Nexo Alpha M3 User Manual

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Nexo Alpha M3 User Manual Manual pdf 14 page image

PAGE 14/37ALPHA ARRAYS - SOME BASIC RULESALPHA SERIES USER MANUAL V1.0DATE: 14/01/00 18:27Directivity - CoverageThe 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 thesteadiness 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 thesize of the interference region.The Alpha series cabinets were designed to respect these two criteria. Particularly, the Alpha M3features a constant coverage angle +/- 5° from as low as 800 Hz up to 12 kHz, with high values ofdB 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 relatedto 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 waveThis gives: λ(20Hz) = 17 m, λ(100Hz) = 3.4 m, λ(1kHz) = 34 cm and λ(20kHz) = 1.7 cmInterferenceThe distance between sources generates a pathlength difference between the two signals that is nilon axis and increases with the listening angle. If thispath length difference increases to half thewavelength in a specific direction, the two signalswill cancel in that direction. This phenomenon isoften described as « interference ».Resulting directivityAt 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 betweensources, the directivity polar plots will show dips and lobes. The amplitude of these lobes willdepend 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 nominalcoverage angle. If the angle is less than the coverage, the interference region will be larger, and ifthe 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 theirnominal coverage angle (Alpha M3: 35°x35°, M8: 75° x 45°; EM/EF: 75° x 30°).Path lengthdifference