18
ARA series Users Manual
As sound propagates through the atmosphere its energy is
gradually absorbed by a number of energy-exchange process-
es in the air called atmospheric absorption.
The degree of absorption depends on the frequency of the
sound wave and the composition of the atmosphere.
At low frequencies, sound waves are generally not absorbed by
the atmosphere and can travel long distances. However, as the
frequency increases, the atmosphere becomes more effective
at absorbing sound. This is because the air molecules them-
selves start to absorb some of the sound energy.
In most conditions, dry air can produce high attenuation of
sound at high frequencies. Therefore, in the case of a predom-
inantly high-frequency range, measurements made under dry
conditions can differ considerably from measurements made
under more humid conditions.
The compensation of the absorption of sound by air (humidity
and temperature) implemented on ALMA is based in the ISO
9613-1: 1993 “Attenuation of sound during propagation outdoors
-Part 1: Calculation of the absorption of sound by atmosphere”.
For each ARA system, SARAs and LARAs the atmospheric cor-
rection can be activated independently per unit considering the
target distance of it and the ambient conditions (air tempera-
ture and humidity). ALMA includes a family of more than 500
filters that are activated at different frequencies with different
gain levels depending on the distance and ambient conditions.
For example, shown above the air absorption at 25ºC - 55% rel-
ative humidity. A percentage of correction parameters is pre-
sented:
ARA series
Atmos. Correction
