Wharfedale Pro WLA Fly Frame - Manual (Page 7)

WLA Series

Air Absorption
The high frequency performance of the medium and long throw sections of
the line array will be limited by air absorption. As the distance from the source
increases, the mid and high frequencies will be reduced. The amount of
absorption at a given frequency is a function of pressure, temperature, and
relative humidity. By far the most important factor is the relative humidity,
although temperature does have a significant effect.
To achieve minimal spectral variance across the coverage of the array, the groups
of loudspeakers covering the middle to rear of the venue will require equalization
to compensate for this effect. Air absorption is difficult to predict with any great
accuracy, and in any case the relative humidity of the venue is likely to change
when the audience arrive or the weather changes. It is important that the venue
is monitored during the event as the equalization applied during the design stage
with an empty venue may not be suitable during the event.
As a general guide, boost above 8KHz for a long throw group of loudspeakers,
and KHz for a medium throw group of loudspeakers is recommended.
The chart overleaf shows attenuation in dB/m for a given frequency and humidity.
It is important to note that the effects of air attenuation are linear, unlike the level
attenuation from distance from the inverse square law. For example, at 00m a
source would be 40dB down compared to it’s SPL at 1m. Assuming 50% relative
humidity and 20ºC the level at 10KHz will be an additional 16dB below this
(approximately 56dB down).