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Data was recorded using the set-up detailed above for the METEK DSDPA.90-24

SODAR. Measurements were made with and without the SODAR's baffles although for measurements made with the baffles attached the angle range was reduced to 8-30º as stability is an issue when using full baffles. For future measurements a stronger platform made from a metal frame and a method for supporting the top of the tilted SODAR would be required to make measurements of the whole SODAR for a larger range of angles.

4.5.2.1 Results Recorded without the SODAR's Baffles.

The first set of results was recorded without the baffles attached to the SODAR. For each tilt angle ten pulses were recorded for each beam. The measurement period was approximately an hour long as a result so increasing the number of pulses recorded should not be a time problem for future measurements. The SLM recorded an FFT every 0.1s. This is the shortest integration time allowed by the SLM software and was used to minimise the comb filtering effect caused by reflections from surrounding objects. In effect this is a form of pseudo anechoic time gating that relies on the data filtering and averaging processes applied after the measurement to remove data points where the influence of reflections occur as either values that are too high due to the capture of reflected energy in addition to direct energy or too low due to comb filtering effects or only partial capture of the direct signal. Filtering was carried out on the data to remove all the points that occur in between the SODAR pulse peaks resulting in a reduction of the number of data points from approximately 30000 to 280 giving ten points for each angle from 8º to 35º. Figure 4.5.3 shows the data in this form.

Figure 4.5.3 – Raw data from measurements of METEK DSDPA.90-24 SODAR directivity using a tilting platform and without baffles attached.

In order to improve the quality of the data further the two largest and smallest values for each tilt angle were removed and the remaining six points averaged. Due to the movement of the SODAR array towards the SLM, as the tilt angle was increased, a correction to the data to account for the small changes in distance and angle is applied. This correction is calculated using trigonometry to find the change in angle and

distance and the inverse distance law to calculate the change in SPL due to the distance changes. Figure 4.5.4 shows this data with a comparison to the predicted directivity based on the model shown in Chapter 2.

0 50 100 150 200 250 50 60 70 80 90 100 110 120 130

Data Point (Number)

S P L ( dB ) Untilted Beam Tilted Beam Reference Beam

Figure 4.5.4 – Measured directivity of METEK DSDPA.90-24 SODAR array using tilting platform method for a tilted and untilted SODAR beam compared with modelled beam

patterns of the same array shape.

From these result it can be seen that the FWHM is 16.3º for both the model and the measured data. There are some discrepancies between the data and the model. For the untilted beam the first side lobe energy is more than 10dB higher than in the model and the first null occurs at a narrower angle. This is due to the reflection of the pulse from the ground near the tilt platform. The SODAR computer stated that the tilt angle used for the second beam was 16.8º. It is unclear from this plot whether this is true or not however it is likely that the angle is slightly different since the SODAR computer assumes that the SODAR's baffles are attached. A quadratic fit can be applied to the section of measured data that represents the main lobe of this beam in order to find to tilt angle. Figure 4.5.5 shows the data from the tilted beam fitted with a quadratic curve. -25 -20 -15 -10-8.4 -5 0 5 10 -50 -45 -40 -35 -30 -25 -20 -15 -10 -6 0 Angle (º) N or m al iz ed S P L ( d B )

Measured Untilted Beam Measured Tilted Beam Modelled Untilted Beam Modelled Tilted Beam

Figure 4.5.5 – Measured data for tilted SODAR beam fitted with a quadratic curve to estimate the acoustic tilt angle.

The peak of the quadratic curve is found to be 16.38º which is 0.42º lower than the angle stated by the SODAR computer. This is a significant difference that could be caused by the presence of echoes from the ground and nearby buildings distorting the beam at the position of the SLM. It does show that it should be possible to find the acoustic tilt angle of a SODAR for a given set-up using a tilting platform and a SLM attached to a tower.

4.5.2.2 Results Recorded with the SODAR's Baffles.

A second set of results was recorded with the SODAR's full baffles attached. This set of data was recorded in cooler temperatures than the first set and the beam tilt angle employed by the SODAR was 14.7º instead of 16.8º. The range of angles

measured was from 7º to 28º as the SODAR with baffles on the platform becomes unsteady above 28º. Figure 4.5.6 shows the data recorded with the baffles attached. There is no comparison to the modelled behaviour of the array since the model does not include the baffle effects.

-22 -20 -18 -16.8 -14 -12 -10 -8 -8 -7 -6 -5 -4 -3 -2 -1 0 Angle (º) N or m al iz ed S PL ( dB )

Measured Tilted Beam Quadratic Fit

Figure 4.5.6 – Measured directivity of METEK DSDPA.90-24 SODAR array with full baffles using tilting platform method for a tilted and untilted SODAR beam.

From these measurements the FWHM of the untilted beam is 13º. All the recorded side lobe energy is at least 30 dB below the main lobe peak, although the exact peak of the main lobe was not recorded due to the physical restrictions of the platform. The FWHM of the tilted beam is 12.4º. The reduction in width is due to the baffles

reflecting the edges of the beam back into itself resulting in a narrower width. Figure 4.5.7 shows the tilted beam fitted with a cubic in order to estimate the beam tilt angle. A cubic fit is preferred this time due to the more complex shape resulting from the baffle influence. Both quadratic and cubic fits were tried for the unbaffled results with no change to the tilt angle identified.

-25 -20-18.9 -15 -10 -6.5 -5 0 -45 -40 -35 -30 -25 -20 -15 -10 -6 0 Angle (º) N or m al iz ed S PL ( dB )

Measured Tilted Beam Measured Untilted Beam

Figure 4.5.7 – Measured data for tilted SODAR beam with baffles fitted with a quadratic curve to estimate the acoustic tilt angle.

The peak of the cubic fit is found at 14.48º which is 0.22º lower than the angle stated by the SODAR computer. This could be due to measurement position errors and errors in fitting along with reflections from the surrounding buildings. The positioning error due to the human control of the tilt platform is 0.2º so this result is close to the

SODAR angle if position error is a contributing factor to the measurement result. There is significant distortion of the beam shape between -7º and -11º which would have altered the fitting although it is clear that the peak of the measured data is lower than 14.7º. The angle reported by the computer is a theoretical angle rather than the true acoustic angle so this may also contribute to the difference measured here.