Yamaha NEXO Alpha & Alpha E Series Owner's Manual download pdf (Page 14)

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ALPHA ARRAYS - SOME BASIC RULES

ALPHA SERIES USER MANUAL V1.0

DATE: 14/01/00 18:27

Directivity - Coverage

The two main qualities one might expect from a cabinet for array constructions are:

• A good directivity control in the mid and high frequency region, which guarantees the

steadiness of the interference region where dips and lobes occur;

• A strong roll-off of the directivity function at the -6dB cut-off angle, which minimises the

size of the interference region.

The Alpha series cabinets were designed to respect these two criteria. Particularly, the Alpha M3

features a constant coverage angle +/- 5° from as low as 800 Hz up to 12 kHz, with high values of

dB loss / degrees at cut-off angle.

Directivity of Multiple Sources - What HAPPENS?

In order to understand the coverage behaviour of combined sources, wavelength must be related

to the space between sources.

The wavelength

λ (in meters) of o sine wave is determined by:

λ = C/f where f is the frequency of the sine wave

This gives:

λ(20Hz) = 17 m, λ(100Hz) = 3.4 m, λ(1kHz) = 34 cm and λ(20kHz) = 1.7 cm

Interference

The distance between sources generates a path

length difference between the two signals that is nil

on axis and increases with the listening angle. If this

path length difference increases to half the

wavelength in a specific direction, the two signals

will cancel in that direction. This phenomenon is

often described as « interference ».

Resulting directivity

At low frequencies -because wavelength is much larger than the spacing between sources-

interference is generally not significant. However the directivity index will increase.

In the mid and high frequency range, where wavelength is comparable to spacing between

sources, the directivity polar plots will show dips and lobes. The amplitude of these lobes will

depend on the directivity of the individual sources and on their angulations.

Small Arrays (less than 4 Alphas M3 / M8)

The amplitude of the dips and the lobes is minimised when angling the cabinets at their nominal

coverage angle. If the angle is less than the coverage, the interference region will be larger, and if

the angle exceeds that value, there will be a « hole » between the cabinets.

It is therefore strongly recommended when using a small amount of cabinets to angle them at their

nominal coverage angle (Alpha M3: 35°x35°, M8: 75° x 45°; EM/EF: 75° x 30°).

Path length

difference

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Yamaha NEXO Alpha & Alpha E Series Owner's Manual Page 14

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