Proper Surround Loudspeaker Selection and Placement...

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  Many factors come into play in designing a dipole loudspeaker to maximize the null zone. The off-axis responses of each driver, their location and the type of crossover slopes that will be used must all be weighed into the design by the loudspeaker engineer. It is then a matter of adjusting the crossover to achieve the frequency response that will give the desired cancellation or destructive interference effects. Angling the baffle on the opposite ends of a cabinet is a means for the engineer to widen or narrow the effects of the null zone.

Tomlinson Holman pioneered the concept of using dipolar surround loudspeakers for THX home theater systems, and dipolar surround loudspeakers are an important part of that application. The dipolar loudspeakers used in THX home theater systems are classified as THX-approved and have a flat power response when measured in a reverberant room. This means that when the energy around the loudspeaker is averaged together, the result is a flat frequency-response curve. This loudspeaker is not easy to design because of the null zone effect. Once the design is complete and the loudspeakers are set up, however, the result is one of the best surrounds for recreating a sense of envelopment and ambiance. Dipole loudspeakers are usually set up on opposite walls so that the null zone is aimed toward the listener position with the loudspeakers mounted on the wall approximately 2 feet to 3 feet (610 mm to 914 mm) above ear level.

Click for Figures 6 & 7

I have had the good fortune to design and work on two THX-approved loudspeaker system projects. When I found out that the surrounds had to have a flat power response curve to pass the THX requirements, I immediately thought of using horns to produce the high frequencies in the surround loudspeakers. This was a good fit for the system because the left, center and right loudspeakers used horns as well, so timbre matching the system would be easier than if dome tweeters had been used. Horns would also recreate the dynamics and performance of the majority of today's movie theaters because they also use horns for the front-channel loudspeakers.

Using a horn-loaded tweeter instead of a dome tweeter also had the advantage of controlling the directivity of the high-frequency energy. This benefit increased the null zone several degrees over dipole loudspeakers that use a dome tweeter. The null zone at high frequencies was also more consistent and was influenced less by the side wall and room reflections thanks to the directivity control of the horn. A main requirement of the THX dipole surround loudspeaker is to ensure a diffuse radiation pattern while maintaining a smooth frequency response. Not all units can make this claim.

Dome and horn tweeters
High-frequency devices commonly come in the form of a dome or horn. Each has its own dispersion and sound quality characteristics. Horns are known for their high sensitivity and pattern control. They usually cost more than the dome tweeters, but horns, because of their unique air-coupling capabilities, can also have low distortion levels when compared to direct-radiator dome tweeters (see Figure 5).

Dome tweeters have a wide dispersion pattern throughout most of their operation range. Their radiation pattern then begins to narrow sharply as the wavelength of the frequency it produces approaches the piston diameter (the moving area of the diaphragm) of its diaphragm. When the wavelength produced by the tweeter is equal to or less than its diaphragm cross-sectional piston area dimension, the dome tweeter begins to control directivity (see Figure 6).

Figure 8. Radiation patterns for a horn and dome tweeter.

In the case of a horn, the vertical and horizontal wall angle of the horn shape the radiation pattern (see Figure 7). Horns get their efficiency by channeling their emitted energy to a predetermined radiation pattern. Dome tweeters, on the other hand, are free to emit their energy and do so over a 180 area. The horn tweeter and the dome tweeter each have their unique characteristics, and the debate as to which is better will ultimately be determined by the particular application.

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Reprinted with permission from Sound & Video Contractor Magazine, August, 2000
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