The frequency is the identification of a note and is usually defined by the number of vibrations per seconds.To measure frequency one should use a frequency meter. In cases where the electrical contractor does not have such a meter, it is usually sufficient for the ambient noise frequency in an environment for our purposes to be identified gengenerally. For example, the noise in a machine shop in which a grinder is being operated would be of a high frequency, while that of forge with a drop hammer would be of a low frequency.

Time Rating

Account must be taken of the time cycle over which the alarm is required to operate and a signal has to be selected which has an adequate time rate. It should be noted that sounders used as fire alarms are required to be continuously rated. Sirens for example, are products available on the market which have time ratings of either one minute or more, but these products are not accepted by the Fire Authorities. The contractor should keep this in mind when modifying or inspecting existing installations.

Noise Level Required

Having established the ambient or background noise and frequency level, the sound pressure level needs to be defined in order for the sound to be heard over distance that the sound is required to travel. Tests show that the hynab ear can distinguish a warning signal which is ten decibels below that of the ambient nose level, provide that there is an adequate frequency differential.

Noise Attenuation

In selecting the correct sound pressure level required to cover an application, it is necessary to appreciate that as a “rule of thumb” sound is absorbed and reduces at the rate of 6 decibels as the distance from the signal is doubled. This factor is known as attenuation. Where the operating conditions are difficult, for example where there is a likelihood of high winds or where there are solid objects in the noise path, attenuation of 8 – 10 decibels to avoid “blind spots” or inadequate coverage. Before finally choosing the signal to be used, ensure that the same or similar sound is not used in an adjacent system for other applications. Should the sounder be installed outdoors and be subjected to exterior elements, then a weather proof version must be selected. It should be remembered that there are also in-door situations that require weatherproof enclosures. Explosion or flameproof signal devices are essential if the sounder is required to be sited in a location where there are explosive or fire hazard conditions.

Sounders Available

There are many basic types of signalling devices available but in this selection we are covering:

  • Sirens
  • Buzzers and Hooters
  • Electronic Sounders
  • Beacons
  • Bells