The earliest form of police siren was made of two perforated disks. One of these disks revolved and when airflow was guided to these disks from a tube, air passage through the disk resulted in a sound. The disks were soon replaced by two concentrically placed cylinders one of which revolved. This too created a similar sound and with the low level of noise pollution in olden days, this type of police siren did a good job.
While these conventional sirens were pneumatic sirens, electronic sirens soon came into use as police siren. The energy requirements of electronic sirens were less, with different sounds being created using amplifiers and sound modulators. Wail, yelp, hi-lo, phaser, manual, air horn two-tone etc. are the more commonly used sound patterns in a police siren today. Besides that, present day sirens come with software that allows the synthesizing of more sounds according to user preference.
The control head of the siren can be programmed to toggle between different siren sounds, and also to automatically switch to more intense intersection sounds when the vehicle is turning the corner. Most police siren heads are also programmed to stop the sound when the driver is exiting from the vehicle. However, some vehicles make use of manual sirens also along with electronic sirens.
The type of police siren that is in common use these days make a sound that is shrill enough. But the problem is that the fast evolving technology is giving many drivers the choice of shutting out other sounds of the road. Many cars are sound proofed to block out other noises of the road. Then all cars have radios and options for playing CDs or cassettes. And people have the option of shutting out the world with the help of their earphones, not to speak of the problems created by mobile phones.
In this context the existing sound of the police siren often become insufficient to attract the attention of other vehicle drivers enough to warn them. So, now police siren technology is seeking higher levels to compete with the sound proofing of the cars, and the music listening devices that people have at their disposal. The result of this is the development of two different types of siren amplifiers known as howlers and rumblers.
Howlers or rumblers are not a new police siren in itself but something that can be added to a siren. It works by emitting piercing low frequency sounds. These are sharper than the ordinary sound of a siren and its vibrations will reach the other vehicles on the road, however high the noise level is. Since these are more vibrations than sounds, the other vehicles on the road will be able to feel it, even if they do not hear it, and the drivers will be alerted to the situation.
As a police siren, both howlers and rumblers do a very good job in that the warning is carried to the other vehicles on the road fully, notwithstanding how distracted the other drivers are. But the sounds these sirens produce are very sharp so that those operating it have to wear hearing protection. The inbuilt timers of these sirens also have to be kept on, to minimize the operator’s exposure to it.