Detailed Analysis of a Temporal Sequence

A sequence of 32 successive 10-minute averages of meteorological conditions is represented by {a_log,ai_m) values plotted in Figure 8.31 to illustrate the temporal changes in sound speed profiles one might observe. The lines track trajectories in {a_hg,ai,„) space during the selected 320 minutes.

Figures 8.32-8.35 show examples of the impact of the varying sound speed profiles on the predicted spatial behaviour of excess attenuation in the 1/3 octave bands with centre frequencies 250 Hz and 1000 Hz, for sound propagation from a 2 m high point source over ‘long grass’ (see Table 8.6). In the 250 Hz band, there is a distinct zone of destructive interference associated with ground effect (see Figure 8.32) which is predicted to be very sensitive to the changing meteorological conditions.

Figure 8.34 indicates that the minimum in this zone is predicted to move by roughly 100 m during the selected 320 minutes. When the effective sound speed profiles become more linear (i.e. the a_hg value approaching zero while a_u„ increases, corresponding to periods from 13 to 18 in Figure 8.31), the destructive interference zone is predicted to become less pronounced since there are less strong sound speed gradients near the ground. Excess attenuation in the 1000 Hz 1/3 octave band is predicted to have a more complex spatial behaviour (see Figures 8.33 and 8.35), with clearly identifiable zones of constructive interference. When the sound speed profile becomes more linear or slightly upward refracting, a small shadow zone is predicted very close to the ground (e.g. during periods 20-22). Although all of these conditions could be categorized as downwind, some remarkable features are predicted. At receiver heights on the order of 2 m or less, going further away from the source will not necessarily lead to a decrease in sound pressure level.

Figure 8.32 Predicted distribution of sound levels relative to free field in the 250 Hz 1/3 octave band, for a source height of 2 m and propagation over “long grass" (see Table 8.6).The axes are not to scale. For the significance of the T in the top left hand of this and Figure 8.33, see Figure 8.31.

Figure 8.33 Predicted distribution of sound levels relative to free field in the 1000 Hz I/3 octave band, for a source height of 2 m and propagation over “long grass” (see Table 8.6).The axes are not to scale.

Figure 8.34 A sequence of plots of the spatial distribution of sound pressure level relative to free field propagation in the 250 Hz I/З-octave band, in 32 successive 10-minute intervals of moderate downwind conditions.The upper left figure is identical to Figure 8.32, and similar axes were used. The numbers in each subplot correspond to those in Figure 8.31.

Figure 8.35 A sequence of plots of the spatial distribution of sound pressure level relative to free field propagation in the 1000 Hz I/З-octave band, in 32 successive 10-minute intervals of moderate downwind conditions.The upper left figure is identical to Figure 8.33, and similar axes were used. The numbers in each subplot correspond to the ones in Figure 8.3 I.