attenuation (although small) will disappear, and the jet orientation will
change in the vertical direction, potentially directing the rear lobe of the
directivity pattern more towards the ground. It is believed that these are
two factors responsible for a sudden increase in noise level, as shown in the right-hand portion of Figure 2-9, introducing a second noise peak. An additional factor may well be the presence of temperature or wind gradients that become significant above a few hundred feet. The spectrum of the noise heard in the community behind the runway end is shown in Figure 2-10 at the times of the two peaks in the time history. The low-frequency content changes little; the higher requencies are diminished by air absorption.