Attenuation of sound waves
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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.


 
point source line source

 

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 und humidity. High frequencies are much more affected by atmospheric absoprtion than low frequencies. Therefore, one can hear only the bass tones at greater distance from an open air concert.  

Attenuation by air absorption at 20 °C and 70 % relative humidity 
(after ISO 9613-2)
125 Hz 0,3 dB/km
250 Hz 1,1 dB/km
500 Hz 2,8 dB/km
1000 Hz 5,0 dB/km
2000 Hz 9,0 dB/km
8000 Hz 76,6 dB/km