The groups of products selected in this catalogue have been available in the market for over 20 years. With many years experience in the area of sounder selection your local NHP representative is always available to offer advice in helping to choose the correct product. When selecting a sound device a careful study of your requirements should be made.

The following points should be considered: 1. The nature of the proposed signals, including sequences, duration of blasts, intervals and length of signal. Each type of hazard should be given its own code to ensure the correct response. On-site signals must not be confused with off-site warnings. 2. The nature of other signals in the locality. Hazard signals must not conflict with emergency services or civil defence signals. 3. Area and range of audibility to be covered by the system. The signal must be clearly audible to all persons, inside and outside the plant likely to be affected. 4. Test facilities. Siren motors, shutter and signal sequences should be regularly tested to ensure that they are still functioning properly. It should be possible to carry out testing without actually sounding the sirens. 5. The nature of the terrain and construction and heights of the buildings on the site. Undulating ground and enclosed or noisy areas must be taken into account. 6. Availability of adequate power supply. 7. The type of system to be installed. Plants with high levels of machine noise - or covering large sites - may be better covered by a series of smaller sirens than by one large one. 8. Local meteorological conditions. For example, valley sites are likely to be affected by fog, mist and wind currents. 9. The positioning of sirens. The ideal height above ground level for a siren depends on the individual type and power of the instrument. Sirens should not be mounted too high above ground level: 4.5 m to 6 m is usually recommended. Putting sirens on top of high buildings often has the effect of deflecting the sound waves upwards because of negative temperature gradients. Sirens should not be located close to tall buildings. Ideally there should be at least a clear 50 m radius around each instrument.

Evacuation Sirens Electronic Sounders Bells

Noise Attenuation

In selecting a sounder for a particular application, the table below 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 may be significantly less.

m		Decibel level (dB)
1		90	92	94	96	98	100	102	104	106	108	110	112	114	116	118	120	122	124	126	128	130	132	134	136	138	140
2		84	86	88	90	92	94	96	98	100	102	104	106	108	110	112	114	116	118	120	122	124	126	128	130	132	134
3		80	82	84	86	88	90	92	94	96	98	100	102	104	106	108	110	112	114	116	118	120	122	124	126	128	130
5		76	78	80	82	84	86	88	90	92	94	96	98	100	102	104	106	108	110	112	114	116	118	120	122	124	126
10		70	72	74	76	78	80	82	84	86	88	90	92	94	96	98	100	102	104	106	108	110	112	114	116	118	120
20		64	66	68	70	72	74	76	78	80	82	84	86	88	90	92	94	96	98	100	102	104	106	108	110	112	114
30		50	62	64	66	68	70	72	74	76	78	80	82	84	86	88	90	92	94	96	98	100	102	104	106	108	110
50		56	58	60	62	64	66	68	70	72	74	76	78	80	82	84	86	88	90	92	94	96	98	100	102	104	106
100		50	52	54	56	58	60	62	64	66	68	70	72	74	76	78	80	82	84	86	88	90	92	94	96	98	100
200		44	46	48	50	52	54	56	58	60	62	64	66	68	70	72	74	76	78	80	82	84	86	88	90	92	94
300		40	42	44	46	48	50	52	54	56	58	60	62	64	66	68	70	72	74	76	78	80	82	84	86	88	90
500		=	38	40	42	44	46	48	50	52	54	56	58	60	62	64	66	68	70	72	74	76	78	80	82	84	86
1000			=	=	=	38	40	42	44	46	48	50	52	54	56	58	60	62	64	66	68	70	72	74	76	78	80
2000						=	=	=	38	40	42	44	46	48	50	52	54	56	58	60	62	64	66	68	70	72	74
3000									=	=	38	40	42	44	46	48	50	52	54	56	58	60	62	64	66	68	70
5000											=	=	38	40	42	44	46	48	50	52	54	56	58	60	62	64	66

Sounders or sirens for alert and alarm purposes are too often chosen from the wide range of models available on the market without sufficient attention being given to the particular application for which they will be used.

Factors determining choice

The selection of the sounder for a particular application is usually determined by several factors.

These are:

• Ambient noise in the environment
• The quality and type of sound in the environment
• The duration of signal required
• The noise level required
• The type of electricity supply available
• Is visual indication also required

Environmental factors

The type and intensity of sounder chosen for any application will be determined by the environment in which they are used. Thus, sounders for certain applications in hotels might be unsuitable for similar applications in a factory; a sounder for use in a dockyard may be quite inappropriate for use in a school.

There are four broadly different types of environments.

These can be categorised as follows:

• Industrial, manufacturing. This category 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 (including the North Sea oil industry).
• Buildings; commercial and public. Schools, hospitals, residential homes, office complexes, airports and military sites are covered. Building sites, too.
• Priority and public service vehicles. This category includes ambulances, fire and police authority 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 amongst his equipment, but it is usually sufficient for the frequency of noise in an environment for our purposes to 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 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. Hooters are available on the market which have time rating of either one minute or two minutes. Such hooters will not be accepted by the Fire Authorities. When modifying or inspecting existing installations, contractors should bear this in mind as many existing fire alarm installations have hooters with non-acceptable time ratings.

Noise level required.

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, for instance 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 too. Explosion protected 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.