Arcline 8/212 User Guide V1.0
Page 56
A.3 Climatic considerations
Atmospheric (air) absorption
When things sympathetically vibrate in a sound field they absorb sound energy, converting
some of that energy to heat and re-releasing an attenuated and phase-shifted version of the
sound. Although we tend to think about solid objects when we think of vibration and sound
absorption, the elements that make up air also exhibit sound absorbing properties – through
viscous losses due to the molecular collisions of all the atmospheric gases plus the vibrational
and rotational energy absorption processes of the two main gases, nitrogen and oxygen.
Air’s absorbent properties are mainly affected by temperature and relative humidity in typical
concert venues or festival sites.
These processes are per cycle (per wavelength). This is a problem for long-distance sound
propagation as it means that the resultant absorption is per metre, not the radial-type per
doubling of distance. And the absorption increases with frequency.
Relative humidity
Temperature °C
10°C
20°C
30°C
40°C
10%
0.0696
0.176
0.262
0.220
20%
0.154
0.217
0.167
0.116
30%
0.188
0.168
0.114
0.0838
40%
0.179
0.130
0.0883
0.0707
50%
0.157
0.105
0.0741
0.0646
60%
0.136
0.0889
0.0654
0.0619
70%
0.118
0.0776
0.0599
0.0610
80%
0.105
0.0695
0.0563
0.0611
90%
0.0936
0.0634
0.0539
0.0619
Absorption coefficient (usually shown as
α
) in dB/metre* at 8 kHz
(8 kHz = minimum bandwidth for good “out of the box” vocal presence)
*Multiply the dB/metre figure by the distance (in metres) required (e.g. 20% RH, 20°C gives a
21.7 dB 8 kHz loss at 100 m).
Red
= 8 kHz greater than 20 dB loss at 100 m,
Orange
= greater than -15 dB at 100 m,
Brown
=
greater than -10 dB at 100 m.
11 Appendix A - Line array basics
Summary of Contents for Arcline 212
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