wired and wireless methods to control and issue air defense alarm signals. The control equipment is divided into wired control and wireless control. The power supply system is divided into mains power supply and self-power supply. The main The types of alarm facilities include:
Electric alarm
Electric alarms use electric motors as the main component of the alarm to convert mechanical energy into sound. The siren drives the sounding wheel to rotate at high speed through the motor. The general speed is around 2880r/min. When the motor drives the sounding wheel to rotate, the air enters the sounding wheel from the side air inlet, and the absorbed air generates high speed and high pressure and squeezes the window of the fixed wheel at the same time. When it comes out, it produces a buzzing sound. When the air defense siren pauses, it is actually not that there is no sound, but due to the inertia of the motor rotor, it still emits a low-frequency buzzing sound.
For example, the pre-alarm sounds for 36 seconds and stops for 24 seconds. In fact, what you hear is that the sound rises and falls for 36 seconds and falls for 24 seconds. The motor-type air defense alarm is composed of main components such as a sounding wheel, a fixed wheel, a positioning base, a positioner, a stainless steel cover, an electric motor, and a base. There are two types of electric alarms: discrete and horizontal.
1. Sounding wheel: It is the most important part of the alarm. It operates at high speed and generates sound source.
2. Fixed wheel: a component that is highly matched with the sounding wheel. It usually has about 10 sounding windows; the sounding window is equal to the sounding blade of the sounding wheel.
3. Positioning seat: the support for fixing the positioner.
4. Positioning device: When the chime wheel is in non-working state or after shutdown, the sound window can automatically close to prevent mice, birds, spiders, airborne debris, etc. from entering the alarm and triggering the alarm. to protect the alarm.
5. Stainless steel cover: on the one hand, it amplifies the sound after refraction, and on the other hand, it protects the entire alarm and prevents rainwater from entering the alarm;
6. Electric motor: The power of the siren drives the sounding wheel to rotate and plays the role of sound.
7. Base: the bracket of the entire alarm.
Electro-acoustic siren
The electro-acoustic siren consists of a host function and a speaker. Through the electro-acoustic siren terminal, it can carry out language communication, shouting, etc. Electric sirens and electroacoustic sirens focus on covering main urban areas and important target areas, and are the main body of the air defense warning network.
Multimedia alarm terminal
That is, LED multimedia alarm terminal, focusing on covering residential areas. It can not only issue air defense warning signals, but also issue emergency announcements and convenient information through text, voice, images, etc., enriching its functions.
Mobile alarm vehicles
Mobile alarm vehicles focus on covering blind spots and blind spots in the city. In addition, researchers are actively developing new air defense warning facilities that can be applied to buildings, underground spaces, etc., and gradually establishing and improving a multi-means, full-coverage air defense warning network. Under normal circumstances, an air defense alarm can cover an area of ??1 square kilometer, and the civil air defense department sets the alarm based on this distance.
However, 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, alarms are installed relatively densely; in open areas, there are not so many alarms. . In addition, squares or schools and other places where people gather are key places to install alarms.
The reason why the air defense sirens have a strong sense of presence and can be heard clearly in every corner of the city is precisely the result of the simultaneous cooperation of multiple sirens.