Alarm Sirens - Various Types of Sirens - Siren Sounds

The Sound of Sirens - About Alarm Sirens

Alarm sirens - you will find them on pylons and rooftops, in industrial quarters, prisons and even on beaches. Alarm sirens are installed around the world to warn about potential threats and dangers.

For some, the sound of a siren is an everyday reminder that work is over; to others, sirens are a nuisance -- particularly if they are being tested frequently (and if they happen to be installed next to your home).

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High Performance Alarm Siren - Civil Defence Siren

While the main purpose of sirens is to save lives, they can also become a symbol of death and destruction during wartime. For victims of war, the unnerving sound of an alarm siren is a harbinger of terror, deeply ingrained in their traumatised souls.

These are good reasons to take a closer look at the characteristic sound of an alarm siren. Traditionally, sirens were entirely mechanical, motorised constructions, but nowadays they are increasingly being replaced by electrical signals or even mobile phones.

The Avosound Online Archive contains more than 1,000 recordings of alarm sirens, police sirens, Aerosol air horns, recordings of a Klaxon horn and many other alarm sounds.

Alarm and Siren Sounds back in the Stone Age

The necessity for a signal sound to inform or warn members of a society is not a recent invention. Even in the Stone Age, drums and animal horns were used to spread messages far and wide. After the discovery of various metals, people began to manufacture signal horns for military use. Instruments like trumpets and trombones are derived from these early horns. As we can see, alarm systems and siren sounds have been part of our society since the dawn of mankind.

Alarm Bells were sufficient pre-Industrialisation

The use of alarm bells goes back all the way to the Egyptians and the time of the Pharaos. These so-called plate bells were hung from shelves so that their distinctive sound could be heard from far away. This technique is still being used today, except that plate bells and forged bells have been replaced with cast bells. For many centuries, storm bells and alarm bells were used to warn village and city dwellers from fires and attacking barbarians.

After the Industrialisation, the ambient noise of a mechanised society necessitated a new kind of alarm system. The first sirens were steam-powered pipes used on steamships, because traditional bells were no longer loud enough to overpower the noise of the engines.

The simultaneous high and low point: WWII air raid sirens

Thanks to electricity and the invention of the motorised siren, this type of siren was widely adopted before and during the Second World War. The undulating, howling sound of the siren was highly characteristic, which made it an efficient warning signal ahead of air raids. It also became synonymous with the terror of falling bombs -- death dealt from the skies. An entire generation was scarred forever by the sound of the motorised siren.

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Air Raid Siren Sound Recording: This original, historical recording is an original recording from 1944, made during an air raid on London and preserved in the BBC sound archive. It is almost spooky to hear the motor sounds of the approaching bombers. The siren sound begins at 1:40, which is easily identified in the spectral display.

More original WWII recordings are available in the BBC sound archive and the Avosound Online Archive.

In England, each village and city had its own network of sirens. Some of them were electrical (e.g. the famous brand Klaxon) while others were manually operated (particularly in rural areas).

The Cold War: From Air Raids to Nuclear Alarm

During the Cold War, sirens were no longer used to warn of air raids. Instead, they became alarm systems for possible nuclear attacks. In Germany, the term 'alert' replaced the previous 'air raid warning'. A shortened, one-minute siren alarm was devised to alert citizens of a possible attack.

In post-WWII England, alarm sirens were assigned a new meaning: alerting people to the threat of a nuclear attack. They were also used to warn people of floods and freak weather, as well as escaped prisoners or highly dangerous mental-health patients.

Sirens in America

In the United States sirens became commonplace after the country officially joined the Second World War. Most of the sirens could only produce a single tone. Their warning signals were also an octave higher compared to their European counterparts.

During the Cold War period, America had to be prepared for a nuclear attack. In Canada and the United States, alarm sirens would signal 'Alarm' and 'Attack' when a threat was imminent. Nowadays, the United States use various signals, but the most common ones are still 'Alarm' and 'Attack'.

American Sirens: Sirens for Alarm and Attack

The siren signal for Alarm is a constant sound, while the signal for Attack is an undulating howling siren sound. The purpose of the siren is to get people to inform themselves about the current situation (e.g. by turning on the radio).

Alarm Siren - Types and Construction

Electronic High-Performance Siren

Picture: Electronic High-Performance Siren, Source: Mde, Wikipedia

There are various types and builds of alarm sirens. Depending on their construction, alarm sirens will give off different alarm signals. Big alarm systems used in civil defence make use of pneumatic and electric alarm sirens. These large-scale sirens are built to project the alarm sounds over a big distance in order to reach as many people as possible in case of an emergency.

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Mechanical Sirens

Mechanical alarm sirens create a howling noise by rotating a shovel-studded drum within a container that sports openings. The higher the number of the rotor's revolutions, the higher the pitch of the howling. The result is an undulating siren alarm sound. The siren can be operated electrically or manually. These mechanical alarm sirens are widely used all over the world. In the United States, very big and loud mechanical alarm sirens equipped with V8 cylinder motors were used during the Cold War.

Picture: Mechanical Alarm Siren - Motor siren testing, Source: Youtube

In Germany, the mechanical siren is called unit siren E57 (although East Germany and countries of the former Soviet Union used their own type of similar construction during the time of the German Democratic Republic). This alarm siren creates a howling alarm sound that can allegedly be heard across very long distances (it reaches a sound level of 105 dB). In rural areas, the sound of the siren is said to reach 70dB even at a distance of 700m. In cities and industrial areas, these levels can obviously not be reached. One can imagine that the American alarm siren outfitted with a V8 motor was probably quite a bit louder than that.

Mechanical alarm sirens are often mounted on rooftops, high rise buildings and pylons. As these sirens are exposed to the elements and, in some cases, animals (particularly birds), they have to be tested several times a year. The sirens in Switzerland, for example, get tested annually on the first Wednesday in February -- which results in an impressive, country-wide howling concert.

Pneumatic Alarm Siren

Pneumatic alarm sirens can create enormous sound pressure and spread it across big distances thanks to the horns mounted on the top of the siren. The construction of pneumatic alarm sirens is different from mechanical sirens. While the mechanical siren creates the airstream with the help of centrifugal forces, the pneumatic alarm siren uses a compressed air canister to pump the airstream into the siren head.

Picture Source: Pneumatic High-Performance Siren, Federal Archive, B 422 Bild-0010 / CC-BY-SA 3.0

Alarm Siren mounted on a roof

Pneumatic Alarm Siren - Principle

Video: Principle of the air-operated alarm siren - the explanations are in german but the example how it produces the siren sound can be seen at 1:13, Source: Youtube

The air-powered alarm siren (pneumatic siren) is easily identified by the horns mounted to the siren's head. Similar to a mechanical siren, the air-powered alarm sirens create their signal by interrupting the air-stream cycle with the help of a perforated disc. The spinning disc inside the siren's head is called a rotor; it is electrically powered. The air-stream is directed through the holes in the rotor and external vents. The spinning motion of the rotor either opens or closes the holes, which creates the sound of the siren that is then directed to several horns. The air-stream that is needed for the operation of the siren is created by an Aerosol canister underneath the siren.

Benefits of Pneumatic Sirens compared to Mechanical Sirens

The main benefit of this type of siren is the extremely high sound level it creates. Pneumatic sirens can reach a sound pressure level of 130db (!) at a distance of 20 meters. The other benefit is the use of pressurised air. With the help of compressed air from a tank, these sirens can create a very high sound level almost immediately, which is very important for the proper operation of a siren. Pneumatic sirens also need less electrical power than mechanical sirens (but they do require more maintenance).

Many of these high-performance sirens were de-installed after the Cold War because they were no longer used. You can still see the pylons that those big old sirens used to be attached to (although nowadays they are more likely to house a mobile antenna than a siren).

Pneumatic Siren - Two-tone Horns

Unlike mechanical sirens, pneumatic alarm sirens can create a two-tone sound. Pneumatic sirens with horns are typically used on ships. The bigger the ship, the lower the sound of the horn. The Sounddogs Archive features some very nice recordings of ship horn sounds and pneumatic sirens. Below you will find an impressive example of a big ship's signal horn.

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Klaxon Alarm Sirens

The name Klaxon is almost synonymous with alarm sirens. The British company Klaxon manufactured the first mechanical sirens and horns in Birmingham. Before they were eventually replaced by electrical versions, these mechanical sirens were standard issue in cars, trains and ships. Klaxon alarms were typically used on submarines as well (diving alarm).

Klaxon Alarm Siren - Mode of Operation, Technical Drawing

Klaxon horns and sirens create their characteristic sound entirely through mechanics. A rotating, corrugated gear wheel rubs against a rivet that is fastened to a spring-steel membrane. The rotation of the corrugated wheel triggers movement in the rivet and in the spring-steel membrane, which leads to the typical Klaxon sound. The horn then adds a natural amplification to the membrane sound.

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The typical Klaxon Sound

Klaxon alarm sirens were often used in the harsh environments of submarines, ships and industrial buildings. It comes as no surprise then that the unique siren sound of the Klaxon can be heard in many movies. A submarine movie without a Klaxon diving alarm is not a real submarine movie!

The Klaxon horn made another famous appearance in George Lucas' Star Wars films. Just one example: Ben Burtt, the sound designer and creator of the Star Wars sounds, used a Klaxon horn for scenes set inside the Death Star.

The Klaxon siren sound in the Star Wars movies

Ben Burtt is the granddaddy of Star Wars sounds. He not only created the famous sound effects of the light sabres, laser guns and starships, he also created a unique acoustic environment for this epic science-fiction trilogy. Ben's contribution to the Star Wars films is now part of film history -- as well as a major influence on new generations of sound designers.

As Ben mentioned in various behind-the-scenes documentaries, he has a preference for mechanical sounds and likes to record everything that makes a sound by being rattled, turned or cranked. In some documentaries you can even see him record the sound of wires being hit in order to create the effect of a laser gun. This sound eventually became the trademark Star Wars laser effect and is still being used on the latest episodes in the series. For many fans of the genre, the effect pretty much defines what a laser gun should sound like.

Ben Burtt uses the sound of an old Klaxon siren: 2:40, Source: YouTube

The Klaxon horn is present in every Star Wars movie, although sometimes its sound has been modified in post-production. One of the most prominent appearances is in Rogue One, during the scene where the gigantic laser weapon on the death star is installed.

Ben used the Klaxon horn as a foundation for the creation of new sounds and effects. He bought old, rusty versions of the siren and recorded them in his studio.

It is quite impressive to hear him talk about how these old sirens can create interesting sounds. By turning the crank, different sounds can be created -- depending on the speed and force of the cranking. Because the siren operates mechanically, the acoustic quality of the sound is much better than if it was created digitally.

With digital, you sooner or later reach the limits of sonic definition, the sound becomes 'pixelated'. Mechanical sounds, on the other hand, always deliver 100% 'natural' definition.

Electrical Alarm Sound - Burglar Alarm

Electric alarm sirens are common in households and burglar alarm systems. Modern alarm systems are almost entirely electrically operated.
Electric alarm sirens use a small speaker to produce very high sound volumes.

Electric Sirens for Civil Defence

Electric Sirens for Civil Defence

Electric alarm sirens are not only used in household alarm systems or fire alarms, they are also quite common in civil defence structures (although in larger versions). Electric alarm sirens create the siren signal with the help of a loudspeaker and an amplifier. A horn directs the siren sound into whatever direction is desired. Electric sirens come with the benefit of having few mechanical parts that need maintenance (except for the loudspeaker). These sirens are therefore much lighter than their mechanical counterparts. Thanks to the electric transmission of the siren signal, undulating sounds can be created. It is also possible to use the siren as a public-address system.

Talking Sirens: Using Alarm Systems as a PA

Unlike mechanical sirens that can only produce a howling noise, electronic sirens can also transmit voice announcements. The usefulness of this technology is debatable, though, because wind and other noise will often make the transmission inaudible. Another problem with voice transmission for alarm purposes is echoing. As sirens are generally fitted to pylons or rooftops, the sound from several units transmitting simultaneously can create echoes and reverb between the houses and streets. The resulting cacophony renders the alarm message incomprehensible. Anyone who has ever listened to announcements delivered through cheap PA speakers at railway stations or airports will know how difficult it is to understand those messages.

When the Storm breaks, the Siren speaks

Noise disturbances and inclement weather are detriments to voice announcements. Sirens don't just go off on sunny days, after all (particularly not if they happen to be tornado, flood or hurricane warnings).

One major benefit of electronic sirens is their capability to be run off batteries or solar panels; this allows the siren to work independent of AC current. Electronic alarm sirens use considerably less power than mechanical ones.

Siren Sounds - Alarm Sound Examples

Siren Sounds - Alarm Sound Examples

American Alarm Siren in Bear Park, Milwaukee, Type Tempest T-135. This siren manages a sound pressure level of 135dB at a distance of 30 metres - Picture: Source Wikipedia, Copyright © 2009 Fedsigtbolt

This is the Sound of various Alarm Sirens

Motor Siren mounted on a high Tower - Motor Siren for Air Raid Alarm

In German and English cities, air raid alarms were used to warn of attacking bomber planes during the Second World War. The air raid alarm was supposed to warn both civilians and troops of a new attack. The air raid alarm is a series of undulating howling noises that last for about two minutes.
The air raid alarm was mainly created with mechanical motor sirens mounted on pylons and rooftops. For added mobility, the air raid sirens were also mounted on cars and trailers.

Picture: Mechanical Alarm Siren mounted on a tower in Crowthorne, England, © Copyright Brendan and Ruth McCartney

Air Raid Siren: All-Clear Signal

To give the all-clear after an air raid, the siren would produce a constant sound for about a minute. In some countries, this constant alarm sound is used to warn of natural disasters; in Israel it is also used to announce the sabbath.

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Flood Alarm

In Switzerland, the flood alarm consists of twelve constant low-frequency sounds that last 20 seconds each (with 10 seconds intervals between them). If you hear the flood alarm, you should get out of the affected area immediately. Flood alarm sirens are only installed in high-risk areas such as dams. If a dam breaks and a flood wave is released, a general (undulating) siren alarm will go off in the areas at risk.

Alarm Sirens: What to do in case of a Siren Alarm?

Sirens do not just exist to blight beautiful beaches or make ugly buildings even uglier. Alarm sirens have a purpose: to warn us of dangers. If you hear the sound of a siren, you shouldn't automatically assume that it's a false alarm.

If you leave near a dam, you should immediately seek higher ground when you hear a flood alarm go off. The majority of people, however, do not live near dams but in villages and cities. In case of a general alarm, citizens are advised to turn on the radio and await official communication. If the alarm turns out to be false, the authorities will spread the news and give the all-clear.

Mobile Phone Alarm

The purpose of a siren is to alert as many people as possible. An alternative to the alarm siren is the mobile phone alert. Even in countries that have insufficient infrastructure and some highly remote areas, mobile phones are readily available. As long as the mobile phone antennae are working, the mobile phone alert is a highly efficient and far-reaching tool. The antennae, however, are highly susceptible to natural disasters (e.g. earthquakes, tornados, tsunamis). But as long as the system managed to send out the alert BEFORE the actual disaster, it proved useful.

You can subscribe to a number of alerts and warning signals from different service providers. A text message is enough to subscribe to the global tsunami alarm, for example:

Tsunami Alarm

The term 'tsunami' became a household word after 2004 when a massive sea-quake in the Indian ocean triggered a series of devastating flood waves. Indonesia was hit particularly hard, along with other countries on the Indian ocean: Thailand, India and Sri Lanka all suffered massive devastation from the tsunamis.

To prevent future disasters of a similar kind and warn coastal populations about sea-quakes and tsunamis, many countries installed early warning systems. Sirens and info displays were set up along beaches to alert people of approaching tsunamis by sounding an alarm or displaying a text message. Tsunamis are the result of sea-quakes or underwater rock-falls. Depending on the location of the quake, the shock wave of the tsunami might take quite a bit of time until it hits the shore. Early warning systems use satellites and buoys to identify shockwaves and trigger alarms.

In Thailand, tsunami sirens alert the population and the tourists of the approaching danger in more densely populated areas. Sirens are also used to warn of tsunamis in other countries. The type of siren and alarm used depends on the region and the country. The goal is always the same, though: to warn of the approaching danger.

Tsunami Alarm in Christchurch

Tsunami Alarm - Alarm Siren in Christchurch, New Zealand. Source: Youtube

Mobile Phone Tsunami Alarm

Covering vast distances of ocean front with tsunami alarm sirens is quite a massive undertaking. For this reason, the mobile phone alarm system was created. It warns quickly and efficiently of approaching tsunamis. Many countries that do not have a well-developed infrastructure do boast a good mobile phone network, which makes it possible to alert people with a cell phone alarm.