The X-axis on the graph depicts frequencies from 0.1 Hertz (Hz) to 100 Hz and the Y-axis corresponds to the strength of the measured sound in decibels (dB). The figure is an example of a measurement taken at 2.5KM from four wind turbines on a clear summer night. The peaks in the figure at 0.8, 1.6, 2.4, 3.2, 4.0, 4.8, 5.6, 6.4, 7.2 and 8.0 Hz confirm that the measured sound is from the wind turbines because these particular wind turbines are known to produce sound in these specific frequencies.