# Air Raid Sirens: A Sound of Vigilance with Profound History and Practical Significance Air raid sirens, the sounds that trigger a strong sense of alert in people during special times, boast profound historical roots and great practical significance. They are not only crucial signals for protecting people’s lives during wars but also important early warning tools for responding to natural disasters and emergency incidents in peacetime. In war movies, the shrill and piercing sound of air raid sirens often makes people feel the tension and cruelty of war. However, in real life, the role of air raid sirens goes far beyond this. As a key component of urban defense systems, they provide strong support for the government to conduct civil defense command and guide the evacuation of the public. Whether facing air strike threats or natural disasters, air raid sirens can promptly warn people to take protective measures and safeguard the lives of themselves and their families. ## 01 Classification of Air Raid Sirens The air raid siren system consists of an alert information processing and control center and multiple alert notification networks. The alert information processing and control center is the "heart" of the urban air raid siren system, responsible for receiving and processing air situation and disaster information, as well as controlling and monitoring the alert notification networks. The alert notification network is the main body of the air raid siren system, including the air raid siren control system, radio and television alert notification networks, and other alert notification networks. - The air raid siren control system issues air defense and disaster prevention alerts to urban residents via sound signals. It mainly comprises electric (electroacoustic) sirens, centralized siren control equipment, siren control transmission channels, and siren power supply facilities. - The radio and television alert notification networks use radio stations and TV stations to transmit air raid siren signals, and can also utilize wired radio networks and TV networks. - Other alert notification networks include wireless mobile alert networks, satellite alert communication networks, and civil air defense mobile siren vehicles. Together, they form a civil air defense alert notification system that integrates multiple means and levels of sound and voice alerts. ### 1. Classification of Air Raid Sirens Air raid sirens can be categorized based on their energy conversion methods and usage modes: #### By Energy Conversion Method - **Electric Sirens**: These drive a wind wheel to rotate via an electric motor, converting mechanical energy into sound energy to emit alert signals. - **Electroacoustic Sirens**: These convert alert audio voltage signals into audio current signals through a main amplifier, which then drives a loudspeaker to produce alerts. - **Pneumatic Sirens**: These use an air source as power to blow a steam whistle and generate sound alerts. - **Hand-Cranked Sirens**: These require no power supply; instead, manual clockwise cranking of a handle generates alert signals. #### By Usage Mode - **Fixed Sirens**: Installed at fixed ground locations for long-term use. - **Mobile Sirens**: Mounted on vehicles or used in a mobile manner through other means. - **Elevating Sirens**: Normally hidden underground and raised to the ground for alerting when needed. ### 2. Working Principle of Motor-Type Air Raid Sirens Motor-type air raid sirens consist of key components such as a siren wheel, fixed wheel, positioning base, positioner, stainless steel cover, electric motor, and base. - The **siren wheel** is the core component, generating sound waves during high-speed rotation. - The **fixed wheel** is equipped with sound-emitting windows, which work in conjunction with the siren wheel. - The **positioner** ensures that the sound-emitting windows close automatically when the siren is not in operation, preventing small animals and debris from entering. - The **stainless steel cover** not only refracts and amplifies sound but also protects the siren from external environmental impacts. - The electric motor drives the siren wheel to rotate at a high speed of approximately 2,880 revolutions per minute. Air is sucked into the siren wheel from the side air inlets, then squeezed out from the fixed wheel windows under high speed and pressure, producing resonance and emitting the alert sound. ## 02 Types of Air Raid Alerts and Signal Regulations Air raid alerts are generally divided into three types: **preliminary alerts**, **air strike alerts**, and **all-clear alerts**, each with specific sound characteristics and purposes: - **Preliminary Alert**: Issued when an enemy air strike is imminent but has not yet entered the airspace above the area. Its sound pattern consists of 36 seconds of continuous alert followed by 24 seconds of silence, repeated three times, with a total duration of 3 minutes. It reminds residents to quickly enter a protective state. - **Air Strike Alert**: Issued when enemy aircraft enter the air defense zone and an air strike is about to occur. The sound pattern is 6 seconds of alert followed by 6 seconds of silence, repeated 15 times, totaling 3 minutes. Residents must immediately take action to enter bunkers or protective facilities. - **All-Clear Alert**: Issued when the air strike ends and the danger is lifted. It features a continuous alert sound for 3 minutes, informing residents that protective measures can be lifted. ## 03 How Air Raid Alerts Cover an Entire Area The coverage of air raid alerts depends on multiple factors, with the following key measures ensuring full-area coverage: 1. **High-Altitude Installation**: Sirens are usually installed at elevated locations, such as building rooftops, radio towers, or dedicated siren towers. The height advantage allows sound to travel farther and be less likely to be blocked by obstacles. 2. **Precise Layout Planning**: The placement of sirens is calculated and planned meticulously. In cities or large areas, the distance between air raid sirens is controlled within a specific range to ensure that the sounds emitted by individual sirens overlap, forming a complete sound field that can penetrate buildings and other obstacles. 3. **Multi-Channel Integration**: Modern air raid siren systems are closely integrated with multimedia platforms such as radio, television, and mobile networks. Through these channels, even if residents are in enclosed buildings or underground spaces, they can receive alert information via mobile phones, radios, or TVs. This multi-channel, multi-level alert coverage mechanism ensures that air raid alerts can fully cover an area and that all people receive timely early warning signals. ## 04 Management and Protection of Air Raid Sirens To better fulfill the role of air raid sirens in air defense during wars and disaster prevention in peacetime, their use must be standardized in accordance with laws and regulations, and enhanced protection measures must be implemented: - **Wartime Operation**: Civil air defense departments activate the alert issuance control system and issue air raid alerts through various notification networks. - **Peacetime Management**: Civil air defense departments are responsible for the planning, construction, maintenance, and management of air raid siren facilities, and organize regular air raid siren tests on scheduled dates. Local people’s governments at or above the county level may organize air raid siren tests and must issue a public announcement five days before the test, notifying relevant entities such as communication, radio, and television systems to coordinate the transmission and issuance of air raid siren signals. Meanwhile, relevant laws and regulations clearly specify protection requirements for air raid siren facilities, including: - No unauthorized demolition of air raid siren facilities; - No occupation or misappropriation of dedicated frequencies and sound signals; - Building owners must provide dedicated spaces for air raid siren facilities. As a key tool for urban safety protection, air raid sirens continue to develop with the support of modern technology, becoming more efficient and intelligent. Their advancement is not only a progress in technology but also an important part of the social security system, playing a crucial role in protecting people’s lives and property.