The Sound of Sirens - About Alarm Sirens

Author: Guido Helbling, Avosound - Last updated November 7th, 2018

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).

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.

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.

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).

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.

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'.

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).

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.

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).

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 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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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.

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.

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.

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.

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.

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.

Air raid siren: undulating howling created by a mechanical motor siren.

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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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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.

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.

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: http://www.tsunami-alarm-system.com/

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.

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.