How to buy Audio outdoor events: Think outside the box

The timing of this article could not be better. With one client I am setting up a variety of temporary outdoor systems to cope with summer crowds, and another client is having to deal with noise complaints from neighbours.

So my work at the moment is typical of the provision of outdoor sound.

Loudspeaker systems

Outside, the absence of walls and ceiling removes a lot of acoustic issues, theoretically resulting in clearer sound. The purpose of good sound design is to increase the ratio of direct to reflected sound. Sound that bounces off a wall or ceiling will be 'coloured' by the impact and travel further than direct sound. This 'late' sound will interfere with the direct sound, perhaps making it less intelligible. The extreme of this is a venue like a church, where dozens of reflections interfere with each other.

However, some reflected sound adds level as the positioning of loudspeakers against ceilings and in corners produces an increase in volume. On the practical side, a room gives you walls and ceilings to hang loudspeakers from.

Outdoor systems give you none of those benefits, meaning you may need a more powerful system to fill the same audience space, and your own loudspeaker supports. Given that there might be other ambient sound outside, as well as wind blowing, this all becomes more of a challenge.

Line arrays

One of biggest changes in audio in the past 20 years has been the arrival of the line array. Based on 80-year-old technology, a number of advances made them able to produce high-power, quality sound. They are de rigueur for almost all large sound systems and the norm for rock tours, festivals and large outdoor events. They can produce a lot of sound for their size. The amount of truck space required to transport them on an average tour reduced so much that this provided the economic argument on its own.

Previous loudspeakers were based on the 'point source' principle, where each speaker covered a specific section of the audience. While a line array may be made up of several loudspeakers, they combine to create a single virtual loudspeaker. They create a 'cylindrical', rather than a spherical, beam.

When you move further away from any sound source, the volume reduces. Normally the volume halves for each doubling of distance, a reduction of 6dB. However with a line array, it only goes down by 3dB for each doubling of distance. This breaks the rules of how sound travels and allows you to project the sound far further from a single point.

And there's more. Once you build your line array column, it will create your cylinder of sound. However, if you curve the column, you can bend the cylinder. This enables you to cover an audience both near and far from a single point and at a relatively even volume. The angle of the curve is critical, and manufacturers have software to calculate the angle that each line array element needs to hang at, according to the venue.

This is all very well for large line-array systems, but what about architecturally sensitive locations? Well, blending is still possible. Active line-array columns are made up of many individual loudspeaker drivers. Each one has its own miniature amplifier and signal processor. By adjusting the level and delay by tiny amounts between each driver you can bend the beam of sound, up or down and left or right. However, such systems come at a cost due to the electronics and software involved.

A line array can fire a cylinder of sound across a large distance, meaning fewer (and smaller) loudspeaker positions. This results in less rigging and a quicker set-up.

Noise pollution

While line arrays (and other well-designed loudspeaker systems) give good control over the direction of the sound, you might still have the problem of disturbing others. The fewer loudspeaker locations you have, the more energy you need to fire out from each one. This is akin to throwing a rock into a pond. The alternative is to throw dozens of very small stones, which will cause less disruption. However, on a practical basis, it is often easier and cheaper outside to rig fewer but larger loudspeaker positions. This is a compromise to which there is no right answer. It simply depends on the situation.

Outdoor sound needs careful monitoring. At most large concerts, acousticians monitor sound levels and direct the sound engineers to adjust volumes accordingly.

Weatherproofing

An outdoor system is exposed to the elements. British and European standards for permanently installed sound equipment specify that equipment must be rated to IP54. This kit may not cope with the extremes of some weather so it is advisable to locate loudspeakers in a sheltered position.

Many manufacturers will produce IP54 versions of their loudspeakers, and a few manufacturers specialise in weatherproofed loudspeakers. Aside from making a physically robust cabinet and providing glanded connections, they have to do something with the loudspeaker cones.

This might mean making them out of an alternative material, as most drivers are still made from paper. A common technique is to 'dope' them, which involves coating them in a sticky layer that never dries.

Weatherproofed loudspeakers will tend to have reduced performance over their indoor counterparts, but this difference can be marginal.