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For example a 50km per hour (31 mph) side gust = approx 13.9m/s.
The temporary change in direction during the gust = arctan 13.9/340
This may seem trivial until you realise that this sudden 2.3º change will shift a poorly
arrayed system's polar pattern undulations about 2m to right at a typical outdoor mix
position enough to swap high- mid and high frequency peaks and troughs several times
in just a few seconds.
Variable combing (phasing) caused by wind effects should be minimised by
avoiding widely spaced, parallel high frequency sections carrying the same signals.
Spaced, parallel loudspeakers will comb (add or subtract their outputs) depending on
their distance or time offset from us. A 150mm/0.5ms offset at the listening position
will cause nulls at 1KHz, 3KHz, 5KHz, 7KHz, 9KHz, 11KHz etc but we wouldn't be
aware of the combing under casual listening conditions because we are used to
listening to natural sounds in the presence of multiple arrivals (echoes) and our ear-
brain system adapts to it. We don't adapt to
especially in the horizontal plane, as our horizontally spaced ears act as a sensitive
interferometer.
Where budgets allow, mono centre columns should be used for lead vocals and
instrumentals. Large ensembles (such as large string sections or large choirs) should
be divided into multiple subgroups which are sent to separate clusters.
Wind gradients
Air movement is slowed by friction so wind is usually lighter near the ground than it
is higher up. Ground level wind speeds can vary from over 90% of the main wind
speed in the daytime, when the air is being mixed by being warmed by the ground, to
under 30% at night, when air - cooled by the ground - looses buoyancy. This varying
wind speed with height is called the wind gradient.
A wind gradient associated with wind blowing towards a loudspeaker will "slow" its
vertical wavefront differentially. The vertical wavefront will be slowed less near
the ground and its sound path will veer upwards.
= approx arctan 0.04 1 = approx 2.3º
varying
All material © 2007. Martin Audio Ltd. Subject to change without notice.
comb structures though,
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