26 



TYPES OF ACOUSTIC MEASUREMENTS 



120 



100° 

 90» 



MAXIMUM 600 



70» 



BEAM 60° 



WIDTH so" 



(KDDBD0WN) 40 . 



ELLIPSE 



MINIMUM BEAM WIDTH (10 DB DOWN) 



2 o jo 4 o 30 6 o 7 o e o 9 O| o 20 o 30° 40° SO 60" 80° 100° 120° 



20» 



120* 



100* 

 90* 



^MINIMUM 

 so* BEAM 

 so. WIDTH 

 _.(IODBDOWN) 



40* 



SO* 



RECTANGLE 



|0 2° 3° 4' 5° 6» ^■^"lO* 20° 30° 40° SCeC 80° 100* 120* 



MAXIMUM BEAM WIDTH (I0D8D0WN) 



Figure 8. Directivity index as function of beam widths for rectangular and elliptical pistons. Maximum beam widths 

 are measured in the plane through 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. 1 ' 



The following additional information can be ob- 

 tained from the directivity pattern: 



1. The angle of maximum response is the angle be- 

 tween the direction of maximum response and the 

 acoustic axis. 



2. Beam 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 maximum. 



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 



t> 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. 



4.1.4 



Projector Efficiency 



Another criterion which can be derived from the 

 response and directivity measurements is of special 

 interest to the designer because in the most funda- 

 mental way it rates his design as an electric motor de- 

 livering acoustic power for the electric power sup- 

 plied. This criterion is the projector efficiency, de- 

 fined as follows: 



The projector efficiency is the ratio in decibels of 

 the total acoustic power delivered by the projector to 

 the electric poioer input into the projector. 



To compute the efficiency E p of the projector it is 

 necessary to know the transmitting response R T , the 

 directivity index A, and the projector impedance z. 

 The projector efficiency 59 then is given by 



R T + A 



10 log |t_ 70.9. 

 "a 



(8) 



