Introduction

When selecting a loudspeaker for a particular Public Address application, there are two basic requirements to take into account.

The first requirement is that the output level should be such that information broadcast can be comfortably heard by all people with normal hearing in all parts of the area that it is intended to cover.

The second requirement, which is just as important, is that the information must be intelligible. If the level of the speech near a loudspeaker has to be set very loud so that it can cover a large area, the area near the loudspeaker may not be intelligible and is likely to be uncomfortable.

Sound Pressure level (SPL)

All loudspeaker data sheets should quote the sound pressure level of the loudspeaker measured in decibels (dB). This is the sensitivity of the loudspeaker at a distance of 1 metre with a speaker input level of 1 watt. The decibel is a unit of relative level and sound pressures are defined in relation to a reference level, normally 20 Pascals. The SPL figure provides a standard from which coverage of a given loudspeaker location can be calculated. The increase or decrease in sound pressure level for various levels of power input compared with the SPL for an input of one watt obeys a square law and can be calculated using the following formula:

dB = 10 log Px   where Px = Power level for which the change in o/p level is being calculated

The basic rule is that each time the power input to the loudspeaker is doubled the output level is increased by 3 dB. For example, a loudspeaker having an SPL of 90 dB (for 1W/1M) will have an output of 93 dB for 2 watts, 96 dB for 4 watts etc.

The decrease in sound pressure level compared with the SPL at a distance of 1 metre obeys an inverse square law and can be calculated using the following formula:

dB = 20 log Dx   where Dx = distance at which the change in o/p level is being calculated The basic rule is that each time the distance is doubled, the sound level will decrease by 6 dB. For example, if the output level of a loudspeaker is 90 dB at 1 metre, at 2 metres it will be 84 dB, 4 metres 78 dB etc.

The relationship between the increase in output with increase in input power and the reduction in output against increase in distance can be demonstrated by the following table:

When considering the number of loudspeakers required and the output level of each loudspeaker for a given area, further consideration should be given to the dispersion angle of the loudspeaker. All data sheets should give a dispersion angle for the loudspeaker which will be in degrees, but it should also state the frequency at which the dispersion angle is quoted. The figure quoted will be the angle over which the output level reduces by 6 dB at a given frequency ie. if the figure quoted is 120 degrees the output will be reduced by 6 dB 60 degrees off axis.

To obtain a full picture of how the dispersion angle and the frequency relate, it is necessary to consult a polar plot for the Ioudspeaker The example shown is for a typical horn loudspeaker and shows the polar plots at frequencies of 500 Hz, 1000 Hz and 3000 Hz. Before the actual level to the loudspeaker can be determined, attention must be made to the level of the background noise. The system should be designed such that the sound level at head height should be 6 to 10 dB above the normal level of the background noise. It is advisable that anybody designing a PA system should have a basic understanding of acoustics as the way that sound pressure waves behave has a considerable effect on feedback, intelligibility and sound coverage. The relationship between wavelength, speed of sound and frequency is shown below. It should be noted that the relation between a particular frequency and its wavelength is not absolute as the sound velocity changes with temperature. The wavelength is the distance from the crest of one waveform to the crest of the next waveform.

When the audible frequency range is taken as 20 Hz - 20 kHz the wavelength becomes 17m - 1.7 cm. The wavelength for 1000 Hz is calculated as follows (sound velocity taken as 340 m/s):

The higher a loudspeaker is positioned, the greater the power drive that can be accommodated and the larger the area that can be covered by a single loudspeaker. In outdoor PA systems the loudspeakers are normally mounted at a height of 4 - 7 metres depending on the area to be covered.

The following table gives a basic idea of the area that can be covered by a number of ceiling loudspeakers, with relation to the height of the ceiling:

Intelligibility

The intelligibility of a Public Address System depends on a number of factors, not least the nature of the area that the system is intended to cover. It is most important to select the right type of loudspeaker to suit the area, have the right number of loudspeakers correctly positioned and set to the optimum output level, if such things as reverberation and time delays are to be overcome. All Public Address Systems should have good intelligibility, but this becomes particularly important when considering systems that are to be used for emergency purposes. Such systems should or may require to meet the requirements:

The CIS System was a product of an Institute of Acoustics working party.

Loudspeaker Selection

To achieve good intelligibility it is important to select the right loudspeaker for the application in hand. Today's advanced PA systems make it much easier to cope with specific and sometimes complex applications which require a PA system or sound reinforcement. The old problem of howl or positive feedback still occurs, but not quite as frequently. Directional loudspeakers allow more freedom to choose the site and still avoid howl as their maximum output can be directed away from the microphone. Loudspeakers may be Omni-directional, bi-directional or Uni-directional. Omni-directional units are typified by flush mounted ceiling, cabinet and sphere speakers. Bi-directional units have sound omitted from each end of, typically, a horizontal cylinder. Uni-directional units vary from wide angle horn speakers to more finitely adjustable cylindrical models with a very precise field of coverage. The applications of these types of unit are quite specific.

In an outdoor site, the sound attenuates with the distance of the listener from the loudspeaker. Sound level meters can be used to test the speaker's theoretical performance on site and to test the effect of the PA system beyond the desired coverage area.

In some football grounds for crowd control and safety reasons PA systems use loudspeakers which send out sound along closely defined paths such that people in one enclosure will hear an announcement which those only feet away in the next enclosure will not hear, or at least nothing intelligible will be heard.

Some horn speakers quote their sound dispersion angle - 120 degrees is a common specification. In other cases speakers more accurately described as Uni-directional sound projectors can be precisely aimed by adjusting the units on their mountings. The edges of the areas covered by such speakers are very sharp with a minimum of spillage.

Well designed directional loudspeakers like these are also used to overcome the inter-speaker echo effect. This happens when people hearing an announcement from the speaker closest to them also hear it a split second later from another more distant speaker. The delay will depend on the extra distance the sound has to travel and the result is to scramble the announcement. Putting sound systems into an indoor site may require speakers to be chosen with even more care.

Although the walls contain the sound from causing nuisance to others, they also reflect it. This will cause people to hear echoes (similar to inter-speaker interference) or reverberation which is in effect a more instantaneous interference which mixes the direct and reflected outputs.

These problems have to be addressed to different degrees depending upon whether the building is empty, sparsely populated or full. Theatre sound engineers have to pay a lot of attention to the variables they produce although an extreme example in a commercial context is a local authority's swimming pool. The echoes which can be produced from a room of which the walls are tiled and the floor is a water surface are very great.

In situations like this, the ambient noise level will be both changeable and potentially high. The difference between ambient noise and desired loudspeaker levels to overcome it is a complex subject, but for PA systems playing only speech and perhaps low level background music about 10 dB is usually adequate.

All speakers however need careful selection and siting. Small cabinet speakers may be best for corridors and approaches but will hardly cope with sports halls. In some applications loudspeakers with protective enclosures may be required, especially in the corrosive atmosphere at swimming pool complexes. Loudspeakers that are required to range long distances will have to do so without mutual interference. This suggests some degree of directional stability though each installation has to be taken on its own merits.

Using small power speakers in the confines of an office should not make inter-speaker interference a problem even if the area covered by one speaker overlaps that of another. If ceiling mounted speakers are used, some degree of overlap will be unavoidable. A neat spherical speaker can be used for public address low level in-fill in offices and other areas of non-extreme ambient noise or reflective surfaces.

In noisy workshops however, the higher volume required may necessitate multiples of speakers carefully chosen and sited to avoid interference.

Whatever type of loudspeaker is chosen to meet an acoustic requirement, consideration must also be given to the electrical performance of the loudspeaker, particularly the impedance of the unit.

Because loudspeaker driver units have a low impedance of typically 8 ohm, it is difficult to group a number of units together on the same circuit and match the output impedance of the power amplifier without wiring the loudspeaker in complex series/parallel arrangements. Using PA systems on low impedance lines also causes serious voltage drops in the loudspeaker cables.

To overcome the problem, most PA systems use high impedance distribution systems which are commonly referred to as 100 volt line systems. The power amplifiers are fitted with a line matching transformer which converts the amplifier output to a higher voltage and impedance. The loudspeakers are also fitted with a line matching transformer which matches the high impedance of the line down to the low impedance of the loudspeaker driver unit. The transformer will normally have a number of power tappings which allow the power input and hence the sound output of the loudspeaker to be adjusted. Using a 100 volt line system overcomes the problem of volt drops in the cables and allows a large number of loudspeakers to be connected in parallel over long cable runs without having to use large diameter cables.

The following table shows the impedance for 100v line and 70v line loudspeakers set at various wattage levels.

Loudspeaker Impedance(ohms)	Input to each Loudspeaker
	100v	70v
160	60W	30W
250	45W	22.5W
330	30W	15W
400	25W	12.5W
500	20W	10W
670	15W	7.5W
1K	10W	5W
1.66K	6W	3W
2K	5W	2.5W
3.3K	3W	1.5W
4K	2.5W	1.25W
5K	2W	1W
10K	1W	0.5W

IP Rating

For loudspeakers that are installed outside, it is important that the unit has the right degree of protection against the weather conditions that it will encounter in service. The IP (Ingress Protection) System provides a means of classifying the degrees of protection to dust and water afforded by electrical equipment and enclosures. The system is recognised in most European countries and is set out in European Standard BS EN 60529:1991. specification for degree of protection provided by enclosures (IP code).

The system has two figures although it can have a third for mechanical impact, but this is not normally used in practice. The first figure shows the degree of protection against solid objects and the second figure shows the degree of protection against liquids.

If a unit has an IP rating of IP43 you can see from the table that it would be protected against solid objects greater than 1mm and against water sprayed to an angle of 65 degrees from the vertical.

First Number	Second Number
Protection against solid objects	Protection against liquids
0 No protection	0 No protection
1 Protection against solid objects over 50mm	1 Protection against vertically faIling drops of water
2 Protected against solid objects over 12mm	2 Protected against direct spray of water up to 15 degrees from the vertical
3 Protected against solid objects over 2.5mm	3 Protected against sprays of water up to 65 degrees from vertical
4 Protected against solid objects over 1mm	4 Protected against water sprayed from all directions.
5 Protected against dust- limited ingress (no harmful deposit)	5 Protected against low pressure jets of water from all directions.
6 Totally protected against dust	6 Protected against strong jets of water eg. for use on ship decks.
	7 Protected against the effects of immersion up to 1 metre for short periods
	8 Protected against long periods of immersion under pressure