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Current International standards for safety require the installation of an audible warning device in order to attract the attention of the operator and to indicate a dangerous or emergency situation.

The suitability of an audible alarm for a specific application is determined by the following factors:

– sound output DECIBEL (dB);
– sound frequency Hertz (Hz);
– the distance between the audible warning device and the operator;
– the existing environmental noise

International safety standards have established that the dB level must be 15dB higher than that of the ambient noise and the siren must, however, have a minimum sound output of 65dB.
The sound frequency of the siren, at the point where the sound output is greatest, must differ as much as possible to the frequency of the ambient noise.
Sound frequency, however, must be between 300 and 3000 Hz.

To put these rules into force the use of a phonometer is necessary – an instrument that allows the measurement of the dB and Hz frequency levels – and by consulting the technical data of the various types of audible signals in the SIRENA catalogue.

The sound output dB (A) level of Sirena’s audible warning devices are accurately measured in an anechoic testing chamber at a distance of one meter from the axis of the device, the ratings given in the catalogue refer to maximum sound levels; on request further information regarding the sound spectrum for all SIRENA audible warning signals can be supplied.

To select the correct product to be installed the sound spectrum of the siren must be superimposed upon the sound spectrum of the ambient noise.
The differential dB level and the differential frequencies Hz are therefore immediately recognized.

Additional factors to be considered when selecting an audible warning device.

Electric Motor Sirens are suitable for short duty cycle and not for continuous operation.  They produce a single tone sound and reach their specified operating frequency quickly. 
A single sound is very effective but can be easily accustomed to and looses its effectiveness after a short time:  to improve the sound output a  modulated or intermittent tone can be obtained by using a modulator.

Horns and Bells produce distinct sounds which are easily distinguishable. These products have low frequencies and are suitable for a variety of signalling applications especially long distance, short call signals or danger signals. They produce a continuous sound that can be changed to intermittent by using a modulator.

Magnetodynamic and exponential horn electronic sirens have a high frequency sound output suitable for short distance use.  In general, electronic sirens have the following advantages over electric motor sirens:

– low power consumption
– greater sound output with volume adjustment
– variable tone in the sound frequency
– progressive sound
– continuous operation
– combined audible/visual signal

DECIBEL dB(1m) – SOUND LEVEL MEASUREMENT – The sound level is measured in decibels.  Exact data relating to the distance the sound can reach are not available as the following factors can influence significantly the sound intensity: type of sound, wind speed and direction, humidity, fog and rain.

The table below shows theoretical values.

The sound perception of an audible signal, therefore, always depends on the application conditions – see table.

Visual warning signals, thanks to the different colours and degrees of brightness, create a language code that allows communication between machines and operators.

The degree of brightness varies according to:

– the distance between the luminous point and the observer

– the type of lens

– the colour of the dome

The light intensity of Sirena warning signals is measured in Cd (p) in a photometric chamber. The Cd (p) represents the peak luminous intensity using a transparent dome that allows 100% brightness. The distance, the colour and type of dome used must therefore be taken into consideration when installing a visual signal.

For example, if the viewing distance is doubled, the light intensity observed is reduced by a quarter and if the distance is quadrupled the light intensity is reduced by a sixteenth; a reduction of the brightness also depends on the colour of the dome. (See the LIGHT TRANSMISSION table). The light output is amplified if a Fresnel lens is used.

International standards regarding visual signals state that the light output of warning signals must be five times greater than that of the surrounding light level and emergency signals must be ten times brighter.

In order to ensure that the correct beacon is selected the ambient light level must be determined (measured in lux) and is obtained by means of a luxometer.
The choice of light signal – on the basis of European standards – depends therefore on the lux measurement and Cd (p) of the beacon:

Lux=Cd/Distance

Example: 10.000 Cd = 10.000 LUX at 1 m = 100 LUX at 10 m

There are three different types of visual signals:

– Flashing Beacon – cyclic ON/OFF of a filament bulb with greater light up times and less light intensity.
The effectiveness of the signal is attributed to the illumination of the whole surface during the light up time with emission at 360°.

– Rotating Beacon – The parabolic mirror revolves around the bulb emitting an intense beam of light.
Each point of observation is illuminated only when the mirror rotates in its direction.

– Xenon beacon – cyclic flash of a discharge bulb powered by an electronic circuit.
Differing from a flashing beacon the xenon discharge has an extremely high peak intensity in a short light up time. Visibility at 360° is guaranteed and can also be amplified by using a Fresnel lens. (See drawing below: THE LIGHT).