CHAPTER 5

Low-Frequency Design Strategies

While wave guides provide isolated control over various mid- to high-frequency coverage areas, the low-frequency section of a M’elodie array still requires mutual coupling

—with equal amplitude and phase — to achieve better directionality.

Low-frequency directionality is less dependent on the array’s relative splay angles and more dependent on the number of elements of the array. At low frequencies, the more elements in the array (the longer the array), the more directional the array becomes, providing more SPL and headroom in this range. The directional control of the array is achieved when the length of the array is similar or larger than the wavelength of the frequencies being reproduced by the array.

Optimizing and Equalizing the Array

Once the design (number of elements, vertical splay angles and horizontal splay angles between arrays) has been designed using MAPP Online Pro, you can effectively optimize the array by driving it with multiple equalization channels, or zones. Typically arrays are divided into two or three zones, depending the design and size of

the array.

To optimize and EQ the array, different strategies are used for:

■High frequencies

-long throws

-short throws

■Low frequencies

High-Frequency Equalization Strategies

For the far field, air absorption plays a critical role. The longer the distance, the greater the attenuation at high frequencies. In this zone, high frequencies generally need a correction to compensate for energy lost over distance; the correction needed is usually proportional to the distance and high-frequency air absorption.

In the near- to mid-field, the air absorption is not nearly as critical; in this zone, high frequencies need little or no additional correction.

TIP: If your M’elodie line array uses a third zone for short throws, high frequencies there

may need to be attenuated to more appropriate near-field levels.

Low-Frequency Strategies

Although the array can (and usually should) be zoned for implementing different equalization curves for high frequencies, similar or identical equalization should be

maintained in all the low-frequency filters. Different low- frequency equalization settings in the same array will degrade the desired coupling effect.

For the same reason, severe gain tapering is not recommended for line arrays, since adjusting various zones with an overall amplitude control for each results in the following:

1.The length of the line array column is effectively shortened

2.Directionality decreases at low frequencies.

3.Low-frequency headroom decreases.

Figure 5.2 shows a series of MAPP Online Pro predictions based on an example M’elodie system design. Please note that small vertical splay angles on the upper part of the array are used to cover longer distances, while greater angles in the lower elements to increase vertical coverage for shorter distances.

125 Hz

250 Hz

500 Hz

1 kHz

2 kHz

4 kHz

8 kHz

Figure 5.2. MAPP Online Pro plots (above right) illustrate the vertical directivity characteristics of the array (above left), with a section view of the venue superimposed.

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