Sounder - Selection
Factors determining sounder selection
- Ambient noise in the environment
- The duration of signal required
- The noise level required and distance of signal required
Decibel level at a distance of 1 meter from source
The type and intensity of beacon/sounder chosen for any application will be determined by the environment in which it is to be used. Thus, what is suitable in applications in hotels might be unsuitable for similar applications in a factory, while a sounder for use in a dock yard may be quite inappropriate for use in a school.
Four broadly different types of environments
- Industrial, manufacturing. Includes not only factory premises but also equipment and facilities used in factories, such as cranes, mechanical handling vehicles, diesel generating sets and control panels. This category also includes industrial hazardous locations such as coal mines and the petro-chemical industry.
- Buildings; commercial and public. Schools, hospitals, residential homes, office complexes, airports, building sites and military sites are covered.
- Priority and public service vehicles. This category includes ambulances, fire trucks and police vehicles.
- Maritime - ships and dock installations. Hazardous sites such as oil terminals are included.
Frequency: pitch of note. The frequency is the identification of a note and is usually defined by the number of vibrations per second. Frequency can be measured by a frequency meter, which in its simplest form is the tuning fork. It is not expected that the electrical contractor will have such a meter, but it is usually sufficient for the frequency of noise in an environment be identified generally. For instance, the noise in a machine shop in which a grinder is installed would be of a high frequency, while that in a forge with a drop hammer in operation would be low frequency.
Time rating. Account must be taken of the time cycle the alarm is required to operate and a signal must be selected which has an adequate time rate. It should be noted that sounders used as fire alarms are required to be continuously rated. Having established the ambient or background noise and frequency level, the signal strength required is the sound which can be heard at the point of listening. Tests show that the ear can distinguish a warning signal which is ten decibels below that of the existing noise level, provided there is adequate frequency differential.
Noise attenuation. In selecting the signal strength required to cover an application, it is necessary to appreciate that as a “rule of thumb” sound is absorbed or reduces at the rate of six decibels as the distance from the signal is double. This factor is known as attenuation. Where the operating conditions are difficult, such as where there is a likelihood of high winds, or where there are solid objects in the noise path, attenuation of eight or ten decibels should be allowed 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. If the sounder is outdoors, then a weatherproof version must be selected. It should be remembered that there are also indoor situations that require waterproof enclosures. Explosion protected or flameproof signal devices are essential if the sounder is required to be sited in a location where there is explosive or fire hazard conditions.