Sound Systems: Outdoor

Outdoor sound systems have many common characteristics:

• They usually provide sound coverage to large areas.
• They are often used for sporting events, which means a cheering crowd. Therefore, the system must overcome the crowd noise with enough volume.
• They're outdoors; so the listener only hears direct sound.

  A Side Note: The sound traveling from the speaker directly to the ear is called direct sound. If inside a building, the acoustic energy also reflects off other areas, such as ceiling and walls. The reflected sound also reaches the ears or is reflected to another surface where it may be absorbed or reflected again. This additional acoustic energy makes the direct sound appear to be louder.

  Outside, there isn't much reflected sound, so the audience hears the direct sound, and that's all, unless there's an echo. However, the echo arrives so much later it doesn't cause the sound to appear to be any louder.

  Result? An outdoor sound system needs more power or more efficient speakers than an indoor system.

Outdoor Echoes

What causes the echo so often heard outside? Echo is caused by two things:

Figure 1

First, it can be a reflection. Sound travels quite slowly. The accepted rate is 1130 feet per second (the precise speed of sound is actually dependent on the temperature). Put another way, the delay for every foot sound travels is .89 milliseconds. If a reflective surface is 300 feet away, an echo would be heard in a half second (.89 X 2 X 300 = 534 milliseconds or .534 seconds). That is a very discernible delay (Figure 1).

Figure 2

Second, an echo can be caused by hearing sound from two different speakers that are separated (Figure 2). The sound first arrives at the listener's ears from the nearest speaker. Some time later, the sound from the farther speaker arrives at the listener's ears. Echo is the result. Unfortunately, the result of hearing an echo is usually poor intelligibility. The echo just confuses the ear/brain and people give up listening after a while.

Preventing echo in an outdoor system is often a challenge. The sound system design goal is to provide even sound level coverage for all the seating areas. The biggest challenge is to have each seat covered from only one speaker location. And, the listener should hear sound from only one speaker. Unfortunately, this is often difficult to achieve because of suitable mounting locations for the speakers. The other challenge is keeping the sound away from reflective surfaces which may cause an echo back to the audience seating. This has to be done by using speakers with the proper directional patterns and accurate positioning.

Figure 3

Wanted: Efficiency and Fidelity

To produce enough amplified sound for the audience to hear properly, outdoor sound systems need two things: First, the speakers must be efficient. High efficiency means the speaker produces a very loud acoustic output with a small electrical input. The greater the efficiency, the less power needed from the amplifier. Horns are usually used in outdoor systems because they are very efficient. Figure 3 shows the wattage difference between a typical horn and a typical home speaker cabinet. Second, an outdoor system needs fidelity. People often complain about unnatural sound for voices and terrible music reproduction when a system has low fidelity. There are systems available which produce good sound and are very efficient. The investment may mean more money, but the results will be worth it.

A Helpful Tool for Outdoor Systems

A common problem in many sound systems is the lack of level control. The sound is either too soft or too loud and distorted. A compressor/limiter helps alleviate this irritation. It also protects the speakers. The limiter controls the peak levels so the amplifier won't be driven into distortion. Distortion can harm ears and speakers. The compressor keeps the speaker volume more consistent even when the announcer gets excited and talks louder. And for the ultimate in controllers, there are even electronics which will adjust the speaker volume up or down, depending on the crowd noise!

Summary

A high-quality outdoor sound system requires much design and installation skill. Even a less expensive system benefits from proper design, selection of components, positioning, and installation. Please give us a call if you need to upgrade your present sound system or if you need a new one. We are always glad to help you.

by Ron Huisinga © 1996 Internet Sound Institute (www.soundinstitute.com). This article is for personal use only. Any commercial reproduction is not permitted without permission. To obtain permission, contact ISI at hopi@soundinstitute.com

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