Attenuation of noise is subject to distance as well as to absortion through the atmosphere as described below:

1. Distance Sound Attenuation

Sound waves that spread into the atmosphere loose more of their intensity, i.e. the sound amplitude decreases. One reason is the geometrical spreading since the emitted sound energy is distributed over a larger and larger volume. Therefore the local sound energy flux (sound intensity) decreases with growing distance. If the sound originates from a point source (e.g. a single car), the waves are spherical and spread into all directions. In this case the sound level reduces by 6 dB as the distance from the source doubles. If, on the other hand, the sound originates from a line source (e.g. a busy road), the waves are cylindrical and the sound level reduces only by 3 dB as the distance from the source doubles. Model aircraft generally fall under the spherical category.


Distance Sound Attenuation (dBA) Examples

3m 6m 12m 24m 48m 96m 192m 384m 776m

95 89 83 77 71 65 59 53 47

98 92 86 80 74 68 62 56 50

101 95 89 83 77 71 65 59 53

110 104 98 92 86 80 76 70 64



2. Sound Attenuation through Air Absorption

A further effect that leads to an attenuation of sound pressure is the atmospheric absorption. As the sound waves propagate through the air, sound energy is lost due to friction between air molecules and because of further properties of the molecules. This loss depends on the air temperature and humidity. High frequencies are much more affected by atmospheric absorption than low frequencies. Therefore, one can hear only the bass tones at greater distance from an open air concert.


Absorption Attenuation Examples - Weather Conditions 29.92” Hg @ 20ºC and 70%RH (ISO9613-2)

83.3 Hz 0.15 dBA/km

100 Hz 0.22 dBA/km

125 Hz 0.3 dBA/km

250 Hz 1.1 dBA/km

500Hz 2.8 dBA/km

2000 Hz 9.0 dBA/km

2666 Hz 12.74 dBA/km

The base attenuation is governed by distance, and air absorption attenuation is additive so a jet producing 110 dBA @ 3m would be attenuated to 64 dBA @ 776m, and given a frequency of 2666 Hz the effects of air absorption would further attenuate by (12.74dBA x 0.776) = 9.88 dBA giving a noise level of about 54 dBA. In other words, less than normal converstion sound levels!

That's the simplified version - sound levels are determined by a number of other factors. An excellent paper on the subject may be found at: http://www.nonoise.org/library/envnoise/#measuring

Dennis