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.