. Basic methods for the calibration of sonar equipment. 26 TYPES OF ACOUSTIC MEASUREMENTS t20' I00» 90° MAXIMUM»°° 70* BEAM 60° WIDTH 50" (lODBDOWN)^^. ELLIPSE MINIMUM BEAM WIDTH (10 DB DOWN) 2" JO 4» 3° S'Te't'lO" ZO' 30° 40" SO° 60° 80° 100° 120° 20°. 120* I00» 90* ^"^ MINIMUM (o'bEAM 90° WIDTH ^.(lODBDOWN) 40° SO* RECTANGLE 1° f 3° 4° 5° 6° 7°B°9°I0° 20° 30° 40° 50°60° 80° 100* I20» MAXIMUM BEAM WIDTH (1008DOWN) Figure 8. Directivity index as function of beam widths for rectangidar and elliptical pistons. Maxiniiini beam widths are measined in the plane throiii;h th
. Basic methods for the calibration of sonar equipment. 26 TYPES OF ACOUSTIC MEASUREMENTS t20' I00» 90° MAXIMUM»°° 70* BEAM 60° WIDTH 50" (lODBDOWN)^^. ELLIPSE MINIMUM BEAM WIDTH (10 DB DOWN) 2" JO 4» 3° S'Te't'lO" ZO' 30° 40" SO° 60° 80° 100° 120° 20°. 120* I00» 90* ^"^ MINIMUM (o'bEAM 90° WIDTH ^.(lODBDOWN) 40° SO* RECTANGLE 1° f 3° 4° 5° 6° 7°B°9°I0° 20° 30° 40° 50°60° 80° 100* I20» MAXIMUM BEAM WIDTH (1008DOWN) Figure 8. Directivity index as function of beam widths for rectangidar and elliptical pistons. Maxiniiini beam widths are measined in the plane throiii;h the acoustic axis parallel to the short side of the rectangle or including the minor axis of the ellipse. Minimum beam widths are measured in the plane through the acoustic axis parallel to the long side of the rectangle or including the major axis of the ellipse. tapering on the directivity index, however, except in extreme cases, is relatively small.'' The following additional information can be ob- tained from the directivity pattern: 1. The angle of maximum resp07ise is the angle be- tween the direction of maximinn response and the acoustic axis. 2. Bea7n width may be defined as the angular sepa- ration between the two points on either side of the main beam which are 10 db below maximmn. 3. Height of side lobes may be expressed in terms of the maximum pressure in any direction within the side lobe in decibels versus the pressure on the axis. 4. Rear response is defined as the maximum pres- sure within ±60 degrees from the rear in decibels versus the pressure on the axis. It should be noted in this connection that, pro- vided the device is linear, the directivity patterns and the directivity indices for transmitting and for •) It has been shown by E. Gerjuoy that the directivity index of a circular plate has its optimum value when no tapering is used. receiving are identical at each frequency. This fol- lows from the reciprocity principle.
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