12 Public address

Part 1

It would be as well to start with a clarification of the term ‘public address’, or ‘PA’ for short. It obviously means having a system of loudspeakers placed so that the output of microphones or other sources can be fed to a relatively large number of people. It is, though, convenient to realize that there are three quite different applications:

1.   The audience is remote from the microphones. A typical example is that of a sporting event where the commentator is usually in a small room some distance from the loudspeakers. Another good example is on railway stations where the aim is to inform people of train arrivals, delays and so on. In both cases intelligibility is (or should be!) the most important factor. Sound quality is a secondary consideration.

2.   The audience and the microphones are in the same room or hall. The term sound reinforcement would be better here than ‘public address’ because that is usually what it is. In other words the audience, or at least much of it, can hear the performers but probably with a degree of difficulty. What is wanted is reinforcement. Perhaps unfortunately, because it isn't quite accurate, this kind of situation is usually referred to as ‘public address’ or ‘PA’, and that is the term we shall use here, simply to follow generally accepted practice.

3.   The amplification of music at a rock concert and similar events. While very important in the entertainment world, this is not a topic we shall try to cover here as the amount of amplification and the scale and size of the equipment needed is not likely to be within the range of interests of the reader. However, many of the principles we outline here will still apply.

The first type of PA can be dealt with quite briefly, as installations are usually permanent and normally the only problem is of having enough power. To avoid confusion we will call the two situations by their likely environments – outdoor PA and indoor PA.

Outdoor PA

In many ways this is the easier of the two, as there will normally be little or no risk of the microphone(s) picking up significant amounts of the loudspeakers’ output, this being the major problem in indoor PA.

A big problem one frequently finds in outdoor PA (and this is sometimes very noticeable on large railway stations!) is that the sound level from individual loudspeakers has to be high to make sure that there is adequate loudness everywhere. Unfortunately, this is apt to mean that the sound levels near the loudspeakers are uncomfortably loud. Also, at some positions there can be annoying multiple repetitions of sounds caused by different distances from the speakers. The solution is usually to have a large number of small loudspeakers, each one producing a correspondingly lower volume. It must be admitted, though, that this piece of advice is sometimes one that for very good reasons cannot be followed: the cost of large numbers of units may be prohibitive, or it may not be possible to find the right positions for them all to be mounted safely.

There are likely to be very long cables to the loudspeakers and there is then a risk of loss of power in the wires. A common answer is to use what is termed 100-volt working. Briefly, this means that the system copies the national grid in a miniature way, where power is distributed at high voltage and low current. Part 2 explains this more fully.

Indoor PA

The problem here, almost invariably, is that the microphone(s) and loudspeaker(s) are not very far apart so that it is easy for the microphones to pick up some of their own amplified output. This then goes round the system again and there is a grave risk of an oscillation, resulting in a (usually) high-pitched squeal. This is often called ‘feedback’. A more descriptive term which we shall use here is ‘howl-round’.

Howl-rounds can be most annoying to an audience, as well as extremely embarrassing to the sound operators! Part 2 explains a little more fully their causes; at this point we will simply suggest some remedies to try. They may not all work!

1.   Reduce the sound levels from the loudspeakers to the lowest that can be accepted. This may not be as drastic as it appears, as it can often happen that the audience near the stage/platform receives a reasonable level of direct sound and therefore little reinforcement is needed.

2.   Angle loudspeakers away from the microphones. This can help but if a lot of the returned sound is via a hard wall then angling may have only a small effect.

3.   Use directional microphones. Cardioids with their dead sides towards the auditorium are an obvious solution. If there have to be loudspeakers close to the stage it is possible that hypercardioids, with their two ‘dead’ regions, may be more effective.

4.   If EQ is available on the amplifier or mixer being used then judicious use of it can help. To be really effective, however, the EQ needs to be comprehensive. For example, the use of presence cut at or near to the howl frequency is likely to be much more use, and possibly cause less deterioration of the wanted sound than simple top or bass cut. (A parametric equalizer, if available, can be very useful here.) Consider, if possible, ‘line-source’, sometimes called ‘column’, loudspeakers. These are stacks of drive units, often an odd number like five or seven, in one cabinet and they have a marked directional effect, albeit with limitations. Briefly, it can be said that if we suppose such a stack to be vertical then the sound radiated from it tends to be confined to a relatively small vertical angle but a wide horizontal angle. If this type of loudspeaker is used so that, for example, it is above the microphone, then there can be quite high sound levels out in the audience area but very little at the microphone.

Some general comments about indoor PA are the following:

1.   There is a great tendency for PA levels to be too high. The performers in front of the microphones are themselves often to blame for this! Many feel that they are not being heard unless they can hear themselves clearly from the PA loudspeakers. Admittedly, it is possible for a speaker to get great reassurance from this, but restraint should be tried.

2.   Personal microphones clipped to the clothing usually seem to give reasonable immunity to howl-rounds. As they are generally omnidirectional, the reason for this is slightly puzzling. Absorption and/or shielding by the clothing and body of the wearer may help to explain the effect. Additionally, the fact that the microphone is a fairly constant distance from the mouth makes the sound levels more predictable.