Environmental Factors

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

What happens to sound over distance

In selecting a sounder for a particular application, the table to the right can be used as a guide as to the sound level expected at a certain distance away. Local conditions such as wind speed and direction or objects masking the sound path will change the end result. In difficult conditions, the distances a sound can be heard amy be significantly less.

Decibel level at distance from source

Decibel level (Db) at source

Distance from source (m)	1m	80	85	90	95	100	105	110	115	120
	2m	74	79	84	89	94	99	104	109	114
	3m	70	75	80	85	90	95	100	105	110
	5m	66	71	76	81	86	91	96	101	106
	10m	60	65	70	75	80	85	92	95	100
	20m	54	59	64	69	74	79	86	89	94
	30m	50	55	50	65	70	75	80	85	90
	50m	46	51	56	61	66	71	76	81	86
	100m	40	45	50	55	60	65	72	75	80
	200m	-	39	44	49	54	59	66	69	74
	400m	-	-	40	43	50	53	60	63	70
	500m	-	-	-	41	46	51	56	61	66
	1000m	-	-	-	-	40	45	50	55	60
	2000m	-	-	-	-	-	39	44	49	54
	3000m	-	-	-	-	-	-	40	45	50
	5000m	-	-	-	-	-	-	-	41	46

Decibel level at a distance of 1 meter from source

180

Loudest possible sound

170

Rocket Launch

160

Ear drum bursts

150

Threshold of pain

140

Rock concert

130

Air raid siren

120

Jack hammer

110

Riveting machine

100

Chain saw

90

Noisy factory

80

Vaccum cleaner

70

Noisy restaurant

60

Normal restaurant

50

Quiet office

40

Library

30

Whisper

20

Leaves rustling

10

Breathing

Environmental factors

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.