Because the ambulance sirens they have just that soundwhich seems to change once the emergency vehicle passes us? And the same could also be asked about the jeeps Fire Brigadefor the police car and for other emergency vehicles. We have all heard these sirens at least once in our lives, but perhaps not everyone knows that it is precisely the frequency that decides the frequencies of each sound. Italian law through two different ministerial decrees.
The main Ministerial Decree regarding sirens for emergency vehicles is that of October 17, 1980 (which is nothing more than an update of the Ministerial Decree of 3 July 1974). Here it is written that all vehicles used for services fire prevention and the ambulances must be equipped with sirens capable of producing sounds in two different tones: Sol (392 Hz) and the Me (660 Hz), with a frequency tolerance of 5%. The sequence consists of an alternation between these two tones, the overall duration of which (therefore of low tone + high tone) must be of 3 seconds – what we have all come to know as “Nino Nino“.
Caution: the choice to use a siren bitonal it’s not random. By doing this it is easier for motorists to identify the position and the direction of rescue vehicle Thank you to the effect Doppler, which occurs because sound waves in front of the object they are compressed (and the sound will have a higher frequency), while behind the object the waves space apart (and we will hear a lower frequency). Precisely for this reason, when the ambulance is coming towards us the sound will seem louder to us acute and when it moves away the sound will seem more serious. Below is a short video to get a clearer idea of the phenomenon:
But the decree does not end here: within it it is also said that the intensity of the sound along the main axis must be between 115 hey 125 dB at a distance of 2 m, while that which reaches a vehicle placed in front of the vehicle at a distance of 30 meters must be at least 90dB. Essentially, therefore, ambulances and firefighters have the same type of emergency acoustic signal.
The situation changes slightly with regards to the vehicles supplied with law enforcement agenciessuch as Police And Carabinieri. In this case the reference decree is the Ministerial Decree 20 March 1979 and the main difference compared to what we have seen so far is that changes have been made to the frequencies:
- The low frequency is equal to 466 Hz and corresponds to a The sharp;
- The high frequency is equal to 622Hz and it’s a D sharp.
The other characteristics are almost identical to what we saw previously, except for thealternation between the two sounds which, in this case, does not present variations in terms of length between high and low frequencies.
