Martin Audio manual Vertical -6dBcoverage, WLX/WLXGS High, Vertical coverage, 47Hz, 94Hz, 188Hz

The medium sized array (left) has significant output to ±90º whereas the large array’s ±90º output is dramatically reduced.

A note on coverage nulls

It is useful to be able to calculate where these first response nulls will occur for various frequencies as they indicate areas where coverage, transient response and directional information would be poor without fill systems. For symmetrical arrays nulls will occur either side of the on-axis line. We can calculate the overall “null-to-null” angle using the simple formula:

As a very rough guide, the null-to-null angle will be approximately twice the -6dB coverage angle.

Interpreting polar plots

i)It is conventional to “normalise” polar plot on-axis amplitudes so that different polar shapes may be readily compared. In practice, the large array (right example) would have a higher on-axis amplitude than the medium array.

ii)It is also conventional to plot polar amplitudes on a logarithmic scale. This is fine when working in sound pressure level terms but is not suitable for superimposing a polar plot onto a venue plan. Venue plans are drawn to a linear scale so polar plots with linear amplitude scales would be more suitable.

iii)Polar plots have been simplified in this article for clarity. Real-world off-axis lobe amplitudes and shapes would vary considerably depending on boundary loading, echoes, reverberation and other audio sources affecting the same space.

Vertical -6dB coverage

The following table gives the approximate vertical coverage angles of typical WLX/WLXGS arrays - ignoring boundary effects (see later).

WLX/WLXGS High		Vertical coverage
	47Hz	94Hz	188Hz
3	Wide	Wide	98º
4	Wide	Wide	69º
8	Wide	69º	33º
16	69º	33º	16º

Use tall stacks for long shots. Useful for long distances in low-roofed venues with raked seating up to the height of the stack.

Use short stacks for short, wide vertical shots.

Use tall, electronically steered, stacks to project to high, distant seating – see later.

Vertical Boundary effects

As mentioned before, a solid floor will act as a reflector. This will cause a vertical stack to perform as if it were double the length, giving a useful low frequency boost and a narrower vertical polar response.

All material © 2007. Martin Audio Ltd. Subject to change without notice.