Ground Attenuation Factor Based on Measurements



Assumptions made on the ground types between sound sources and receivers can significantly
impact the accuracy of environmental outdoor noise prediction. A guideline is provided in ISO
9613-2 and the value of ground factor ranges from 0 to 1, depending on the coverage of porous
ground. For example, a ground absorption factor of 1 is suggested for grass ground covers.
However, it is unclear if the suggested values are validated. The purpose of this study is to
determine the sound absorption of different types of ground by measurements.
Field noise measurements were made using an omnidirectional loudspeaker and two microphones
on three different types of ground in a quiet neighborhood. One microphone was located 3ft from
the loudspeaker to record near field sound levels in 1/3 and 1 octave bands every second. The other
microphone was located a few hundred feet away to record far field sound in the same fashion as
the near field microphone. The types of ground tested were concrete, grass, and grass with trees.
Based on the measurement data, it was found that grass and trees absorb high frequency sound
well and a ground factor of 1 may be used for 500Hz and up when using ISO 9613-2 methodology.
However, at lower frequencies (125 Hz octave band and below), grassy ground reflects sound the
same as concrete surfaces. Trees absorb more low frequency sound than grass, but less than ISO
9613-2 suggested.