This is the third segment in our series on design principles for distributed systems. For additional reading, be sure to check out the first and second articles within the series.

A number of different patterns can be used for laying out ceiling loudspeakers, and each pattern can be implemented in different degrees of layout density.

The pattern and density selected for an installation affect:
• Evenness of coverage
• Sound level capability of the system
• Intelligibility
• Power amplification requirements
• Cost of the system

The two basic, most widely used loudspeaker patterns are square and hexagonal (also called offset or triangular).

Square Pattern
A square pattern lines up the rows and columns and often requires fewer loudspeakers, resulting in a lower system cost.

A square pattern is easy to lay out, especially on a suspended ceiling tile grid.

It may also be easier for zoning large open spaces. It is usually the preferred choice, delivering more even coverage, and often with fewer speakers. It’s usually the best starting point for a design.

Hexagonal Pattern
In a hexagonal layout pattern, the rows are offset from each other. While a hexagonal pattern typically uses more loudspeakers, it may use fewer in some particular cases.

For example, in a room laid out with a square pattern, it the end row of loudspeakers cover only a small area. In a room where the width is 1.5 times the coverage of the loudspeaker, part of a loudspeaker’s coverage is wasted in the square pattern.

In these cases, you might be able to reduce the number of loudspeakers in each row by one and offset the rows from each other, with the offset loudspeaker from one row partially filling in the uncovered area from the adjacent row. This sometimes can reduce the evenness of coverage, however, I recommend using the square pattern when possible.

The three most common densities in loudspeaker layout are edge-to-edge, minimum overlap and full overlap. Edge-to-edge density places the loudspeakers such that the outside edges of their single-loudspeaker 6 dB down points (as projected onto the listening plane) just touch one another.

Minimum overlap is a tighter spacing where all spots are within the 6 dB coverage pattern of at least one loudspeaker. Full overlap is tighter still: the 6 dB down point of one loudspeaker extends to the on-axis point of adjacent loudspeakers, and all listeners are within the coverage pattern of at least two loudspeakers. See Figure 1.

Considerations For Selecting Layout Density
In general, higher loudspeaker density increases the SPL capability, overcomes ambient noise better, and reduces the variation in sound level throughout the space.

Budget may necessitate an edge-to-edge spacing when a higher density would be desirable. In this case, the designer should inform the client of the ramifications before the system is installed.

Higher Sound Levels (SPL)
Tighter loudspeaker densities allow for slightly higher SPL levels due to a greater number of loudspeakers covering each area within the listening space. While the summation between adjacent loudspeakers is incoherent, there is still some increase in SPL. See Table 1.

Level Variation Due to Layout Density
The loudspeaker density also affects the amount of variation of the sound level within the listening space. The tighter the spacing, the more consistent the sound will be from place to place. Some off-axis locations will be lower by as much as the stated figure in Table 1.

For example, in a square pattern installation with edge-to-edge density, if the on-axis maximum SPL for music and speech (considering distance from the speaker and any additional SPL from layout density) is computed to be 92 dB, there will be some locations within the listening space that will be 4.4 dB lower at 87.6 dB.

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