Nearfield Details

Taken a close look at your loudspeakers, lately? Often in the rush to get them in position and then go on to completing the rest of the sound reinforcement system, some important details are overlooked. The nearfield of the loudspeakers (within the first 10 feet) has a great impact on the tonal quality of the sound they produce. Missing a few important acoustical details in this critical area can make a bigger difference in the sound of the system than your choice of amplifier, mic pre-amp or interconnect cable. In fact, large variations in the system's response (measured at the listener's location) can be directly attributed to the interference. In many cases this is caused by interaction with nearby surfaces and other loudspeaker drivers. These nearfield effects can also produce wide variations in the relative frequency response at different seating locations within the audience.

Our auditory system places a great deal of importance on the sound that arrives in the first 10 ms after the direct sound from a source. The comb-filter effects of interaction in this brief period are a major part of what we perceive as the sound qualities of the source. These early reflections colour the sound of an acoustic instrument or voice to give us a sense of their immediate surroundings. However, sound reinforcement loudspeakers are intended to reproduce a sound source (with all their colouration intact) and not to add any further change in sound quality. If we don't clean up the nearfield of the loudspeaker, then we have removed the ability to reproduce a wide range of sound qualities with the system. Instead the system is perceived to have an overriding quality that it imparts to all the sound it reproduces.

A close examination of these nearfield effects can yield a tremendous improvement in the fidelity of the sound from the system. The first place to start is the interaction between loudspeakers. Minimize the audience areas that are covered by more than one device (in each frequency band) by carefully aiming the devices. This is the advantage of using highly directional devices, as they can reduce the area of overlap. It is also necessary to minimize the offset between devices. The real trick in designing a loudspeaker array is the alignment of devices so that the overlapping areas of seating have synchronized arrivals from the associated devices (a four-dimensional problem with mostly one- or two-dimensional solutions). This may include the very delicate adjustment of the physical position of each device and highly accurate measurement equipment to optimize the relationship.

The other nearfield effect to examine is the acoustically reflective surfaces near the drivers. Gaps between the mouths of horns or the sides of full-range boxes should be covered with rigid panels or filled with acoustically absorptive material (fiberglass insulation, acoustical foam, etc.). The rigid panels can be hard if they are not in the direct or reflected (sound) path to the listeners. Changing the orientation of nearby surfaces can also help to reduce the interference effect in the audience. The closer the surface is to the driver: the more important the surface's acoustical qualities and orientation becomes. Keep in mind that absorption is related to wavelength -- higher-frequencies can be absorbed by a relatively thin layer of fuzzy material; while lower-frequencies require very thick layers of fuzzy material.

With a little effort, and a few commonly available materials, you will be able to make significant improvements in the quality of sound the loudspeakers produce. This will allow you to then take full advantage of the improved sound qualities produced by the new mixing, processing, and amplification gear you buy.

United Entertainment Media Inc.
460 Park Avenue South, 9th Floor
New York, NY, 10016
Ph. 212-378-0400
FAX 212-378-2160

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