20 



TYPES of acoustic: measurements 



Figure 1. Circuit referred to by equation (1). 



values are independent of this factor up to the point 

 where overloading sets in. 



It is seen from the above that the power delivered 

 by an amplifier is a function of the load impedance. 

 For this reason, the use of a fixed input power, a fixed 

 applied voltage, or a fixed applied current in deter- 

 mining the variation with frequency of the pressure 

 delivered by a projector (the impedance of which 

 changes with frequency) in general does not provide a 

 response characteristic that is representative of actual 

 service conditions. This consideration has led to the 

 use of cwailable poxcer as a basis. 



411 Available Power 



Available power 47 is defined as the power which a 

 driver having a fixed generated voltage e g and a fixed 

 internal resistance >,, delivers into a matched load re- 

 sistance r L . Figure 1 illustrates the circuit. 



From this circuit it can be seen that the power de- 

 livered into the load resistance r L = r g is 



' A "G»+ J 



curves of this transition loss are plotted against the 

 ratio of the impedance magnitudes r g /z for different 

 phase angles 6. 



If a projector is tuned, so that at resonance its im- 

 pedance is a pure resistance, the maximum power 

 will then be delivered by the driver to the projector 

 if this resistance matches the internal resistance of 

 the driver, that is, when r L = r g . It is seen that under 

 these conditions, the actual power P, equals the avail- 

 able power P A . In all other cases the actual power is 

 less than the available power. The efficiency of the 

 electric system under these conditions, however, is 

 only 50 per cent, since the amount of power dissipated 

 in the output tubes equals that supplied to the load. 

 In practical designs using class B or C amplifiers, it 

 is an advantage to use a lower source impedance, 

 about 14 )'l- This improves the electric circuit effi- 

 ciency and reduces the power dissipation in the out- 

 put tubes, thus permitting smaller tubes to be 

 employed. In testing, the source impedance of the 

 actual system should be simulated. 



412 Transmitting Response 



The transmitting response of a projector of given 

 impedance is expressed in terms of the pressure at 1 

 meter distance on the acoustic axis in decibels versus 

 reference pressure (1 dyne per sq cm) per watt ax'ail- 

 ahlc power from a given generator impedance (as- 

 sumed to be purely resistive). 



In connection with this definition of transmitting 

 response, it should be noted that the pressure de- 

 livered increases as the square root of the available 



4r„' 



(1) 



The actual input power delivered into a projector 

 of impedance z= r + jx= \z\cos9 + j\z\sin6 has the 

 following value: 



P,= 



Hence, 



r„ + z 



P, 



r = Pa 



■irr, 



(r, + rf + x- 



10 log y-= 10 log 



|_(r ff + r/+ x* j 



(2) 



IMPEDANCE RATIO 



The term 10 logP,/P A is well-known in transmis- 

 sion circuit theory 1 "' 9 and is the transition loss between 

 the resistance r B and the impedance z. On Figure 2 



Figure 2. Transition loss between a generator having an 

 internal resistance of /'„ ohms and a load impedance of 

 magnitude : and phase angle 0. 



