A megaphone is a device that amplifies the sound of one’s voice, typically for the purpose of making announcements or instructions. With a siren, it becomes an emergency warning system.
There are several types of this instrument – some are handheld and resemble a small plastic trumpet, while others have permanent mounts on either vehicles or walls. They can also be mounted on motorcycles in order to warn other drivers and pedestrians about an upcoming police checkpoint.
In the United States, large megaphones with sirens are often used by stadium event security staff and by taxi drivers.
When mounted on the front of large trucks, such megaphones serve as an effective form of public address systems. The loud noise of a truck’s exhaust may be enough to cause ear damage to people standing close by, but they are also very effective at silencing small groups of protesters or hostile individuals.
A portable megaphone is often used at political rallies and similar types of demonstrations. The loudspeaker on a handheld model can be directed in order to focus the sound where desired.
For low-power hand-held or worn voice amplification, a piezoelectric microphone or a dynamic microphone is necessary for the conversion of sound into an electrical signal. Once the sound has been converted into an electrical signal by a transducer, it is amplified through an amplifier.
The megaphone is the direct opposite. It converts low-level electrical signals into sound waves which are amplified using a linear amplifier and an electronic crossover. Depending on the design, the overall effect may be that of a wooden trumpet or screaming siren.
An example of this is the “dead cat” megaphone, used by truckers to communicate with other drivers in order to warn them about police checkpoints or speed traps in slow-moving traffic. One or more electronic speed limiters are wired into the amplifiers, which produce an extremely high-pitched squeal when the truck passes through a radar beam.
The name “dead cat” comes from the fact that it is “as dead as a dead cat on the end of a rope”, since any police who hear it will be running for cover to avoid being hit by missiles aimed at them by angry drivers.
“Dead cat” megaphones have been a part of American law enforcement since at least the 1960s when they were first referred to as “speed guns”. They are also commonly used by law enforcement officers, particularly by highway patrol officers, for marking and communicating with one another about the locations of speed traps.
In Canada and the United States, megaphones are commonly used by truck drivers as warning devices to warn other motorists. The term triggers from the sound produced. A low-frequency tone, usually issued by a hand-held megaphone, is used to alert other motorists of impending speed traps along the highway in order to decrease the possibility of accidents.
In Canada and the United States, megaphones can be used as an offensive device by organized protesters. They are often used by police officers during riots, preventing rioters from hearing any commands they make over a loudspeaker. The loud noise produced by this equipment makes it difficult for people to hear what is being said on a loudspeaker. They also very effectively stop a small group of protesters from yelling slogans at gatherings of thousands of people, as the wild and sudden noise can cause the group to stop chanting, and run away.
In Turkey, police use megaphones attached to motorcycles in order to warn drivers about upcoming checkpoints on major highways. In Spain, law enforcement uses heavy-duty megaphones to warn vehicle drivers in busy areas about dangers.
A key component of the ambulance or emergency services vehicle is the public address system. Among its primary tasks is the broadcast of emergency warnings and transport instructions. This may include information on procedures such as traffic accidents and road closures, road conditions (i.e. icy roads), hazardous weather conditions, and impending roadblocks. The messages are often relayed across a larger area so that members of the public well beyond the immediate area of any incident can be alerted.
In addition, warning tones such as reverse beeps and sirens are used to alert pedestrians and other drivers to the presence of an emergency vehicle in situations where an audible message might cause confusion.
The sound generation component of a modern PA system consists of a loudspeaker or two loudspeakers with coaxial driver units. The type of loudspeaker used is dependent on the nature of the vehicle. Normally, a dynamic loudspeaker is preferred because it has a greater range and volume than a piezoelectric loudspeaker. Some units may also contain other sound-producing sources such as different types of whistles, sirens, or bells.
The PA system is mounted under or behind the vehicle’s dashboard so that the operator can easily reach all controls while seated in his compartment while keeping his eyes on the road ahead. The PA system is designed in such a way that all controls are within the operator’s reach. The system is operated from a special control panel installed on the dashboard. The control panel is usually mounted at eye level and the controls are usually lighted or backlit with luminous type markings to make them easily visible under poor lighting conditions.
Most of the PA systems used by emergency services in Hong Kong and Macau use megaphones as their primary sound source.